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Development and verification of an internal
equalization charge system for lead-acid batteries in hybrid power systems
JOHN LJUNGBERG ANDREAS WESTERGREN
Master of Science Thesis
Stockholm, Sweden 2013
Development and verification of an internal equalization charge system for lead-acid
batteries in hybrid power systems
John Ljungberg Andreas Westergren
Master of Science Thesis MMK 2013:35 MDA 457
KTH Industrial Engineering and Management
Machine Design
SE-100 44 STOCKHOLM
Examensarbete MMK 2013:35 MDA 457
Utveckling och verifiering av ett internt utjämningsladdningssystem för blybatterier i
hybrida kraftsystem
John Ljungberg
Andreas Westergren
Godkänt
2013-Juni-11
Examinator
Mats Hanson
Handledare
Mikael Hellgren
Uppdragsgivare
Northstar Sitetel Sweden AB
Kontaktperson
Ulf Krohn
Sammanfattning
Detta examensarbete är utfört hos Northstar Sitetel Sweden AB. Examensarbetet innehåller en produktutveckling av ny teknologi med intern utjämningsladdning för batterier i hybrida kraftsystem till radiobasstationer. De befintliga system som används idag, använder en dieselgenerator för att ladda batterier som sedan förser radiobasstationen med kraft. Dessa system har ofta dålig bränsleeffektivitet på grund av långa utjämningsladdningar med låg last till dieselgeneratorn. Ibland har de även dålig mätnoggrannhet vilket gör att batteri kapacitet och dieselgeneratorer ofta överdimensioneras.
Studier har gjorts av olika typer av hybridsystem och även batterier, fokus har varit på en djup analys av bly batterier och hur dessa ska laddas optimalt med speciella typer av utjämningsladdningar.
En simulering av tänkt system har gjorts baserat på studierna och ett verkligt system har designats och tillverkats bestående av hårdvara med mikrokontroller, elektronik samt mjukvara med laddnings- och kontrollalgoritmer. Testning med utjämningsladdning av parallellkopplade batterier har utförts. Analys av data från tester har gjorts och jämförts med de system som finns på marknaden, speciellt med hänsyn till ekonomi och driftsfall inom speciella laddningsgrader av batterierna.
Resultaten visar att denna nya typ av intern utjämningsladdning fungerar väl och kan spara upp till 13,5 % i diesel förbrukning samt 260 timmars körningstid av dieselgeneratorn per år. Även mätnoggrannheten har förbättrats markant vilket gör att batteriernas kapacitet och dieselgeneratorns storlek kan minskas utan försämrad tillförlitlighet.
Master of Science Thesis MMK 2013:35 MDA 457
Development and verification of an internal equalization charge system for lead-acid
batteries in hybrid power systems
John Ljungberg
Andreas Westergren
Approved
2013-June-11
Examiner
Mats Hanson
Supervisor
Mikael Hellgren
Commissioner
Northstar Sitetel Sweden AB
Contact person
Ulf Krohn
Abstract
This thesis is conducted together with Northstar Sitetel Sweden AB. The thesis contains a product of new technology with internal equalization charge for batteries in hybrid power systems for radio base stations. The existing systems in use today, uses a diesel generator to charge the batteries which then supplies the radio base station with power. These systems often have poor fuel efficiency due to long equalization charges with low load to the diesel generator. They sometimes have poor accuracy, which means that battery capacity and diesel generators are often oversized.
Studies have been made in different types of hybrid systems and also batteries. A deeper analysis of lead batteries and how these should be optimized with specific types of equalization charges have been made.
A simulation of the system based on studies on a real system is designed and manufactured consisting of hardware with microcontroller, electronics and software with charging and control algorithms. Tests of equalization charge of batteries connected in parallel have been performed. Analysis of data from the tests have been done and compared with the systems available on the market, especially with regard to economy and operating conditions in specific charge levels of the batteries.
The results show that the new type of internal equalization charging works well and can save up to 13.5 % in fuel consumption as well as 260 h less runtime of the diesel generator per year. The measurement accuracy has been significantly improved, which means that the capacity of batteries and diesel generator size can be reduced without loss in reliability.
Table of contents
1 INTRODUCTION ............................................................................................................................................ 1
1.1 BACKGROUND ..................................................................................................................................................... 1 1.2 PURPOSE............................................................................................................................................................ 1 1.3 SCOPE AND OBJECTIVES ........................................................................................................................................ 2 1.4 METHOD ............................................................................................................................................................ 2 1.5 CHAPTER STRUCTURE ............................................................................................................................................ 2
2 LIST OF ABBREVIATIONS USED IN THIS THESIS ............................................................................................. 5 3 FRAME OF REFERENCE ................................................................................................................................. 7
3.1 HYBRID POWER SYSTEMS ....................................................................................................................................... 7
3.1.1 Energy sources that provide power to the load and the batteries ........................................................ 7 3.1.2 Different combinations of hybrid power systems ................................................................................ 10 3.1.3 Most common batteries types for hybrid power system ..................................................................... 12 3.1.4 Battery charge controllers ................................................................................................................... 16
3.2 VALVE REGULATED LEAD-ACID BATTERY MODEL ...................................................................................................... 18
3.2.1 Charge and discharge reactions .......................................................................................................... 18 3.2.2 Overcharging process .......................................................................................................................... 21 3.2.3 Aging factors ....................................................................................................................................... 22 3.2.4 Different methods for equalization charge ......................................................................................... 23
4 DESCRIPTION OF THE TASK ......................................................................................................................... 26
4.1 INTERNAL EQUALIZATION CHARGE SYSTEM FOR THE PROTOTYPE .................................................................................. 26
5 MODELING AND SIMULATION OF THE SYSTEM WITH INTERNAL EQUALIZATION CHARGE .......................... 27
5.1 CYCLING AT PARTIAL-STATE-OF-CHARGE ................................................................................................................. 27 5.2 EVALUATING POWER REQUIREMENTS FOR A REMOTE AREA POWER SUPPLY .................................................................. 28 5.3 MODELING AND SIMULATION OF THE SYSTEM IN SIMULINK ........................................................................................ 28
6 DEVELOPMENT OF A PROTOTYPE FOR THE CHARGE CONTROLLER .............................................................. 29
6.1 HARDWARE ...................................................................................................................................................... 30
6.1.1 Charging equipment for the system .................................................................................................... 32 6.1.2 The microcontroller used for the charge controller ............................................................................. 33 6.1.3 Simulation of the hardware in MultiSim ............................................................................................. 34 6.1.4 Design and development of the Printed Circuit Board ........................................................................ 35
6.2 BUDGET AND COST FOR THE COMPONENTS USED FOR THE PROTOTYPE .......................................................................... 36 6.3 SOFTWARE ....................................................................................................................................................... 37
6.3.1 Handling of the communication with external sources ....................................................................... 37 6.3.2 Logging of the important data ............................................................................................................ 39 6.3.3 Handling of sensing data and control of the system ........................................................................... 40
7 IMPLEMENTATION OF THE CHARGE CONTROLLER IN A REAL SYSTEM ......................................................... 42 8 TEST AND VERIFICATION ............................................................................................................................. 44
8.1 PREVIOUS WORK DONE BY SITETEL ........................................................................................................................ 44 8.2 TEST AND VERIFICATION OF THE PROTOTYPE ............................................................................................................ 45
9 RESULT ........................................................................................................................................................ 46
9.1 ANALYSIS ......................................................................................................................................................... 50
10 CONCLUSION ............................................................................................................................................... 54 11 DISCUSSION AND FUTURE DEVELOPMENT .................................................................................................. 55 12 REFERENCE .................................................................................................................................................. 57 13 APPENDIX ................................................................................................................................................... 59
1
1 Introduction
1.1 Background
In non-electrified parts of the world with increasing telecommunication demand, electricity is generated
with diesel generators and/or solar cells. A common way is the use of diesel generators along with
battery storage, so-called hybrid systems. Diesel generators produce power that is many times larger
than what telecom station (usually a radio base station) needs. A diesel generator runs intermittent
charging batteries and supplies the radio base station. When turned off, the station is operating on
battery until it is appropriate to charge the batteries again. This is repeated cyclically. The advantages
are that diesel has a higher efficiency at high power, and that the operating time is sharply reduced,
thereby extending the intervals between refueling and servicing of diesel generator. A further plus is
that the time when the diesel emits noise is reduced and also more environmentally friendly due to less
exhausts.
To optimize the hybrid operation, battery charging time should be as low as possible. This is done most
efficiently by limiting the charge state to not fully charge. During charge the charging current is
substantially proportional to the uncharged ampere-hours, which mean that you can charge at a higher
current through the entire charge if you settle for a not fully charged state. This type of operation is
called partial state of charge (PSoC). A disadvantage of PSoC operation is that the batteries available
energy content is reduced, leading to the need to use a larger battery, another disadvantage is that the
batteries have to be subjected to a more thorough charging with low current for a long time at certain
intervals a so called equalization charge. It may require 12-24 hours of charge and diesel generator
operation in these cases.
Northstar SiteTel is a global company producing batteries and site solutions for radio base stations.
Apart from Northstar SiteTel there are other companies in the business e.g. Eltek, Emersys, PowerOne,
Huawei and Emerson competing for market shares. This thesis work is done together with Northstar
SiteTel to evaluate a new solution to save diesel generator runtime.
1.2 Purpose
With a tough telecom market, everyone wants to sell their system. This work was done together with
SiteTel to build a new hybrid system, which should be a leader on the market with the best fuel saving
technology, cheap and environmentally friendly. Main aspects are listed in the following.
Long equalization charges do not use the diesel full power, which decreases the efficiency
drastically.
Hybrid systems at remote areas require regularly refilling of diesel, which is expensive.
Existing hybrid system lack measurement accuracy, as an effect of this the batteries used are
sometimes over dimensioned.
In order for this to work, a new technology for equalizing batteries needs to be evaluated. A Hybrid
system with internal equalization charge for batteries connected in parallel is built, tested and
compared to other already known hybrid systems, mainly in terms of cost, fuel consumption and
climate. By moving already existing energy in the batteries to another by using a boost converter, the
system can now equalize each battery at a time with low losses and almost no extra uptime for the
diesel generator.
2
The system is built upon an already existing cooling cabinet, which are used when charging batteries, a
commercial charger with rectifiers are used and controlled to cycle the batteries.
This thesis will present different hybrid solutions and charge control strategies, which are used for cyclic
operation. It presents comparisons between different types of batteries including design, operational
performance and maintenance.
Details are provided for Valve Regulated Lead Acid (VRLA) batteries, which are used in this thesis, and a
deeper section about different kinds of equalization charges, especially this new technology with
internal equalization charge.
1.3 Scope and Objectives
The important questions which are addressed in this thesis.
Which are the basic types of batteries used in a hybrid system?
What are the different types of hybrid solutions already on the market?
What is an equalization charge and what happens?
What are the different kinds of equalization systems available on the market?
How can you measure health of VRLA batteries?
How much savings can you expect from this new technology?
1.4 Method
To be able to understand how to build the system and when to charge etc. a pre study was made prior
to the prototype implementation. The frame of reference work was divided between the authors. The
first part about hybrid power systems, different batteries and charge controllers was made by John
Ljungberg. Next part about lead acid batteries and equalization charge was made by Andreas
Westergren.
A simulation was made in parallel with the pre study which continued until the prototype phase. The
work continued using the development process V-model, this process was well suited for this thesis
work. The prototype was designed according to requirements of the system, and the software design
was built upon this system. Changes in requirements, system design and software design were made
after verification. The prototype was tested and the results were compared to earlier data.
1.5 Chapter structure
The frame of reference will go deeper in describing some of the theory, which is needed to understand
how hybrid systems are built up. The section hybrid power system will describe some different
combinations in energy sources used in hybrid applications; different batteries and charge controllers
for these are also mentioned. Next section about battery model will give a deeper understanding in
Valve Regulated Lead-Acid (VRLA) batteries used in this thesis. It will also show different ways of
equalizing batteries, especially this new internal equalization method, and why you should equalize.
Task description describes the task deeper, why this thesis was made, and also some more information
about the internal equalization charge. The project started with a simulation, this section will describe
the whole simulation phase with different data and assumptions.
As the project prolonged, a prototype was built consisting of both hardware and software. The
prototype was tested and verified which is shown in section Verification. All necessary data and analysis
can be seen in the Result section.
3
In the end a conclusion will summarize the work done, and the discussion/Future recommendations
finalizes this thesis with some thoughts about future development. Where references have been
considered necessary they are marked with [ref], where ref is the number referring to correct reference.
4
5
2 List of abbreviations used in this thesis
Table 1, List of abbreviations used in this report
Abbreviation Description
A Ampere
AC Alternating Current
ADC Analog to Digital Converter
AGM Absorbed Glass Mat
Ah Ampere Hours
ARV Array Reconnect Voltage
BMS Battery Management System
CAN Controller Area Network
CAPEX Capital Expenditures
DC Direct Current
DOD Depth of Discharge
EMC Electromagnetic Compability
EMI Electromagnetic Interference
EOD End of Discharge
GPIO General Purpose Input Output
Hz Hertz
LP Low Pass
LRV Load Reconnect Voltage
LVD Low Voltage Load Disconnect
LVDH Low Voltage Load Disconnect Hysteresis
OpAmp Operational Amplifier
OPEX Operating Expenditures
PSoC Partial State of Charge
PV Photovoltaic
RAPS Remote Area Power Supply
RBS Radio Base Station
RTOS Real Time Operating System
SD Secure Digital
SLI Start Light Ignition
SOC State of Charge
SPI Serial Parallel Interface
USART Universal Asynchronous Receiver/Transmitter
V Voltage
VDC Volts of Direct Current
VR Voltage Regulation
VRH Voltage Regulation Hysteresis
VRLA Valve Regulated Lead Acid
W Watt
6
7
3 Frame of reference
3.1 Hybrid power systems
A system consisting of batteries where the batteries are recharged, by for instance a diesel generator, is
called Hybrid Power System, but these systems can be designed in many different ways.
Hybrid power systems can consist of various combinations of solar panels, wind turbines, generators
powered by different fossil fuels and batteries. [19]
The most common combinations are:
Diesel generator with batteries
Wind turbine or solar panels with diesel generator and batteries
A combination consisting of all sources
These types of hybrid power systems are used as remote area power supply (RAPS) systems, which is an
electricity system for locations where electricity distribution systems are unavailable. Typical RAPS
systems are used in telecom for their off grid radio base stations (RBS). With the increased use and
dependence of cell phones and wireless communication, these systems have become much more
important.
Figure 1, Hybrid power system design [1]
3.1.1 Energy sources that provide power to the load and the batteries
Wind turbine
In a hybrid power system with wind turbines, small wind turbines are more common. The wind turbine
generates electric energy from low wind speed to a specified maximum speed. For small wind turbines
the current is often direct current (DC) and larger turbines have alternating current (AC). [1][16]
Generation source
- Solar panel
- Wind turbine
- Diesel generator
Power conversion and control
- Rectifiers
- Inverters
- Battery charger
- Control system
Energy-storage facility
- Deep-cycle batteries
Load
8
Figure 2, Power output at different wind speeds [15]
As seen in Figure 2 the wind turbine has a cut-in speed where the turbine starts to generate electric
energy. When the wind speed increases the output power raises rapidly to the rated output speed
where the electrical generator reaches the limit to generate more power, also called rated output
power. Too high wind speed can damage the turbine structure and therefore it has a cut-out speed
where a braking system locks the wind turbine and power generation stops. [15]
Solar panel
According to Jacobi’s Law the "Maximum power is transferred when the internal resistance of the
source equals the resistance of the load, when the external resistance can be varied, and the internal
resistance is constant." [21]
Figure 3, I-V curve of a typical PV array [17]
This requires an ideal load, but the batteries in a hybrid system are far from ideal. In general the
operating voltage for a photovoltaic (PV) array that will charge a 12 V battery is 17 V. Usually lead acid
batteries are charged up to 14 V and when the discharging starts the voltage will drop to 12 V [17].
Because the battery also is a power source, the operating voltage for the PV array will be pulled down to
12 V because the opposing voltage from the battery due to the difference in resistance [20] (see Figure
4).
9
Figure 4, I-V curve for a typical PV array while charging a 12 V battery without an ideal load [17]
As seen in Figure 4 the delivered power from the array will only be 50 W instead of the specified 75 W,
because the un-optimized power output is 50 W at 12 V. To prevent this problem a voltage regulator
that act as an ideal load is placed between the PV array and the battery to get the specified 75 W and a
DC/DC converter adjust the output from the PV array to the battery voltage. [1][4][17]
Other factors that affect the output power from the array are the solar irradiation and the ambient
temperature.
Figure 5, power change with a decrease in solar irradiation on a typical PV array [17]
Figure 6, power change with an increase in ambient temperature around a typical PV array [17]
If the solar irradiation is decreasing, so will the short circuit current, but the open circuit voltage will only
decrease slightly (see Figure 5). When the ambient temperature is rising there will only be a small
increase of the short circuit voltage, but a relatively larger decrease of the open circuit voltage will occur
(see Figure 6), compared to the decrease in solar irradiance. [17]
Diesel generator
A diesel generator is an internal combustion engine with an electrical generator for electrical energy
generation. A hybrid power system usually has a small diesel generator because it only charges the
batteries or is used for backup.
For small diesel generators it is necessary to run the load no less than 60-70 % of the maximum load to
obtain best possible fuel efficiency. [1]
10
3.1.2 Different combinations of hybrid power systems
A hybrid power system that only consists of renewable energy sources is desirable but not practically
possible for many applications. That is because it is not possible to know the energy generation from the
sources. The reliability decreases significantly and the capital expenditures (CAPEX) cost increases due to
the maintenance and required space. [3]
The simplest hybrid system is a diesel generator with batteries. In this type of system the diesel
generator is charging the batteries with a rectifier or a battery charger in between to convert AC from
the converter to DC. The load is driven by the batteries and when the battery capacity becomes too low
the generator starts to charge the batteries. When the capacity comes to the defined max level the
generator stops.
To get this to work autonomously the system needs to be controlled with a control system. Usually this
control system consists of a microcontroller with some sensor and contactors/relays attached to it. The
important sensing data for this system is voltage and current. With this data the microcontroller can
calculate the capacity of the batteries and control the generator to power on and off.
Most hybrid power system has a backup generator to improve the reliability of the system, which is
important to ensure a solid access to energy.
Figure 7, renewable hybrid power system with a backup generator [18]
These systems can be run optimal with different configurations for the renewable energy source and
generator.
One configuration is to produce 80 % of the energy required for the batteries using a diesel generator to
allow the generator to run at optimal efficiency during the entire charging phase. The remaining 20 %
11
takes time to charge because it requires low power. This part is an ideal application for the renewable
source. [3]
Figure 8, 80/20 fuel to solar configuration [3]
In Figure 8 the power for the load is negative, but to be able to easily compare both load and sources,
the power for all of them are positive.
If the area for installation of the system is sufficient the renewable energy sources can provide 80 % of
the required energy and the diesel generator acts as backup at unfavorable weather. [3]
To determine how the system should look like, the conditions for the location where the system will be
placed must be investigated. The components of the system can be selected after all the data for the
location has been collected. The manufactures for the PV-arrays and the wind turbines have
characteristic curves for their products and with the collected data the choice of right sources can easily
be done, to get the most cost effective and reliable system possible. [4]
An important fact is the relationship between CAPEX and operating expenditures (OPEX) for the system,
where a system is relatively cheap to install the OPEX will be high and vice versa.
Figure 9, illustrative graph showing the relationship between OPEX and CAPEX for different hybrid system combinations
12
Figure 9 above illustrates how the relationship between OPEX and CAPEX change for different
combinations of a hybrid system. A system that is not dependent on renewable energy is costly to run
and maintain while a system only consisting renewable energy sources is instead costly to build, but less
expensive to run and maintain. [3]
Conclusion
When designing a hybrid power system the developer must consider different solutions depending of
the location of the system, economy, natural resources, availability etc. If it is difficult to get access to
the site, there will be hard to run and maintain a system containing e.g. a diesel generator and in some
cases batteries. In these cases the CAPEX cost will increase, due to the dependence of renewable energy
sources, but on the other hand a decrease in OPEX cost will occur. The natural resources like sun and
wind are highly dependent of the location and these resources have to be carefully investigated for each
site. Both sun and temperature are important for PV arrays in order to utilize full power, so there are
many challenges to take into account.
3.1.3 Most common batteries types for hybrid power system
Rechargeable batteries that are used in hybrid power systems are called secondary batteries. Common
secondary batteries for these systems and their characteristics are listed in Table 2 below.
Table 2, Characteristics of secondary battery types [2]
Battery Type
Cost
Deep Cycle Performance
Maintenance
Flooded Lead-Acid
Lead-Antimony low good high
Lead-Calcium Open Vent low poor medium
Lead-Calcium Sealed Vent low poor low
Lead Antimony/Calcium Hybrid medium good medium
Captive Electrolyte Lead-Acid (VRLA)
Gelled medium fair low
Absorbed Glass Mat medium fair low
Nickel-Cadmium
Sintered-Plate high good none
Pocket-Plate high good medium
As seen in Table 2 many types of lead acid batteries are used in hybrid power systems. Because of
differences in performance and design they are often classified in one of the following three categories
[2]:
SLI Batteries Lead-acid batteries that are designed for shallow cycle service, belongs to the category of
starting, lighting and ignition (SLI) batteries. These batteries are not designed for long life under
deep cycle service, but because they are the only manufactured batteries in some developing
countries, hybrid power systems sometimes contains SLI batteries. If the daily depth of discharge
is limited to 10-20 % and the maximum discharge is limited to 40-60 % the batteries can provide
service up to two years in small systems.
13
Motive Power or Traction Batteries Batteries that are designed for deep discharge cycle service belongs to the category called
motive power or traction batteries. The normal use of these batteries is in electrically operated
vehicles. Because of their deep cycle capability they are very popular in hybrid power systems.
Stationary Batteries For un-interruptible power supplies (UPS), stationary batteries are most common. These
batteries are designed for occasional deep discharge and limited cycle service but they have
characteristics similar to the two previously categories. Due to the continuous float charging that
is common for stationary battery applications, battery designs with low water loss lead-calcium
are generally used.
Lead-Antimony Batteries
Lead acid batteries with antimony as alloying element in the plate grid, which is the electrodes in the
battery with a specific pattern, are called Lead-Antimony Batteries. The advantages of using lead-
antimony alloys are high discharge rate performance, excellent deep discharge, better mechanical
strength than pure lead grids, better lifetime at higher temperatures than lead-calcium batteries and the
shedding of active material is limited. The disadvantages are high self-discharge rate and, depend on the
amount of overcharge and temperature, frequent water additions is required. Generally Lead-antimony
batteries are classified as motive power or traction batteries. [2]
Lead-Calcium Batteries
Lead-calcium batteries has calcium as alloying element and in addition to the same advantages as Lead-
antimony batteries, lead-calcium batteries has less loss off water and lower maintenance requirements
due to reduced gassing of the batteries. The disadvantages are reduced battery life at higher operating
temperatures and repeated deep discharges, which also leads to poor charge acceptance. [2]
There are two types of flooded lead-calcium batteries:
Flooded Lead-Calcium, Open Vent
Usually classified as stationary batteries and have a capacity range up to 1000 Ah. Low water loss, low self-discharge and a lifetime of up to 20 years in standby or float service are some advantages.
Flooded Lead-Calcium, Sealed Vent
Have a capacity range between 50 and 120 Ah. Apart from the advantage from other lead-calcium designs these batteries are “maintenance free” which means that water does not need to be added. The disadvantage is limited lifetime.
Lead-Antimony/Lead-Calcium Hybrid
These batteries have lead-calcium at the positive electrodes and lead-antimony at the negative plates
and are typically flooded batteries. This combination has the advantages from both lead-antimony and
lead-calcium batteries. [2]
Captive Electrolyte Lead-Acid Batteries
Very common types of lead-acid batteries for hybrid power systems are captive electrolyte lead-acid
batteries. In this type of batteries the electrolyte is immobilized and under normal conditions the
battery is sealed. The batteries start to gas at excessive overcharging and at that time the valves open
14
under gas pressure. Captive electrolyte lead-acid batteries are often referred to as valve regulated lead
acid (VRLA) batteries.
This battery design is intolerant of excessive overcharge because the electrolyte cannot be replenished
which also is accelerated in higher temperatures. The charge regulation voltage, charging methods and
temperature regulation are very important for this type of batteries. To prevent excessive overcharge,
constant-voltage with compensation of the temperature is a recommended charging algorithm. [2]
Gelled batteries and Absorbed Glass Mat (AGM) batteries are the most common type of captive
electrolyte batteries:
Gelled Batteries
Water loss is reduced by limiting the escape of gas. Improvements of deep discharge cycle performance in some batteries by an addition of phosphoric acid.
Absorbed Glass Mat (AGM) Batteries
Near full state of charge are gasses containing a composition of hydrogen and oxygen produced and an internal gas recombination prevent water loss.
Nickel-Cadmium Batteries
Different kinds of secondary batteries are Nickel-cadmium (Ni-Cad) batteries. Compared to lead-acid
batteries they have several advantages. The advantages are long life, low maintenance, handle excessive
discharges and exhaustive overcharge. Ni-Cad batteries are costly and compared to lead-acid the
availability is limited. [1][2]
Sintered Plate Ni-Cads and Pocket Plate Ni-Cads are the most common primary types of Nickel-Cadmium
batteries:
Sintered Plate Ni-Cads
Immobilized batteries that prevent leakage and are easy to transport and handle. The battery type has so called “memory effect”, which means that the batteries will reduce its capacity due to incomplete discharge cycles. This can partly be reversed by conducting a calibration charge cycle
Pocket Plate Ni-Cads
Much better than lead-acid batteries to manage deep discharges and temperature extremes and do not have the “memory effect”. The high initial cost is a disadvantage.
Lithium-ion Battery
Compared to the standard Ni-Cad the lithium-ion battery has twice the energy density, self-discharge is
less than half and similar behavior in terms of discharge. They don’t have a “memory effect” and don’t
require scheduled cycling, which means it, is a low maintenance battery. The maximum current flow
under cycling is between 1 C and 2 C. A rate of 1 C represents a charge current equal to the rated
capacity.
The disadvantages are the need of a protection circuit, or battery management system (BMS), to
maintain safe operation, which limits the peak voltage while charging and, under discharge, preventing
the voltage to drop too low. To prevent temperature extremes, the cell temperature is also monitored.
15
Another problem is aging of the batteries and because of this, after two or three years [22], the
batteries frequently fail.
Table 3 below is a summarized list of the advantages and disadvantages between the different batteries
from this chapter, which provides a clear overview of them all.
Table 3, Advantages and disadvantages with different battery types [2]
Battery Type Advantages Disadvantages
Flooded Lead-Acid
Lead-Antimony low cost, wide availability, good deep cycle and high temperature performance, can replenish electrolyte
high water loss and maintenance
Lead-Calcium Open Vent low cost, wide availability, low water loss, can replenish electrolyte
poor deep cycle performance, intolerant to high temperatures and overcharge
Lead-Calcium Sealed Vent low cost, wide availability, low water loss
poor deep cycle performance, intolerant to high temperatures and overcharge, cannot replenish electrolyte
Lead-Antimony/Calcium Hybrid
medium cost, low water loss limited availability, potential for stratification
Captive Electrolyte Lead-Acid (VRLA)
Gelled medium cost, little or no maintenance, less susceptible to freezing, install in any orientation
fair deep cycle performance, intolerant to overcharge and high temperatures, limited availability
Absorbed Glass Mat medium cost, little or no maintenance, less susceptible to freezing, install in any orientation
fair deep cycle performance, intolerant to overcharge and high temperatures, limited availability
Nickel-Cadmium
Sealed Sintered-Plate wide availability, excellent low and high temperature performance, maintenance free
only available in low capacities, high cost, suffer from “memory effect”
Flooded Pocket-Plate excellent deep cycle, low and high temperature performance, tolerance to overcharge
limited availability, high cost, water additions required
Lithium-Ion
high energy density, low self-discharge, no memory effect, don’t require scheduled cycling, low maintenance
the need of a protection circuit, problem with aging
Conclusion
When choosing batteries for a hybrid power system it is important to know how the system should
work, what source it will contains and how the load will behave. There are lots of differences between
batteries e.g. cost, charging behavior, temperature dependence, maintenance etc. The most efficient
battery for one solution might not be suited for another operation.
NorthStar SiteTel has since the beginning of year 2000 been developing lead-acid VRLA batteries and
site solutions. They just recently started to develop lithium-ion batteries, but BMS are hard to develop.
For this purpose VRLA batteries are used for this thesis work.
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3.1.4 Battery charge controllers
Battery charge controllers are important to:
Prevent battery from overcharging
By limiting the energy supplied by the source when the battery reaches a fully charged state.
Prevent battery from discharging to a low state of charge
By disconnecting the battery from the load or start charging when too low state of charge is
reached.
A battery charge controller regulates the charge of the battery and related to the performance and life
for the battery, it is the most important function. The main purpose is to, without overcharging, fully
recharge the battery. If the battery charge is not regulated, the voltage will reach to high levels and
cause major gassing, electrolyte loss and heating which will damage the battery.
By limiting or interrupting the current flow from the source the charge controller prevents excessive
overcharge of the battery when it becomes fully charged. This is often achieved by limiting the
maximum value of the voltage and this is called the voltage regulation (VR) set point. If for example a PV
array is used to charge the batteries while it is sunny, the array will be disconnected until the voltage
drops to the array reconnect voltage (ARV) set point. Another method that is used is to calculate the
ampere-hour (Ah) into and out of the battery to determine when to charge or discharge.
Excessive discharging of the battery repeatedly will result in loss off capacity and life. Most charge
controllers include a feature that disconnects the load or force charges the battery when the battery
reaches a too low voltage to protect from this. The low voltage state is a pre-set or adjustable state
called low voltage load disconnect (LVD) set point. If the load is disconnected, it reconnects again once
the battery is recharge to the load reconnect voltage (LRV) Set Point. [1][2][4]
Figure 10, set points for a charge controller [2]
Voltage Regulation (VR) Set Point
VR set point is defined as “the maximum voltage that the charge controller allows the battery to reach,
limiting the overcharge of the battery “[2]. The controller will interrupt the charging or limit the current
17
flow once the VR set point is reached. Dual VR set points are sometime used with for example higher
voltage for the first charge of the day and a lower voltage for the rest of the cycles. This will provide a
more effective float charge of the battery due to the gassing and equalization from the first charge. For
flooded batteries, the voltage selection should be at a point to minimize gassing, but for VRLA batteries
the gassing should be avoided.
If the system is a stand-alone PV system, the VR set point should be higher than the recommendation by
the battery manufactures, this because the need of shorter charging time due to the limited period of
useful time of the arrays. [2]
If the difference between VR and ARV is too large the current flow to the battery will be disconnected
for long period of time. This will make it difficult to fully recharge the batteries again. If the difference is
too small it can damage electro-mechanical switching elements due to the rapid on-off cycling. This
difference is called voltage regulation hysteresis (VRH).
A hysteresis between 0.4 V and 1.4 V is most common in on-off type controllers for nominal 12 V
systems. If a PV system does not have a daytime load the VRH should be smaller and vice versa. [2]
Load Reconnect Voltage (LRV) Set Point
The LRV set point is the voltage, which the controller reconnects the load or stops charging the batteries
after that LVD have occurred. If the load was disconnected, the voltage rises to its open-circuit voltage
(OCV) after reconnection and even more if additional charge is provided. The LVD set point occurs when
the voltage and the state of charge (SoC) is high enough.
The batteries lifetime will be shortened if LRV set point is selected too low so, just like with the LVD set
point, the developer have to consider the charge rates for the loads and sources. [2]
If the difference between LVD and LRV is too small the load or controller can be damaged due to the
rapid on-off cycling, and extending the charging time, if the difference is too large the charging time will
be extended. This difference is called low voltage load disconnect hysteresis (LVDH). [2]
Array Reconnect Voltage (ARV) Set Point
When the current flow from the array is disconnected from the batteries the voltage begins to fall. After
a while the voltage will drop to the ARV set point and the array starts to charge the batteries again. With
this method the array will cycle current to the batteries in an on-off manner and let the batteries be
fully charged repeatedly under the day. Otherwise the batteries would not be fully charged. [2]
In some controller designs of pulse width modulation (PWM) types the current flow is limited by
constant voltage and is not regulated in an on-off manner. [1][2]
Low Voltage Load Disconnect (LVD) Set Point
If the voltage goes to low at discharge the batteries could be damaged. The minimum safe voltage value
before this can occur is LVD set point. Sometimes a unit with LVD functionality is separated from the
main charge controller, especially when the load should be disconnected from the batteries instead of
charging them to a safe voltage. The definition of the LVD set point is “the actual allowable maximum
depth-of-discharge and available capacity of the battery” [2]. In a hybrid power system design the
available capacity must, under the development process, be carefully estimated to get the actual depth
of discharge.
The developer has to consider the rate of discharge to determine a proper LVD set point. For high
discharge rates lower LVD set points is needed but for example small stand-alone PV systems the
battery voltage is not significantly affected by the low discharge rate that is used. When selecting the
18
LVD set point battery manufactures have a specified cut-out voltage that corresponds to 100 % depth of
discharge (DoD) and is typically 10.5 V for a 12 V battery. In order not to shorten the battery service life,
the batteries should never discharge completely, in general the LVD set point should be selected no
greater than 75-80 % DoD. [2]
3.2 Valve Regulated Lead-Acid Battery model
All lead acid batteries consist of lead plates that are covered in electrolyte, each cell produces
approximately 2.1 V. This means that 6 cells are series produces when charged, 12.6 V, this is a typical
12 V-battery.[23]
Figure 11, components of a VRLA battery
The electrolyte that the lead plates are submerged into is a mixture of positive charged hydrogen ions
and negatively charged sulfate.[23][26]
3.2.1 Charge and discharge reactions
Electricity from the charger produces chemical energy, to be able to charge, the charger has to provide
higher voltage than the cell voltage but not too high, that can damage the battery. The current flow
produces electrons to the negative plate, which attracts positive hydrogen ions. Now hydrogen reacts
with the lead sulfate bonded to the plate and forms sulfuric acid and lead, this process goes on until
most of the sulfates have reacted and hydrogen starts to rise from the negative plate.
Water contained in the electrolyte reacts with lead sulfate bonded on the positive plate and forms back
into lead dioxide, when the process is nearly complete remaining oxygen is formed as bubbles which rise
from the positive plates.
19
Figure 12, charging process with gassing and the plates are covered in lead sulfate
During discharge sulfate ions are moved to the negative plate, here the ion loses its negative charge and
combines with the active material to form lead sulfate. Excess electrons now flow through the negative
pole to the applied load and back to the positive pole where they are attracted by the positive plate.
The active material on the positive plate, lead dioxide, contains oxygen that reacts with hydrogen and
produces water. Remaining lead forms lead sulfate that sticks on both the positive and negative plate.
Figure 13, discharging the battery, lead sulfate is formed on the plates
During discharge of the cell the number of ions contained in the electrolyte decreases and more lead
sulfate covers the active material.
The electrolyte that contains sulfuric acid gets depleted until almost only water remains. Sulfate coats
the plates, thus reducing active material surface, which means that the chemical reaction will eventually
stop, and the voltage will decrease until the plates are fully covered. [26]
Charging lead batteries
A normal charging process for VRLA batteries consists of different charging modes, bulk, float and boost
which all have different voltage settings. [5]
20
Bulk mode is the first phase, which provides high constant current. Next phase is float charge, which has
a constant voltage, normally just below the gassing voltage seen at V1 in Figure 14.
Last is the boost, or equalization, the battery is overcharged with high voltage for a short period.
Equalization charge is above the gassing voltage, V2, which will produce essential gassing to remove lead
sulfate. Figure 14 below shows the typical charge curve.
Figure 14, a typical recharge curve. Bulk mode can be seen during charge when current is constant which turns into float and equalization charge
Grid reactions
The chemical process in a VRLA battery that produces the storage is achieved by conversion of lead
sulfate to lead dioxide which occurs on the positive electrode, and on the negative electrode lead
sulfate is converted into sponge lead. This reversible process is shown in equations (1)-(2), (5)-(6), and
net reactions equation (9)-(10). [14]
Positive electrode
(1)
(2)
(3) (4)
Negative electrode
(5)
21
(6)
(7) (8)
Net reactions
(9) (10)
The reversible reactions mean that the battery cell is either discharged or charged depending on the
direction of the current. During a discharge the sponge lead and lead oxide are converted to lead
sulfate. As seen in equation (9), water is produced during discharge; the concentration of acid
electrolyte is reduced when discharging the cell.
The charging process is the opposite; sulfate is converted back into sponge lead and lead dioxide. As
seen in equation (10), acid electrolyte concentration increases when the battery cell is charged.
The cell is fully charged when all lead sulfates have been converted back to sponge lead and lead
dioxide. When the chemical reaction no longer can absorb the applied charging current, the excess
current will be consumed by overcharging processes.
3.2.2 Overcharging process
As Previously described above are the regular charge chemical reactions, and when the reaction stops
the excess current goes to the overcharge process.
The VRLA battery has an internal design, which will prevent the gas from escaping; instead it will diffuse
back to the negative plate where it chemically reacts with the sponge lead to form lead oxide. The
electrolyte reacts with the formed lead oxide, which then forms lead sulfate and water. Lead sulfate will
continue with further chemical reductions to form lead and sulfuric acid. During the whole time the
battery is fully charged, and charging continues, this overcharge process will continue as shown in
equations (11)-(13): [14],[10],[11]
(11)
During the regeneration, water is reverted back from the positive plate and consumed at the negative
plate.
(12)
(13)
When the cell is nearly fully charged, the overcharge process will start which will release oxygen from
the positive electrode:
(14)
22
The oxygen will diffuse through the separator to the negative plate. At the negative plate oxygen reacts
chemically with sponge lead to form lead oxide, equation (11).
Gassing
As mentioned, when the battery is nearly fully charged the oxygen starts to form bubbles. This is called
gassing the battery. During charge until gassing, is called the natural absorption rate, which means the
amount of current that can be applied without the electrolyte is formed to hydrogen and oxygen. After
this level overcharging starts, that means that some of the current is wasted to heat and electrolyte
starts to evaporate. In VRLA batteries, this gas is re circulated back into the electrolyte so it does not
escape, thus VRLA batteries does not need to be refilled with distilled water as flooded lead batteries.
Gassing also occurs at normal charge, but at a much lower rate, this means that more Ah needs to be
charged into the battery than were taken out. [23][24][25][26]
3.2.3 Aging factors
Grid corrosion
When a battery is in a charged state the positive electrode is covered with active material, lead dioxide.
In addition to providing mechanical strength, it gives a low resistance path from the active material
through the plate to an access point. Any potential difference, which is common for the positive
electrode in a VRLA cell, will gradually corrode the grid. This can never be eliminated, but different alloys
can reduce the rate of corrosion. The Corrosion consumes the protecting lead dioxide layer. Equalization
charges are required periodically to rebuild this protecting layer. [26]
Stratification
“Battery Stratification is caused by the fact that the electrolyte in the battery is a mixture of water and
acid and, like all mixtures, one component, the acid, is heavier than water." [25]
Acid can start to settle at the bottom of the battery. Due to the high concentration of acid, more sulfate
will easier be stored on the plates. This reduces both capacity due to less active material, see Figure 15.
Figure 15, acid settles in the bottom of the battery
23
Sulfation
When the battery is cycled with incomplete charging cycles, the lead sulfate crystals left will gradually
form bigger particles harder to dissolve during normal charge. They stick at the plates that will reduce
the capacity due to less active material. Repeatedly performed incomplete charging cycles will increase
the sulfate crystals, which eventually degrade the batteries capacity. [7]
Loss of electrolyte
This occurs due to gassing of flooded lead acid batteries, hydrogen disappears during charge/discharge
specially when performing a full charge with higher voltage. The cell will degrade its performance due to
less current flow. By adding water the cell can be rescued, adding electrolyte will distort the mass and
probably shorten the lifetime through higher corrosion. In VRLA permeation is also a problem, which
occurs during overcharge, these cannot be refilled as easily as flooded. This can be reduced by changing
the float voltage, keeping the right temperature and be careful when equalizing. [24]
3.2.4 Different methods for equalization charge
When cycling batteries at PSoC, as in hybrid systems, battery stratification occurs after some cycles, this
process can partly be reversed with an equalization charge. Gassing is used to recover the battery.
Basically lead crystals grow bigger each discharge cycle and normal charge cannot break these.
Equalization charge is an overcharge sometimes with higher voltage to force gassing resulting in
breaking the crystals. In a normal hybrid solution for RBS off grid, the equalization would typically occur
twice per month and would consist of a long charge. During this the diesel, or other charging
equipment, would charge for approximately 10-20 hours. [6][12]
What happens during overcharge is:
1. Charge of discharged material.
2. Oxygen Development, which recombine into the battery.
3. Hydrogen Development that leads out from the battery.
4. Corrosion of the positive grid.
In point 1, it may happen that one plate is more charged then the other, this is what happens in the
hybrid case with PSoC operation where there will be a severe fraction of the lead sulfate over time. This
is where the equalization is designed to charge and resolve these hard small grains.
Different methods
The normal way of equalizing lead acid battery is with a overcharge, where the voltage is usually higher
than float. The equalization will be complete when the charging current no longer increases. This is
called a passive equalization, which is not effective for batteries connected in parallel. Batteries behave
different and with the passive charge the battery with higher voltage will be more charged and perform
more gassing with higher temperature, which eventually harms the battery.
An active equalization is a special equalization where the charge only applies for each battery, or even
cell level. This method has several advantages for batteries connected in serial. The batteries do not
have to be charged with higher voltage, which will reduce amount of water loss. It will only charge each
24
battery as much as necessary. There are mainly four different types of active equalization systems,
where all of them act as a BMS: [8]
1. Relay array:
Relays connects cells to the charger which can provide extra power if necessary, which will
remove the imbalance between cells. Main problem is that the relay array is very large and the
wiring is complex.
2. Switched capacitor system
Capacitors transfer energy when switched together, thus taking power from the higher voltage
cells to the lower. Mosfet switches in the range of hundreds kHz which reduce the sizing of
capacitors. The charging rate between cells is typically between C/500 to C/200.
3. Multi-output transformer
A multi output transformer transfer energy to the lowest voltage cell. Most of the energy is
transferred via diodes. A disadvantage is the power and duty cycle of the system is limited due to
not saturate the transformer.
4. Distributed structure on DC/DC converters
Both isolated and non-isolated DC/DC converters are used in this system. In the more charged
battery the energy is stored in the transformer when the particular transistor is switched on. A
disadvantage in this method is high cost and lots of components.
By combining the active and passive equalization charge you will get a cheap system which combines
the advantages from the active method but keeps the simplicity from the passive. The method is called
internal equalization charge, it can be seen as the series connected string is "one battery" and the
equalization charge will equalize one string when necessary.
Measure health of batteries
There are many different ways to measure SoC, they often require removing the battery and in some
cases letting it "rest" for at least one hour before measurements. The problem lies in how to measure
the stratification and to know when to equalize.[10]
Voltage
A normal way is to measure the voltage, this works well for lead acid batteries, but is quite inaccurate.
When measuring voltage to estimate SoC it is important to let the battery rest, as the acid diffuses
through the cells the voltage will drop. A problem with the voltage measurement is that it only
measures SoC, not state of health (SoH), which means a battery can seem to be fully charged but may
have less capacity because of battery stratification. Temperature also matters when measuring voltage,
higher temperature raises the open circuit voltage and lower lowers it.
Internal resistance
A way is to measure the internal resistance, the more sulfate left on the plates, the higher internal
resistance due to the lead sulfate acts as an insulator. High resistance of a battery allows it to accept a
higher current charge than when fully charged. This method requires the battery terminals to be
disconnected and a special instrument to be used.
25
Coulomb counting
In hybrid systems to be able to cycle batteries in particular PSoC window are of great importance. The
normal way is to use coulomb counting, that means "counting" Ah. Measuring current during
charge/discharge multiplying with time gives the Ah produces some problems:
A problem is measurement accuracy with high current.
Batteries have internal losses, which need to be taken into account, and these vary during the
battery lifetime.
The batteries have a lower efficiency during equalization charge, which is hard to define.
Basically some of the energy is lost due to internal resistance and gassing. This efficiency is typically in
the range of >95 % during PSoC. For the batteries used in this thesis the efficiency goes up as high as
99%.
During overcharge, the voltage is above the gassing range, which means the efficiency is lower. This
efficiency is hard to measure. Theoretically, an accurate measurement of the emitted composition and
the amount of gases can help to determine the efficiency at the charging. However, one should have a
measure of the change in the battery before and after equalizing charge by conducting an accurate
capacity measurement.
Energy counting
Counting energy in and out from batteries is also common amongst hybrid systems; losses in form of
heat generated from the battery meaning temperature must also be taken into account. The difference
in voltage when charging and discharging gives is a significantly higher amount of energy put into the
battery than taken out. This energy counting efficiency is typically around 80-90 %.
In general counting energy, both in terms of watt and Ah, in and out from the battery is the easiest way
of determining SoC of a lead battery during cycling. Although this requires knowing the initial SoC
otherwise it has to start with a full charge. Depending on how well the system is calibrated it might also
periodically calibration by discharge and fully charged. [9]
The ability to determine SoC / SoH can be done but to a cost, the system must be disconnected or an
external reading has to be done.
Designing algorithms for controlling SoH to know when to perform equalization charge, or replace the
battery, is done by lab tests. Batteries are cycled and measured, the data would show that at a certain
PSoC operation the battery will need equalization charge after XX cycles and after XX cycles it is time to
replace the battery.
26
4 Description of the task
The existing systems from different manufacturers are not as energy efficient as they could be, the
measurement accuracy is sometimes poor and the long equalization charge increases diesel cost. This
thesis aims to continue the development and improve the efficiency both in measurement and diesel
cost. SiteTel does not sell or develop any hybrid controller, so the task is to build a charge controller to
cycle batteries in a particular PSoC window and the batteries should switch between equalization charge
and normal charge at specified time.
The system should include a climate solution consisting of SiteTels outdoor cabinet Site Star-Tall. This
system will include four battery strings delivered by SiteTel. Diesel generator should be simulated by a
power supply that turns on and off from the system. The electrical components required to implement
internal equalizing charge has to be produced and tested.
The system's use windows need to be examined, mainly in terms of average power output, and
temperature changes. The system's competitiveness against what is known technologies in hybrid
energy solutions should be analyzed.
4.1 Internal equalization charge system for the prototype
The complexity of the equalization on cell levels and higher cost lead to that it will not be used. Instead
the equalization will be performed within string level, which is four 12 V batteries connected in series to
provide a 48 V “string”. This method has been tested by SiteTel in laboratory environment. The basic
idea is very simple, disconnect one string and perform an equalization charge with higher voltage during
a certain specified time.
During tests performed at SiteTel, they came up with optimal time, voltage and current. That is to
charge the 48 V string with 64 V during three hours with around 7.5 A, which should apply each 28 cycle.
Just as the long 12 hour overcharge this new method with higher voltage will boost the batteries enough
to reduce the stratification and almost restore the original capacity. In laboratory with only one string
the charging process is easy, just connect a 64 V power source with current limiter and disconnect after
three hours.
Our solution is a real system with four parallel-connected strings with some requirements:
The output voltage must never be disconnected, i.e. the load must always be powered.
Output voltage should not be increased due to the maximum in a 48 V telecom equipment is
around 56 V. This means that the string to be equalized should be disconnected from the rest
during this charge.
When in PSoC cycling mode, all batteries are connected in parallel; the charger charges all batteries,
which are also connected to the load. After 28 cycles, one string is switched to boosting stage; the two
(depending on how many used, it can also be three) other strings are now connected to both the load
and boost converter, which will then equalize the string for three hours. The idea is that during this time
the diesel generator should not have to be switched on.
27
Advantages when using internal equalization charge
Main advantage of using equalization charge is of course to improve battery lifetime. With this kind of
internal equalization charge, the diesel does not have to run as much as the normal systems where the
equalization would typically be with diesel running for 12 hours twice per month.
In these kinds of hybrid applications this is of great importance, the cabinet is often placed in remote
areas where roads might be rough etc. so improving lifetime of batteries and increasing time between
refueling is an advantage.
The other advantage is in the environmental question, these hybrid systems are supposed to be
environmentally friendly, so reducing the diesel running time is important.
5 Modeling and simulation of the system with internal equalization
charge
Radio base stations have variable load depending on the traffic load. One purpose of this system is of
course to reduce fuel cost in terms of less diesel run time. Another aspect could be to have "quiet
hours" during the night to reduce disturbing noise. In order to measure the battery behavior and
possible savings in diesel run time, i.e. fuel, a simulation was made using the computer program
Simulink. The block model can be seen in Appendix 1.
A battery model included in Simulink was modified with values from the batteries supposed to be used
in the real system. Just as the real system will look, four strings are used one string includes four
batteries connected in serial, to make the simulation as realistic as possible.
Batteries are dynamically changing, meaning they behave different depending on ambient temperature,
pressure, power outage etc. Each battery is also not the other alike as they might have slightly different
compound or different filled electrolyte. Therefore battery dynamic is hard to simulate and predict.
For this thesis, the model in Simulink is adequate; a simple battery model does the job, as the system
will be tested in real test conditions.
5.1 Cycling at partial-state-of-charge
SiteTel have performed several tests showing that ideal PSoC window is somewhere at 60-90 % SoC. This
is the basis of how the hybrid charge control should work for best performance.
Figure 16, PSoC window normally used in cyclic operations
Cycling at PSoC is graphically displayed in Figure 16 above; LVD set point is usually at 42 V for a 48 V
system and should never be reached during a normal cycle. To be able to maintain performance, the
PSoC window should not drift more than ± 5 % within these boundaries. In this range the charge
acceptance is high which will improve efficiency of the diesel generator. The DoD is fairly low which will
provide a longer lifetime of the battery.[7][13]
28
5.2 Evaluating power requirements for a Remote Area Power Supply
To get as close to reality as possible, real data from a radio base station in India acquired during one
week worked to simulate the variable load. This data was provided by the telecom company Ericsson.
India is a typical RAPS area where this kind of system would possibly be installed. At some places the
service providers have lower fees during the night that of course gives different power load, but the
data used serves as a good mean value of RAPS areas. [13][31]
Figure 17, shows how power consumptions for a RBS varies over time
Maximum power requirement is around 1240 W and drops down to 1050 W during nights. These values
were put into a signal builder inside Simulink to provide the correct load.
5.3 Modeling and simulation of the system in Simulink
This simulation is not to compare to reality, assumptions have been made about the charge acceptance
during overcharge, the battery model is far too simple which makes it hard to predict the charge
acceptance etc. The DC/DC converter is not modeled from reality, so during equalization charge we have
little ideas of how much current will flow to the battery and the efficiency of the DC/DC converter.
By combining the RAPS operating schedule together with the PSoC operation and building the switching
system, i.e. contactors and relays, it was possible to make a simulation of the systems switching
between equalization and normal cycling.
Together with the simulated diesel generator the simulation gave some results, which makes it possible
to have this kind of equalization charge without too low SoC. This data provided us with ideas how to
build up the switching in the prototype. The best time would be during night when the traffic load is at
its lowest to not let SoC sink too low, this is something which will have to be implemented in a real
system. Simulation showed that the best time to switch to equalization charge should occur directly
after the charging cycle to not let the SoC decrease more than necessary. The simulation also provided
some good hints about the possible current and voltage spikes which will occur during each switch
between equalization charges.
29
Figure 18, data from simulation showing voltage during cycling
In Figure 18 above, the graph from the simulation shows cycling during three days during which the equalization would occur. Cycles between equalization were in this simulation set to zero.
One battery string will equalize each cycle, so the whole process in this simulation takes around 1.5
days. In a real system considering the power requirement of the RBS and if set to only equalize during
nights this might take longer time. It can clearly be seen that the cycle time between charge and
discharge decreases during equalization charge, this was of course expected due to that one battery is
disconnected thus reducing the amount of available energy or Ah.
6 Development of a prototype for the charge controller
To be able to test the hypothesis, that this equalization charge will work, and compare how well it
follows the simulation, a first prototype was built.
The system includes a climate solution consisting of SiteTels outdoor cabinet SiteStar-Tall. This system
can include four battery strings delivered by SiteTel. Diesel generator will not be used but instead a
power supply that turns on and off from the system. This prototype will also serve as real hybrid test
equipment to be able to compare to the existing ones on already on the market, the system's
competitiveness against what is known technologies in hybrid energy solutions will be evaluated further
down in this report.
In order to expand the systems user-friendly, a number of additional features listed and ranked, and will
be implemented as time permits.
30
Table 4, ranking of features
Extra features Rank
Log data 1
Extern reading of data 2
Quiet hours of diesel generator 3
Present data graphically 4
Touch display 5
Autonomous control from logged data 6
User customization of parameters 7
6.1 Hardware
Hardware is a critical part in the design of the system, there will be high current and voltages that needs
to me measured with high accuracy, and the switching to equalize charge needs to withstand high
switching capacities, i.e. current spikes. All this needs to be controlled via a microcontroller.
Battery cabinet as well as batteries are provided from SiteTel consisting of:
12 pcs of 12 V 170 Ah lead batteries, four batteries are connected in serial to form a 48 V "string"
which in turn is parallel connected to get higher capacity.
Battery cabinet SiteStar Tall including four battery shelves and cooling compressor.
To control the equalize charge, a contactor with high current rating is used which in term is activated
from a mosfet transistor controlled via microcontroller. The contactor has a two-way switch, which
allows current to flow through one way when closed, and once activated the current flows via the
DC/DC and back to the battery. This allows us to control which battery should be equalized and which
battery should provide extra power to the boost. With this setup the charging is modular and
expandable; the system is built for four strings, but in this application there will be three strings. When
three strings are used, two battery strings will equalize the third.
In the prototype the generator is replaced by rectifiers connected to three-phase wall outlet. The RBS is
replaced by a resistive load, which is controlled by a relay activated via a mosfet connected to the
microcontroller.
The load is fixed to a mean value of a RBS; this means that the load is constant during the whole day.
The simulation showed that the best time for equalization charge is during the night when the load is
lower; in this prototype the SoC might have to go deeper.
31
Sensing part
The prototype has different sensors for:
Current measurement:
Current is measured all the time in and out from the batteries, the range is from -100 A to 100 A.
According to specialists at SiteTel, the current resolution needs to be accurate to approximately
±0.1 A. The total ampere-hours are calculated from this measurement so a bad accuracy will
dramatically change the accumulated ampere-hours after some time. The zero point needs to be
accurate and calibrated; otherwise the difference in ampere-hours during charge and discharge
will differ too much. This will make the whole cycle algorithm skew and drift either up or down
resulting in bad efficiency or reduced battery lifetime.
Voltage measurement:
Is measured all the time to be able to shut down if there are any anomalies, the main criteria
here are LVD which is set to save the batteries from totally draining. The system is a -48 V
system, but the measurement range will be 0 to 64 V.
Temperature measurement:
Is monitored inside the cabinet, in-between two batteries, to be able to shut down if
temperature climbs to high during charge or equalization charge. The temperature sensor is
sealed with built in analog to digital converter (ADC) just sending digital values, which we read in
the microcontroller. The working temperature for these systems can vary from between below
zero to 45 C, but since the cabinet cools the batteries the temperature inside should be around
30 C. The temperature sensor should sense when temperature inside cabinet climbs over 50 C
and should then shut down the system.
Figure 19, block diagram of how the system is built up, one battery string with current and voltage measurements. The contactor is also connected to visualize how the current should flow
32
Current shunts used, one for each string, are rated 100 A 60 mV which in turn is connected to an
operational amplifier (OpAmp) to amplify the voltage from 60 mV to correct level in order for the ADC to
handle. Care had to be taken to be able to handle both positive and negative current during charge and
discharge. Voltage can go as high as 64 V during equalization charge and the ADC can handle maximum
5 V. To be able to get 5 V a voltage divider is used before the ADC.
6.1.1 Charging equipment for the system
The charger, or rectifier, is built for -48 V systems and have earth connected to -48 V which means that
everything which is not built for -48 V systems have to be potential free, otherwise that will short circuit.
Figure 20, DELTA charger with rectifiers
The charger has rectifiers that produce 2 kW, depending on the applications rectifiers can be added to
the right power requirements. This charger automatically switches between float and boost charge
depending on the settings. In this case with hybrid application and internal equalization charge, the
charger is set to float, i.e. 54 V all the time, this will reduce gassing of batteries. For this prototype three
rectifiers are used producing in total 6 kW.
Load
The load used is Lingo dry type load bank, 48V max 300A. This is used to simulate RBS.
Figure 21, resistive load used to simulate the RBS
33
Charge controller
To activate the charging, a relay box consisting of a 24 VDC relay and a three-phase contactor is used.
The relay is activated via a mosfet through the microcontroller which activates the contactor for three-
phase and controls the charger and load.
Figure 22, relay box which controls the charging
64 V DC/DC converter
The equalization charge should be at a higher voltage than a regular float charge, in this case 64 V. Since
DC/DC converters are complex, the only alternative was a off the shelf solution.
Figure 23, DC/DC converter Eltek PSC305-LV/60-8
The telecom DC Power manufacturer ELTEK provided one with the right criterion; it should be powered
through the batteries inside the cabinet, i.e. somewhere between 40-56 V and an output of 64 V. This
model can provide an output in the range of 45.2 to 82.5 V with maximum 8 A. The DC/DC is mounted
inside a 19" rack and came with a CAN-dongle which allows different settings of both voltage and
current limits.
6.1.2 The microcontroller used for the charge controller
The evaluation kit EVK1101 is used to be able to receive sensing data, do calculations and control the
system. The kit contains an Atmel AVR UC3B, which is a low power 32-Bit microcontroller with a core
frequency up to 30 MHz and 256 KB internal memory. Other features that are used are an inbuilt SD-
card reader, several General Purpose Input/Output (GPIO) ports and Serial Peripheral Interface (SPI)
connection.
34
6.1.3 Simulation of the hardware in MultiSim
Since the current measurement is most critical, this was simulated first in computer program called
MultiSim from the company National Instruments. The current shunt will give 60 mV at 100 A, and of
course -60 mV at -100 A. This is amplified so that 100 A, 60 mV, equals 5 V and -100 A equals 0 V. There
are four different current measurements, one for each string used, and this OpAmp has four individually
amplified channels. OpAmps have a common mode rejection ratio (CMRR), this can be good or bad
depending on both the printed circuit board (PCB) design and the OpAmp itself, and this is also
something to see during simulation.
Figure 24, connection diagram of one of the four channels connected to LM324
The resistors are chosen as high precision, 0.1 %, to have as high accuracy as possible. To get a reference
0 at 2.5 V a voltage reference is connected directly to the positive input of the operational amplifier that
feeds constantly 2.5 V, which is added to the measured value. If the current reaches outside of the
shunts range, the output value is nonlinear and will reach above 60 mV. This is something that might
occur during the switching from equalization charge. By adding a 5.6 V zener diode, the input voltage to
the ADC will never reach above the maximum input range and the ADC will not take any damage.
Table 5, values from simulated operational amplifier
Real [V] Simulated [V] Difference [V] Shunt [mV]
5 5.083 0.083 60
3.75 3.834 0.084 30
2.5 2.584 0.084 0
1.25 1.3356 0.086 -30
0 0.08613 0.086 -60
35
The common mode fault for operational amplifier LM324 and the difference to the "real" values are
shown in the Table 5 above. The difference is almost constant and the simulated values are slightly
higher and without any correction this will give significantly higher current.
To cope with this problem the values were plotted and compared to the real values, a simplified linear
equation for calibration was made.
(15)
This calibration is easy to adapt into the microcontroller, but these are of course just simulated values
and are theoretical values, so a small calibration has to be made in the microcontroller when everything
is connected.
6.1.4 Design and development of the Printed Circuit Board
Prototype started with a breadboard, which then was transferred to a PCB using MultiSim/Ultiboard.
The PCB includes all Low Pass (LP) filters, mosfet, ADC, OpAmp, voltage regulators, and connectors for
analog channels. Communication to the EVK1101 is done via SPI and GPIO.
Figure 25, external PCB
Some thoughts were spent on how to make it less sensitive to disturbances etc. All low pass filters are
placed as close to the ADC as possible, decoupling capacitors are placed close to all integrated circuits
(IC) also keeping the ground plane on the bottom as intact as possible to reduce radiated emissions and
maintain the capacitance. The circuit drawing can be seen in Appendix 2.
Low Pass filter
To improve measurement accuracy a LP filter is used on each of the eight channels to the ADC. Current
is most critical, an oscilloscope helped identifying possible disturbances, and since the sampling rate is
high the filter is set to a cut of frequency at 200 Hz. The high cut of frequency can be doubtful because
the change in current is a slow process, but simulation showed that the switching back from
equalization charge gives a high current spike which we thought would be nice to be able to see,
anything left is then filtered in the software. Since this prototype is optimized for lab environment with
a particular load and charger, the filters have to be redone since we have no clue how and which kind of
disturbances a real RBS might have.
Measurement accuracy
As mentioned earlier, the current needs to be nearly precise in order for the SoC to stay within its
boundaries. Even a small fault in measurement will accumulate into a quite large over time. It might not
seem as a terrible thing, but since this system should be a fuel saver a small deviation each charge cycle
might end up with a lot of more diesel run hours.
36
The bottleneck in this design is the current shunt, which has an accuracy of 0.5 %, all other parts are
chosen to be better or as good as possible. Resistors for the gain on the operational amplifier are high
precision 0.1 %, they are chosen so as the maximum voltage at 100 A will be 5 V.
The evaluation board EVK1101 has a built in 10 bit ADC, but this is not accurate enough. An extern ADC
12 bit communicating via SPI is used. The maximum nr. of bits in the voltage range of the ADC, 5 V, will
be:
(16)
Since we are measuring 100 A during both charge and discharge it will have a range of 200 A, which
gives:
(17)
The maximum resolution in the ADC is ±0,049 A.
Display
Values are needed to be displayed somehow, a simple pixel matrix LCD was implemented. This cheap
display communicates via SPI, which was good solution since there are spare ports on the EVK1101. It
has 48 rows, 84 column outputs that will be enough to show values of current, voltage etc.
The small size limits the view of lots of data; to enhance the user performance a simple menu system
was built, see software implementation. By using four buttons the user can navigate through the menu
to see different data and settings.
These extra features were added after the PCB was sent for manufacturing. The buttons used for
navigating through the menu had horrible bouncing effect; this meant adding external PCB with button
debounce. This can normally be handled by the software, but since we do not use real time operating
system (RTOS), this gave lots of problems with delays etc. so all debounce had to be fixed by hardware.
The solution was to add debounce circuit consisting of a resistor and capacitor LP filter.
6.2 Budget and cost for the components used for the prototype
The prototype is designed for use within telecom industry, they are reluctant to pay more for high tech
features, so a goal for this prototype was to make is as cheap as possible. This will also help to make it as
competitive as possible on the telecom market. The prototype should also prove that it is possible to
have high accuracy measurements and a switching part for the equalization with cheap components. All
this together with the internal equalization charge, the system does not need to be over dimensioned in
terms of both diesel and battery capacity.
The DC/DC converter is the most expensive part in this solution; this of course included a CAN dongle,
which is a onetime cost. If this will be productified a special DC/DC for this purpose needs to be ordered.
On the other hand, more parts, like four contactors and a PCB, is something that would have been used
in a normal hybrid application anyway.
Different solutions were discussed about this implementation with contactors, how many to use etc. if
the system would be used to only have switching between two strings, i.e. two strings equalizing the
remaining two. This solution would not be much cheaper; the only difference would be two contactors.
37
That combined with the lack of functionality, it will only work with four strings in parallel, lead to the
solution with four independent switching contactors.
For this project / thesis there were no real budget, only that it should be as cheap as possible. The total
expenditure for this prototype reached 12200 SEK where the DC/DC converter was most expensive, this
also included the evaluation board EVK1101, LCD display etc. A bill of materials used for the PCB can be
found in Appendix 3.
6.3 Software
The software is divided in several parts with different functions for the prototype. Every important part
has its own C. and H. files to make it easy to find and make changes. This is also preferable in order to
save resources if that part does not have to be used at the moment in the program. To obtain an overall
view of how the software is working, it is divided into three categories or subsections. These subsections
are further subdivided in several parts. Under subsection 6.3.1 is where the parts about communication
with external sources are explained. The parts which is about logging of data are under subsection 6.3.2
and under the subsection 6.3.3, the most important parts that handle the sensing data, timing and
control of the main function of the system (charge controller), are placed.
6.3.1 Handling of the communication with external sources
Serial Peripheral Interface
The hardware has different interfaces to communicate with the microcontroller. The most central
communicating interface in this solution is SPI. The ADC, the SD-card reader and the display are all using
SPI to send and receive data. The ADC has a sampling rate at 100 kHz and the clock frequency for the
display should be no more than 4 MHz. Due to this the ADC and the display are using the same SPI clock
at 1 MHz, because it is 10 times the sampling rate for the ADC, while the SD-card reader is using a
separate SPI clock at 1.2 MHz. Two different SPI options are used, one for the ADC and the display, the
second for the SD-card reader.
SPI options for the ADC and the display
SPI options for the SD-card reader
spi_options_t spiOptions = .baudrate = 1000000, .bits = 8, .spck_delay = 0, .trans_delay = 0, .stay_act = 1, .spi_mode = 0, .modfdis = 1 ;
spi_options_t spiOptions = .reg = 0, .baudrate = 1200000, .bits = 8, .spck_delay = 0, .trans_delay = 0, .stay_act = 1, .spi_mode = 0, .modfdis = 1 ;
To be able to choose between the display and the ADC, spiOptions.reg is set to 2 for the ADC and 3 for
the display.
GPIO
GPIO pins are used to control parts of the hardware. Apart from the pins for the display, five GPIO pins
are used to control the five mosfets, which are used to throughput current to the contactors and the
38
relay for the charging. One pin is also used to communicate with the temperature sensors ADC. Four
pins are connected to buttons, up/down and enter/return for the menu system.
Display
Apart from the SPI communication the display have a reset and a Data/Command pin that has to be on
separate GPIO pins. In the initialization the reset pin has to go high and low in a specific order to be able
to communicate with it. Before data can be printed out on the screen The Data/Command pin has to go
low to send important commands to the display. To send data this pin has to be high. On the display a
menu system has been implemented, to easily and independently of a computer, have the ability to see
how the system is working.
Figure 26, flowchart of the menu system
Important information like current, voltage and ampere-hour for each battery can be selected and are
then printed out on the screen. Status information like uptime and system temperature can also be
selected.
Figure 27, different display screens
Some variables might be important to change if something in the system has been changed or needs to
operate differently. This can be done by via the menu system, e.g. number of battery strings, high state
of charge, charge factor etc.
39
6.3.2 Logging of the important data
Serial communication
To obtain an overall view of the system, data is transmitted using serial communication or USART. Same
information as for the display is printed out on the computer monitor.
Figure 28, data via USART
This is useful when all the data needs to be displayed at once in real time. The use of USART is also
preferred when you need to import data to computer programs like MatLab.
SD-Card
With a SD-card, logging of all the important data is done using the SD-card reader. This is necessary to
provide an overview of the system behavior over a long period of time. In addition to all the data that
are displayed in real-time like current, voltage and ampere-hour for all the battery stings as well as the
system temperature, it also logs the runtime of the system to provide a timescale.
Figure 29, data in the logging file on the SD-card
With the use of the timescale, custom made graphs can easily be plotted with different selection of
data.
All the data is logged in a .csv file on the SD-card to easily put the different battery strings and type of
data in separate columns. The advantage of a .csv file is the simplicity of column separating, just by
adding a semicolon after each value. This type is perfectly suited for computer programs such as
Microsoft Excel. In order not to fill storage with data too quickly the logging is done only every 30
seconds.
40
6.3.3 Handling of sensing data and control of the system
Analog to digital converter
To get some values from the ADC, bytes of eight bits have to be sent and received in a specific order.
Before receiving data, two bytes containing information about which channel it is, must be send to get a
“data_high” byte. This data_high byte has to be bitmasked into four bits. A dummy byte has to be sent
to get a “data_low” byte. These two bytes are then merged to get a full 12 bit ADC value.
Bytes send and receive for the ADC
byte=0b00000110; if(ch>3) byte|=0b00000001; spi_selectChip(&AVR32_SPI, 2); spi_write(&AVR32_SPI, byte); byte=ch<<6; spi_write(&AVR32_SPI, byte); spi_read (&AVR32_SPI, &data_high); data_high&=0b00001111; spi_write(&AVR32_SPI, DUMMY_BYTE); spi_read (&AVR32_SPI, &data_low); spi_unselectChip(&AVR32_SPI,0);
result = ((data_high<<8)|data_low);
In the C. file for the external ADC a software LP needed to be implemented due to some high frequency disturbances were detected. This filter has a cutoff frequency of 500 Hz. To further improve the measurements, mean value calculations of all samples from the ADC have been implemented.
Interrupts
Several types of interrupts are used for different kinds of functions. The GPIO pins for the buttons
triggers an interrupt to set various variables to be able to maneuver in the menu system. A real time
counter interrupt is used to get an accurate clock for the system and a timer/counter interrupt is used to
get an accurate timing. The real time counter that uses the external oscillator on the EVK1101 triggers
its interrupt every second to make it simple to generate a real clock.
Temperature sensor
To keep track of the temperature of the system and the batteries a temperature sensor is important. To
communicate with the ADC built into the temperature sensor, data has to be sent in a specific order
with predefined delays in between. The temperature value is divided in two parts, one digit part and
one decimal part. These two are put together to get the full temperature value.
Main program
The software’s core is the main program where the actual control of the system is done. The main
purpose of the system is to control the charging, discharging and equalization charge of the batteries. To
have the ability to do this, information about the batteries current, voltage and Ah are received or
calculated. The status of the voltage and current are received from the ADC that is running constantly
but timed by the timer/counter interrupt. The Ah of the batteries are then calculated using the real time
counter clock and current for each battery string. In order for the program to know when to charge, stop
to charge or perform equalization charge, they are divided in different states. In addition to Ah, some
other variables are in use with information about high and low state of charge and what state it is. The
state information is used to know what to do next and to prevent conflicts. The logging and USART
printing is done in the main program after all the calculations and control.
41
Figure 30, flowchart of the main loop
42
7 Implementation of the charge controller in a real system
The prototype is implemented in the cabinet provided by SiteTel and three battery strings, with the
possibility to add a fourth. The diesel generator is still not used and the RBS is replaced by a resistive
load.
Figure 31, block diagram showing the connection and setup of hardware, as it is supposed to look like when implemented in a real system
The cabinet has one cooling compressor with a cooling capacity of approximately 150 W at 25 °C
ambient. To be able to have lots of data and measurements we want to decrease the time between
cycles, but the cooling capacity of the cabinet limits both external load and charging current, otherwise
the batteries produce too much heat. The batteries have an energy efficiency of 85 % which means 15 %
of the total energy is reverted into heat. During the equalization charge, the energy efficiency is lower,
which means the temperature rises in a faster rate. In a real system, the cycling will be slower resulting
in less heat from batteries; a different cabinet with higher cooling capacity can be used.
Since the batteries are placed inside a closed unventilated container, venting tubes are drawn from each
battery and outside of the cabinet (see Figure 32), this is critical since the batteries produce high
amount of gas during equalization charge which otherwise could be ignited by one of the contactors.
43
Figure 32, gas ventilations are set up on batteries [29]
The cabinet have place for four battery strings as seen in Figure 33 below, we are only connecting three for this test purpose, due to the amount of accessibility of batteries.
Figure 33, showing SiteStar Tall cabinet equipped with four battery strings [29]
The switching part is mounted in the bottom shelf of the cabinet, this allows all contactors to fit and
provides easy access. Voltage measurements (wires), and cables going to the boost DC/DC converter are
also placed at the bottom shelf. This solution is a good way of proving that the system easily can be
implemented on a real system.
Figure 34, bottom shelf inside cabinet and modified bottom shelf with three contactors mounted and measurement wires for voltage
44
8 Test and verification
8.1 Previous work done by SiteTel
Various long time tests were conducted previously in lab environment verifying that the equalization
charge works as proposed. [28]
Those tests were only cycling the batteries, and after certain cycles increasing the voltage to 64 V. No
tests have been performed previously with this internal equalization, taking energy from batteries
equalizing others.
The test case looks as follows:
Discharge 5hours 11.4 A
Charge one hour 100 A
Each 28th cycle perform equalization charge at 3hours 7.5 A
The test measured end of discharge (EoD), which are plotted to see how the battery degrades, and how
much it gains from the equalization charge. Figure 55 shows the EoD vs. cycles plotted. The test case
was changed as the test prolonged, so the graph only shows the last 700 cycles, which have the same
cycling periods.
Figure 35, showing EOD of the batteries during cycling with equalization charge
Figure 35 above shows around 700 cycles with measured EoD, each equalization charge clearly raises
the EoD back to a higher level. The negative trend that can be seen is normal; the batteries are
decreasing in performance. The spike seen at 480 cycles is a capacity measurement, which is a full
discharge, followed by a full charge. As seen this raises the EoD even higher than the normal
45
equalization charge, this is due to the long charge removing some large particles and increasing the
active material.
The test will continue until approximately 2000 cycles, then the weight will be measured and conclusions can be made on the battery SoH. These batteries are allowed a weight loss of 500 g of electrolyte until they are rated as bad. Any real conclusions and comparisons are hard to make, as the batteries are not the same as the one used in this thesis, but similar 150 Ah instead of 170 Ah. [28]
8.2 Test and verification of the prototype
The prototype was turned on and left for cycling to see that everything works as expected. Via the menu
system the equalization cycles were set to three to be able for a more rapid test, the initial test were
mainly to see if the switching part i.e. contactors worked as they should.
This test case looks as following:
1. One time charge until all strings have a charge current < 1.0 A
2. Discharge with 25 A until SoC reaches 50 % (load is always on)
3. Charge with 111 A until SoC reaches 90 % (minus the 25 A supplied to the load)
4. Continue cycling at PSoC for two more cycles
5. Perform first equalization charge (battery string 1)
6. Charge until 90 % SoC
7. Next equalization charge
8. Charge until 90 % SoC
9. Last equalization charge
10. Start from point 5.
After five days the system was shut down and SD card removed to analyze the collected data.
46
9 Result
The key point of this prototype was to evaluate if it is possible to build this kind of internal equalization
device, it should work properly and not be too expensive. We proved this is possible to build and it
works as it is supposed to.
As the system turned on, a calibration charge was made as seen in the beginning of Figure 36 which
charged until the current to all strings were below 1 A. After calibration charge, the accumulated Ah
were reset to number of strings (three) multiplied by maximum Ah per string (170), which gave 510 Ah,
also seen in Figure 36 below.
Figure 36, accumulated capacity, Ah, for all strings and total capacity for whole system during five cycles, where the last three are equalization cycles
As seen in Figure 36 above the Ah counting works as it is supposed to, even with the initial calibration
charge. A small jag can be seen at the second cycle, this is nothing more than a power failure and our
safety system shut down the charger and load.
After three cycles, the first string is performing equalizing charge (Battery string 1 shown in blue), the Ah
increases up to more than fully charged (170 Ah is maximum) while the other two decreases in a faster
rate. The same goes for next two strings; one equalizes each cycle and the other two decreases faster.
The total Ah is still the same, energy has not been lost, just transferred energy from two batteries to
another. In the Figure 36 the equalizing battery reaches above maximum, which should be impossible.
This might be due to the columbic efficiency during overcharge; this is something we expected. During
normal charge efficiency is around 99 % and this is something we are taking into account when
calculating Ah. During overcharge there are very few, if any, measurements of the efficiency so we are
using the same 99 %, which obviously is wrong.
47
Something that we thought of already at the start of the project was the systems high current during
charge and we expected high current spikes during the switch back from equalization charge. The
equalized battery is now fully charged whilst the other are discharged to about 60 % SoC.
Figure 37, current plot looking at one battery during cycling, one equalization charge and two cycles where next battery is equalizing
In Figure 37 current is shown from battery string 1 during four cycles, after the first equalization cycle
there is a current spike when battery string 1 is discharged rapidly to charge the other batteries. In
Figure 38 this spike can be seen even more clearly, since logging on the SD-card only occurs each 30
second. Current reaches (down to) approximately -205 A during the first couple of seconds. The current
shunt is a 100 A 60 mV and is calibrated to show maximum 100 A, this is why the measurements looks
distorted, but battery string two and three accepts around 90 A each while the load is set for 25 A.
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Figure 38, screen dump from Realterm, new value each second, showing more accurate current since the logging only occurs every 30 s
To be able to see if we are cycling correctly, i.e. counting Ah correctly, looking at the EoD voltage gives a
good indication. Figure 39 below shows the voltage from battery string 1 during cycling and equalization
charge.
Figure 39, voltage change during equalization charge for one battery
As seen in Figure 39 above the EoD voltage is stable, this means that we are taking out approximately
the same amount of energy each time, which is a good indication that everything works as it is supposed
to. Compared to the simulation, the voltage does not increase as much and fast. This is due to the
49
simplified battery model used. But the simulation gave an idea of how it would look which at least looks
like reality.
Figure 40, zoomed view of equalization with included temperature
Temperature is logged during the whole time to be able to analyze how much more heat is generated
during the equalization charge cycle. Figure 40 above shows one equalization charge, the test was
performed in room temperature and with an open cabinet. As seen the temperature increases during
the whole charging, the temperature sensor was located in between the batteries.
By analyzing data from the previous cycling, the energy efficiency during equalization charge can be
calculated. This is done by dividing the power outtake in watts by the amount of watts charged. Doing
this in two cycles, one regular cycle and, one equalization cycle will give some difference in energy
efficiency.
The first cycle gave approximately 90 % efficiency and during equalization it decreased to 83 %. This
means that more energy is reverted into heat during overcharge.
50
Figure 41, voltage and current for battery string 1 compared during an equalization charge
As seen in graph above current is limited during the first two hours equalizing, this is due to the DC/DC
converter and should be limited to approximately 7.5A. When the voltage increases up to 64 V the
current starts to drop, this has to do with the overcharge of the battery.
During the test we measured gassing at the same time as equalizing the battery. This was done with
specially made bags, which were connected to the gas ventilation tubes. Each bag could be filled with 5
liters of gas; so two bags were connected in "parallel".
The stoichiometry for full overcharge i.e. pure water electrolysis is 1 A, and a cell produces 0.630 L gas
during one hour. With 24 cells (48 V string) and 5 A, we get up to 75 L per hour. [28]
The first two and a half hours, when the voltage still increased, filled the two bags i.e. 10 l. Bags were
emptied and one was connected again, this was filled during the last half hour. In total 15 l was
produced, but it was only when the voltage reached 64 V as the amount of gassing increased up to 10 l /
h. This means that out of 75 L, 65 L are recombined back into the battery.
9.1 Analysis
By comparing both the simulation and data from our tests to existing hybrid system results show how much this new system with internal equalization charge will save.
If we compare to other hybrid systems already on the market, why is this better or how much will you
save?
To start with fuel savings, the existing hybrid systems have a kind of boost/topping charge as lead
batteries need to work properly during cycling. That system will at a preset time charge the batteries for
maybe 12 hours, diesel generator running constantly, at boost voltage, thus equalizing the internal
51
differences between the cells. Without this topping charge, the EoD will slowly sink to a critical level
where the batteries have to little active material left.
The idea with this new internal equalization charge is to reduce the equalization time by increasing the
charge voltage above the gassing voltage, this way the time can rapidly be reduced. The batteries are set
for more gassing and resolving the crystals increasing the active material.
In a real system the charging time will not be limited by the ampere available from the wall outlet fuse.
According to SiteTel the charging time is expected to be approximately one hour and with a discharge
rate at 11 hours the diesel will run for two hours per day. By looking at the graph in Figure 39 and
comparing the time between a recharge one can see that during equalization the time is shorter. During
this (one equalization cycle per string) the diesel runtime will increase to about 2.5 hours per day. With
a setup of four battery strings, charging time will be up to 60 hours of charge each month (30 days).
The regular system with a 12 hour charge two times per month gives (during the 12 hour charge one
charge cycle is reduced) 58 hours of charge and two times 12 hour boost charge:
(18)
With expected life of 3 years, or 36 months, this equals
(19)
By comparing this with the new expected diesel runtime
( ) (20)
This new system will reduce the diesel runtime for almost 800 hours during 3 years. This can be
compared to the old system and can reduce the diesel runtime with up to 27 %. Diesel generators
require maintenance regularly, and with 800 hours decreased runtime, many service intervals are saved.
Looking at fuel consumption, for these calculations a 20 kW generator is concerned with expected fuel
consumption of 4.9 l / hour at high load and 2.3 l / hour during low load (low load is during the 12 hour
long charge when the current drops). [27]
60 hours with high load, i.e. 4.9 l / hour:
(21)
Considering the old system, 58 hours high capacity charge and 24 hours of slow charge:
( ) (22)
(23)
The total savings will then be:
(24)
This represents a yearly saving of:
(25)
The total fuel savings can reach 550 liters/year which equals 13.5 % less per year.
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Why not just set the diesel/charger for high volt charge at 3 hours?
Both ways are making the same effect on the batteries, but the new internal takes energy from the
other batteries already charged which reduces the runtime of diesel generator at low efficiency. During
the equalization cycle, the charge time increases to approximately 1.25 hours / day, by just changing to
the three hours boost charge, this new internal system will save:
(26)
up to 60 % time per cycle.
With 2000 cycles, nr of equalization cycles will reach 72. With 4 cycles / equalization times 1.25 hours,
this gives:
(27)
Compared to:
(28)
The difference by using internal equalization charge compared to using diesel to charge for three hours
will be 500 hours during the expected lifetime of the batteries i.e. three years. Compared to the old
system, three hour equalization with diesel running can save 10 % in diesel runtime which is not as good
as the internal equalization system.
How is battery lifetime improved?
As stated previously the expected lifetime when cycling batteries and conducting equalization charges
with 64 V would be at least 2000 cycles. Comparing this with data from RBS (in RAPS areas), average 2
cycles per day, will give a lifetime expectancy of closer to 3 years. According to previous replacement
intervals of hybrid batteries, this is somewhere between 2-3 years meaning battery lifetime might be
increased slightly.[29]
Another factor is how much gas the batteries produces, by comparing the amount of gas in a 12 hour
charge with boost voltage (56 V) to an equalization charge of three hours at 64 V can give an indication
on if this new equalization charge will improve the battery lifetime. The batteries is said to have reached
end of life when lost 500 g of electrolyte.
Presume that a valve-regulated battery is operating at float voltage and current,
(29)
where 20 % of the current generates gas. H2 per hour is 0.461 l / Ah and a 12 V battery has 6 cells, 48 V
string has 24 cells:
(30)
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For a 48 V string with 170 Ah, this is equal to:
(31)
As the voltage increases, so does the amount of gas. The International Electrotechnical Commission IEC
has guidelines for batteries and gassing levels. Amount of gas during boost will reach to about 1.8 l /
hour.
Figure 42, normal conditions gassing levels assumed 170 Ah/48V [28]
Batteries used for this thesis are new but they have been stored for almost a year at room temperature
above average, without any float charge. This makes it hard to draw any real conclusions, but we have
seen in the data analyzed that the batteries are regaining some of the original capacity, the equalization
charges clearly has a positive effect.
Due to this the recombination phase might not be 100 % correct. But comparing the results we got gave
some results about how much gas can be expected. During a normal equalization charge at boost
voltage the batteries will produce 1.7 L / hour:
(32)
Compared to the internal equalization charge where we got 15 L. With these calculations and
assumptions we can expect a decrease in gas emission, which might imply a slight improvement in
battery lifetime.
54
10 Conclusion
The prototype built is working as it is supposed to, which means that it is possible to productify in the
future. Main purpose of this product is to have as low fuel consumption as possible and to be as
competitive as possible on the market. This is also an advantage in both environmental aspects as well
as disturbing noise from the diesel generator will be reduced. Savings in terms of fuel can, according to
simulations and calculations, be up to 550 l and 264 hours of less runtime per year, which equals a
saving of 13.5 % in fuel consumption compared to the old system.
The battery lifetime is also an important aspect with this internal equalization charge. According to tests
conducted to similar batteries, the batteries will last for at least 2000 cycles with 40 % DoD. [29]
We have also proved that this system is possible and easy to build with cheap components. In a mass
production, special order a DC/DC converter will reduce the price. All the switching is made by four
contactors, which can be mounted on the already existing bottom shelf. Everything can be controlled
through the AVR microcontroller, which also is cheap.
Some of the important questions which are addressed in this thesis are summarized below:
Which are the basic types of batteries used in a hybrid system?
- Flooded Lead-Acid
- Captive Electrolyte Lead-Acid, VRLA
- Nickel-Cadmium
- Lithium Ion
What are the different types of hybrid solutions already on the market?
The most common combinations are:
- Diesel generator with batteries
- Wind turbine or solar panels with diesel generator and batteries
- A combination consisting of batteries, wind, solar and diesel generator
What is an equalization charge and what happens?
An equalization charge is a forced overcharge that aims to reduce the stratification and remove
sulfate from the plates to recover the capacity in the battery.
What are the different kinds of equalization systems available on the market?
There are active and passive methods. The passive method is a forced overcharge with no
sensing mechanism. The active methods can consist of different sensing mechanisms and charge
each cell or battery which needs to be recovered. The active equalization charge system is not
available on the market but it is developed for each specific purpose. The system developed in
this thesis is an active equalization charge system.
How can you measure health of VRLA batteries?
The most common ways are:
- Voltage measurement
- Internal resistance
How much savings can you expect from this new technology?
This active internal equalization charge system can save, compared to old systems, 27 % in
diesel runtime and up to 13.5 % in diesel consumption which would be 550 l / year.
55
11 Discussion and future development
To find a correct DC/DC converter was a struggle in the beginning of the project, since this is supposed
to run on voltages that aren’t standard. The solution was found from ELTEK, but was more expensive
than anything that can be used in the real system, but for this prototype it worked fine. A problem was
also that the voltage settings had to be manually configured via a CAN dongle, sold separately, which
increased the price even more, of course just a onetime cost. During the search to find the right DC/DC
converter, a lot of companies were found who could build these especially for the purpose. This was not
an option at this time due to time and cost, ordering just one is always much more expensive.
A robust temperature sensor should monitor the temperature inside the cabinet in between batteries.
Our solution was a sealed sensor with built in ADC just sending digital values, but due to not using RTOS
or something likewise, the delays needed for the send and receive functions disturbed other non-
critical, like the display of values, functions in our program. A quick fix for this at the time was to not
enter temperature function when using the display.
As this prototype contains many batteries, lot of power, it needed some kind of safety system. If the
system reaches a thermal runway or something in our software locks or voltage reaches LVD the load
and charging should turn off. Problem is that our system is powered via batteries, so if all batteries are
disconnected via the contactors the system will stop. A solution to this would be to have a small backup
battery to power the system.
As for now, an external safety system is used; this measures both temperature and voltage and
disconnects both load and charging if high temp or LVD occurs.
When designing the PCB, thoughts were spent on how to have high EMC and small EMI, something went
wrong during the auto routing and the card was sent for manufacturing, a lot of wires were not
connected directly to the power plane but instead routed like normal wires. The bottom copper power
plane was cut up quite much that may give some disturbances. This is something we encountered when
testing the system; we found some high noises, which were transferred into the ADC resulting in
oscillating values. There was no time to make another PCB so this had to be filtered out with a digital LP
filter.
During the design of LP filter we only measured disturbances we could encounter in lab environment,
only in hindsight we thought about how a real radio base station will send out strange signals. This does
not make any effect on this prototype, but something to think about in future development.
This system relies on having accurate measurements, thus time must also be correct. On the EVK1101
evaluation board there is an external crystal which we use to form an interrupt each second. Something
in the software is disturbing this that changes the time drastically. The crystal should have 30ppm
accuracy, but our time differed up to 5 s / min. For now the fix was in the software manually calibrating
the prescaler for the interrupt so that it matches real time as close as possible. In a future development
an external clock would be recommended, or if RTOS is used better scheduling routines could be applied
for the different functions.
This prototype should run over a long time, so data has to be reached somehow, the SD card logging
works as expected but leaves usability. The card needs to be removed when you want to see data,
during this time the system cannot continue logging. Real time data can of course be read through the
serial terminal, but since we have not implemented anything that is just for monitoring real time. The
56
good thing is that it logs to a .csv file which can directly be opened in excel. One can easily build a macro
that reads the data and presents a graph of the interesting data. A better solution to this would be to
have a web server or similar which you can access via computer to read values.
Some thoughts have been spent on how to know when to equalize the batteries etc. this is hard to
predict, therefore this system will equalize on specified cycle nr. An adaptive algorithm could measure
the power to the RBS and then calculate when, time of day, the system would perform its equalization
cycle. This could then be configured to have “quiet hours”. This will increase the performance because it
will perform the equalization when the power requirements are at its lowest. This algorithm could be
even smarter by measuring power output to the RBS. So with a RBS having low power requirements the
equalization can be set different cycles which also will increase performance and reduce fuel
consumption.
We have also noticed that a so called capacity measurement will increase the capacity because of the
long slow charge. This is something that could be implemented, that after “half time” of the battery
lifetime, perform a capacity measurement. A capacity measurement is a full discharge followed by a full
charge. This will of course require the diesel to run for a long time, but if the batteries can last longer it
might be worth it. This can also give some interesting data, you can compare to the original Ah, which
would give an idea of how many percent of the battery lifetime is left. Another idea could be to have a
setting in the menu where the user can set a capacity measurement whenever he wants to.
Depending on how long the batteries have been stored, a capacity measurement can have a rewarding
effect on the batteries. Long time storage will reduce the capacity due to buildup of large crystals that
are hard to break during normal charging. A capacity measurement with higher boost voltage would
reduce these crystals and improve charge acceptance.
During the equalization charge, the current increased more than we expected. The current shunts used
are rated 100 A 60 mV, this does not mean they cannot handle more, when reaching above 100 A the
scale is nonlinear. We also designed the amplification so that at 100 A the output to the ADC would be
the maximum 5 V, meaning going above 100 A will not make any effect. This is something seen in
chapter 9, Result, where the current during equalization charge is described more. In a future
application the current shunts should be considered changed.
57
12 Reference
[1] R.H. Newnham, W.G.A. Baldsing, ”Benefits of partial-state-of-charge operation in remote-area
power-supply systems”, Journal of Power Sources, 2002, Vol.107(2), pp.273-279
[2] James P. Dunlop, P.E., “Batteries and Charge Control in Stand-Alone Photovoltaic Systems
Fundamentals and Application”, Florida Solar Energy Center, 1997
[3] Arthur D. Sams, “Various Approaches to Powering Telecom Sites”, 2011 IEEE 33rd International
Telecommunications Energy Conference, Oct. 2011, pp.1-8
[4] Gebriele Seeling-Hochmuth, “Optimisation of hybrid energy systems sizing and operation
control”, Kassel University Press GmbH, 1999, ISBN 3-933146-19-4
[5] Y.S.Wong, W.G. Hurley, W.H. Wölfle, "Charge regimes for valve-regulated lead-acid batteries:
Performance overview inclusive of temperature compensation", Journal of Power Sources, 2008,
Vol.183(2), pp.783-791
[6] K. Mamadoua,c, T.M.P. Nguyenb, E. Lemaire-Potteauc, C. Glaizea, J. Alzieub, "New accelerated
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[7] Patrick T. Moseley, "High rate partial-state-of-charge operation of VRLA batteries", Journal of
Power Sources, 2004, Vol.127(1), pp.27-32
[8] Kong Zhi-Guo, Zhu Chun-Bo, Lu Ren-Gui, Cheng Shu-Kang, "Comparison and Evaluation of
Charge Equalization Technique for Series Connected Batteries", 2006 37th IEEE Power
Electronics Specialists Conference, June 2006, pp.1-6
[9] Kong-Soon Ng, Yao-Feng Huang, Chin-Sien Moo, and Yao-Ching Hsieh, "An Enhanced Coulomb
Counting Method for Estimating State-of-Charge and State-of-Health of Lead-Acid Batteries",
INTELEC 2009 - 31st International Telecommunications Energy Conference, Oct. 2009, pp.1-5
[10] M. Thele a, J. Schiffer a, E. Karden b, E. Surewaard b, D.U. Sauer a, "Modeling of the charge
acceptance of lead–acid batteries", Journal of Power Sources 2007, Vol.168(1), pp.31-39
[11] Gu W.B, Wang G.Q, Wang C.Y, "Modeling the overcharge process of VRLA batteries", Journal of
Power Sources 2002, Vol.108(1), pp.174-184
[12] Yutian Pan, Yanqiang Ma, Qingzhang Qin, Peiying Li, "Study on an Active Voltage Equalization
Charge System of a Series Battery Pack", Proceedings of 2011 International Conference on
Electronic & Mechanical Engineering and Information Technology, Aug. 2011, Vol.1, pp.141-144
[13] R.H. Newnham, W.G.A. Baldsing, "Advanced management strategies for remote-area power-
supply systems", Journal of Power Sources 2004, Vol.133(1), pp.141-146
58
[14] Hunter, Phillip M., " VRLA battery float charge : analysis and optimisation", University of
Canterbury. Electrical and Electronic Engineering, 2003
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http://www.wind-power-program.com/turbine_characteristics.htm
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[17] Solar Power (Technology and Economics), Retrieved 2013-02-27, from:
http://www.mpoweruk.com/solar_power.htm
[18] Can I Go “Off-Grid”?, Retrieved 2013-03-01, from: http://windpower.facts-and-review.com/can-
i-go-off-grid
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http://exploringgreentechnology.com/solar-energy/hybrid-energy-systems/
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operation/series-resistance
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http://en.wikipedia.org/wiki/Moritz_von_Jacobi#Jacobi.27s_Law
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http://batteryuniversity.com/learn/article/is_lithium_ion_the_ideal_battery
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[27] Diesel fuel consumption chart, Retrieved 2013-05-14, from: http://www.perfectfuel.ca/pdf/Diesel%20Consumption%20Litres%2012092009.pdf
[28] Gunder Karlsson, NorthStar SiteTel Sweden AB
[29] NorthStar SiteTel Sweden AB
[30] About Northstar SiteTel, Retrieved 2013-06-19:
http://www.northstarsitetel.com/northstar/about/index.php
[31] Kent Westergren, Ericsson AB
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Hård
vara
97
910
100%
Mju
kvar
a10
89
1210
0%Im
plem
enta
tion
155
155
100%
Verif
ierin
g/te
st18
416
710
0%Ra
ppor
t9
158
1610
0%
Pro
jekt
plan
erin
g
