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i
MPPT DC/DC Converter Design
for a
PV Powered Electric Vehicle Charging System
_______________________
Fearghal Kineavy
B.E. (Hons) in Energy Systems Engineering
Department of Electrical & Electronic Engineering
College of Engineering & Informatics
National University of Ireland Galway
2013
ii
Abstract
It is apparent that energy supply will be one of the major challenges for the world during the
21st century, particularly for Ireland where we import 90% of our energy. With petroleum
resources running out and prices at petrol and diesel pumps continuing to climb, the
transport industry is being forced to look at alternatives to petroleum engines and electric
vehicles are the most promising solution to date. Electric vehicles (EVs) will allow us to break
the link between transport and fossil fuels as the electricity to power EVs can come from
energy sources other than fossil fuels such as renewable sources like solar panels and wind
turbines. If EV technology is to have an impact on the transport industry, effective and
efficient charging strategies will be required.
iii
Acknowledgements
I would like to begin by thanking my project supervisor Dr. Maeve Duffy. Her supervision
and advice throughout the year was invaluable and without her this project would not have
been possible. I would also like to thank my co-supervisor Dr. Edward Jones for his feedback
at key points during the year.
I would like to give a special thanks to the Electrical and Electronic lab technicians, Myles
Meehan and Martin Burke. Their help and practical advice over the last year and 3rd year
was priceless and is very much appreciated. Without their input the demonstration system
would not have been possible.
I would like to thank my classmates for their help and support over the past 4 years.
I would also like to thank Dr. Marcus Keane and IRUSE for providing me with solar
irradiation data for Galway, and the gobetwino website for their open source gobetwino
software.
Finally I would like to thank my family and friends for their constant support and
encouragement throughout the past year.
iv
Declaration of Originality
I declare that this thesis is my original work except where stated
Signature:__________________________________________________
Date:______________________________________________________
v
Table of Contents
Abstract ...................................................................................................................................... ii
Acknowledgements ................................................................................................................... iii
Declaration of Originality .......................................................................................................... iv
1. Introduction ....................................................................................................................... 1
2. Project Approach ............................................................................................................... 2
3. Review of Similar Work ...................................................................................................... 3
3.1. Solar Photovoltaic Charging of Lithium-Ion Batteries [2] ........................................... 3
3.2. Solar photovoltaic charging of high voltage nickel metal hydride batteries using DC
power conversion [1] ............................................................................................................. 5
3.3. Direct solar photovoltaic charging of a high voltage nickel metal hydride traction
battery [3] .............................................................................................................................. 6
4. Electric Vehicles ................................................................................................................. 9
4.1. Electric Vehicle Technology ......................................................................................... 9
4.2. Electric Vehicles in Ireland ........................................................................................ 10
4.3. Nissan Leaf ................................................................................................................ 11
5. Solar Photovoltaic Panels ................................................................................................. 12
5.1. Photovoltaic Theory [11] ........................................................................................... 12
5.2. Galway Solar Irradiation ............................................................................................ 13
5.3. Solar Array Sizing ....................................................................................................... 16
5.4. Solar Array Characteristics ........................................................................................ 19
6. DC-DC Converters [8], [9] ................................................................................................. 25
6.1. Basic Switching Converter ..................................................................................... 25
6.2. Pulse Width Modulation (PWM) ........................................................................... 26
6.3. Buck Converter ...................................................................................................... 27
6.4. Boost Converter ..................................................................................................... 29
vi
7. Lithium-Ion Batteries ....................................................................................................... 31
7.1. Lithium Ion Technology ............................................................................................. 31
7.2. Charging..................................................................................................................... 31
7.3. Demonstration System Li-ion Cell ............................................................................. 32
7.3.1. Average Charge Voltage ..................................................................................... 35
8. Charging System Design ................................................................................................... 36
8.1. Converter Design ....................................................................................................... 36
8.1.1. MOSFET .............................................................................................................. 36
8.1.2. Inductor .............................................................................................................. 37
8.1.3. Diode .................................................................................................................. 38
8.1.4. Capacitors .......................................................................................................... 39
8.2. PSPICE Model ............................................................................................................ 39
8.2.1. Synchronous Boost Converter Model ................................................................ 39
8.2.2. Final PSPICE Model ............................................................................................ 41
8.3. Model Results ............................................................................................................ 45
8.3.1. System Efficiency and Power Losses .................................................................. 45
8.3.2. Boost Converter Waveforms ............................................................................. 48
9. Demonstration System .................................................................................................... 50
10. Conclusion ..................................................................................................................... 57
11. Bibliography .................................................................................................................. 58
vii
List of Tables
Table 3.1 – Sanyo 190W module characteristics at 52°C .......................................................... 3
Table 3.2 – Test charge results .................................................................................................. 6
Table 3.3 – Test charge results .................................................................................................. 7
Table 5.1 – No. Sanyo 210W panels required to charge Nissan Leaf each day ....................... 16
Table 8.1 – Variable Inductor ................................................................................................... 38
Table 8.2 – Model Results ........................................................................................................ 45
Table 8.3 – Converter Losses ................................................................................................... 47
Table 9.1 - Charger operating at different input voltages ....................................................... 55
Table 9.2 – PV Charging of Li-ion Cell ...................................................................................... 56
List of Figures
Figure 3.1 – Solar energy to battery charge efficiency .............................................................. 4
Figure 3.2 – Direct solar PV charging of a 15 cell li-ion battery pack ........................................ 4
Figure 3.3 – Charging System ..................................................................................................... 5
Figure 4.1 – Leaf Battery Module ............................................................................................ 11
Figure 5.1 – PV cell [12] ........................................................................................................... 12
Figure 5.2 – Daily Solar Irradiation for Galway ........................................................................ 14
Figure 5.3 – Predicted Solar Irradiation in Galway from model by S. Armstrong and W.G.
Hurley ....................................................................................................................................... 14
Figure 5.4 – 9am-5pm Instantaneous Solar Irradiation for May ............................................. 15
Figure 5.5 – No. Sanyo 210W panels required to fully charge a Nissan Leaf each day ........... 17
Figure 5.6 – Desired travel range from PV charge ................................................................... 18
Figure 5.7 - Simplified Single diode model ............................................................................. 19
Figure 5.8 – Array I-V curve at varying temperatures ......................................................... 21
Figure 5.9 – Array P-V curve at varying temperatures ........................................................ 22
Figure 5.10 – Array I-V curve at varying irradiations .......................................................... 23
Figure 5.11 – Array P-V curve at varying irradiations ......................................................... 23
Figure 6.1 – Basic Switching Converter .................................................................................... 25
Figure 6.2 – Output voltage as function of time ...................................................................... 26
viii
Figure 6.3 – Pulse Width Modulation (PWM) .......................................................................... 26
Figure 6.4 – Buck Converter ..................................................................................................... 27
Figure 6.5 – (a) Inductor voltage and (b) inductor current ...................................................... 28
Figure 6.6 – Boost Converter ................................................................................................... 29
Figure 6.7 – Boost converter waveforms ................................................................................. 30
Figure 7.1 – Common li-ion battery [17] ................................................................................. 31
Figure 7.2 – Li-ion battery charge profile [4] ........................................................................... 32
Figure 7.3 – Li-ion charger ....................................................................................................... 33
Figure 7.4 – Li-ion cell test charge ........................................................................................... 34
Figure 7.5 – Li-ion cell charge voltage and current .................................................................. 34
Figure 7.6 – Cell voltage vs cell charge .................................................................................... 35
Figure 8.1 – Variable Inductance and Current ripple............................................................... 38
Figure 8.2 – Synchronous boost converter .............................................................................. 40
Figure 8.3 – Large current spikes around high frequency loop ............................................... 40
Figure 8.4 – Charging System circuit diagram ......................................................................... 41
Figure 8.5 - Maximum power transfer for solar cell unit [14] ................................................. 41
Figure 8.6 – System Efficiency throughout a typical day in May ............................................. 46
Figure 8.7 – System efficiency vs the output power of the PV array ...................................... 47
Figure 8.8 – Breakdown of converter losses ............................................................................ 48
Figure 8.9 – Boost Converter Waveforms from PSPICE ........................................................... 49
Figure 9.1 – Original System Layout ........................................................................................ 50
Figure 9.2 – 15uH Wurth Inductor ........................................................................................... 52
Figure 9.3 – bq24650 typical application ................................................................................. 52
Figure 9.4 – QFN package ........................................................................................................ 52
Figure 9.5 – Demonstration System Circuit Diagram .............................................................. 53
Figure 9.6 - Initial circuit board layout ..................................................................................... 54
Figure 9.7 – High frequency part isolated from rest of circuit using a ground plane ............. 55
Figure 9.8 – High frequency part encased by grounded metallic box and ground plane ....... 55
Figure 9.9 – Solar charging outside using 10W Spectra PV panel ........................................... 56
1
1. Introduction
There are an increasing number of vehicle choices available that utilize batteries and electric
motors to reduce tailpipe emissions and increase fuel economy. Here in Ireland the
government is actively encouraging the uptake of electric vehicles. By replacing petroleum
with electricity, the eventual production of electricity in a renewable fashion, such as from
solar panels or wind turbines, can achieve a carbon free transportation sector. It will also
reduce dependence on dwindling fossil fuel reserves. At the same time energy efficiency has
never been more important than with the current scarcity of energy. Therefore it is
important that charging systems for future electric vehicles are well-designed to maximise
efficiency.
The main aim of this project is to design a solar photovoltaic (PV) system for charging
electric vehicles (EVs) using direct current (DC) exclusively without any DC/AC (alternating
current) or AC/DC conversion stages. This will be a system that would be suitable for a
building such as the Engineering building in NUIG where there are cars parked during the
day when the sun is shining. A small scale demonstration system will also be built to
demonstrate the concept.
2
2. Project Approach
The project is largely research based, containing some practical elements for the building
and testing of the demonstrator model. Initial research into electric vehicles in general was
carried out. This included research into the number of EVs on the road now, future uptake
trends, current technology (specifically the kind of batteries used in EVs, charging profiles,
and limitations) etc. There was also significant research done on solar PV panels. Using this
research, the author was able to gauge what size PV systems would be required in various
EV charging schemes.
Once the author had an idea of the scale of a PV charging EV system, it was scaled down to a
size that is testable in the lab. This was the basis for the demonstration system. The
demonstration system is a DC-DC converter with maximum power point tracking (MPPT)
that also regulates the charge for a lithium ion (li-ion) battery. The demonstration system
was designed with the aid of a computer circuit simulation program, PSPICE. Appropriate
values for the DC-DC converter components (capacitor and inductor) were calculated and
the circuit simulation program enabled different values to be tested before any circuit was
built.
PSPICE was used to model a real size PV charging system capable of charging an electric
vehicle. Real component models from the manufacturers were used in the PSPICE model to
make it as realistic as possible. Detailed simulations were carried out by running the model
under various operating conditions to investigate how the efficiency varied.
Finally this detailed simulation of a PV charging EV system using only DC current, was
compared against a similar system which has DC-AC and AC-DC conversion stages between
the PV panel and the EV.
3
3. Review of Similar Work
Journals and scholarly websites were searched to find previous research relating to PV
charging of electric vehicles. Very few papers were found. This may be because high voltage
EVs are still relatively new and little research has been done on using renewable sources to
charge them. In fact in one of the papers reviewed the authors state that they believe their
work was the first time that high voltage batteries were charged using solar energy to prove
the concept of solar-powered charging for EVs [1].
3.1. Solar Photovoltaic Charging of Lithium-Ion Batteries [2]
In this paper solar PV charging of batteries was tested by using high efficiency crystalline
and amorphous silicon PV modules to recharge lithium-ion battery modules to serve as a
proof of concept for solar PV charging of batteries for EVs. The li-ion cells charged were
lithium iron phosphate (LiFePO4) 3.3V 2.3Ah cells. Charge tests were carried out using 10,
12, 13, 14, 15, and 16 cells connected in series to make a battery pack. The authors used a
single Sanyo HIP-190BA3 PV module to charge the battery pack. During the tests the PV
module was at a temperature of about 52°C. Typical module values at 52°C are shown in
table 3.1.
Table 3.1 – Sanyo 190W module characteristics at 52°C
Pmax 175W
Voc 62V
Isc 3.77A
Vmpp 50.2V
Impp 3.49A
PV efficiency 14.8%
The battery pack is connected directly to the terminals of the PV module, without any
intervening circuitry. For each test charge the authors continuously recorded the solar
irradiation, the temperature, the system voltage, and the system current. Using these
measurements, the overall solar to charge efficiency of the system was calculated for every
time step of each test charge. Figure 3.1 shows the efficiency during charging for the 13, 14,
4
15, and 16 cell battery packs. Figure 3.2 shows the charge profile for the 15 cell battery
pack.
Figure 3.1 – Solar energy to battery charge efficiency
Figure 3.2 – Direct solar PV charging of a 15 cell li-ion battery pack
The charging was most efficient for the 15 cell battery pack, 14.5%. This is because its
average charging voltage most closely matched the maximum power voltage of the PV
module and therefore the PV module was operating at a higher efficiency than with the
other battery packs. This set up also has a self-regulating feature as the charge terminates
itself as the battery voltage approaches the open circuit voltage of the PV module.
5
This paper showed that it is possible to efficiently charge li-ion batteries with PV panels by
utilizing a direct connection between the two. The direct connection eliminates efficiency
losses associated with intervening circuitry such as DC-DC converters, inverters, and
rectifiers. The efficiency is maximised when the battery charging voltage closely matches the
maximum power voltage of the PV panels and this also provides a self-regulating charge
feature.
3.2. Solar photovoltaic charging of high voltage nickel metal
hydride batteries using DC power conversion [1]
This paper describes the solar PV charging of a high voltage nickel-metal hydride (NiMH)
battery pack used in a hybrid electric vehicle. A DC-DC converter is used to boost the PV
output voltage ( 50V) to over 300V to charge the battery pack. There is no other
intervening power circuitry such as inverters or rectifiers as the whole system is DC. Figure
3.3 shows a schematic of the system. The system is made up of 4 solar arrays each made up
of 2 Sanyo 190W modules connected in parallel. Each array has its own boost converter and
the outputs of the boost converters are connected in parallel and connected to the NiMH
battery terminals.
Figure 3.3 – Charging System
The battery pack has its own battery pack control module (BPCM) which monitors the
battery conditions and can be programmed to open the internal battery high-voltage
contacts when specified conditions are met, thereby terminating the charge. The boost
6
converters did not act as charge controllers. They simply boosted the low 50V PV voltage up
to the voltage required to charge the battery pack but did not control the output voltage or
current to the battery. Instead, the BPCM was used to terminate the charge when the
battery voltage reached 340V or if the battery temperature was outside of its limits. This
simplified the charge control protocol. The two best experiments were the final two,
performed on 8 Feb. and 19 Feb. 2010. Table 3.2 shows the important results.
Table 3.2 – Test charge results
8th Feb 19th Feb
Duration of experiment (minutes)
66.3 66.7
Average solar irradiation (W/m^2)
1,089 1,132
Avg. charge rate 0.58C 0.58C
Energy to battery (kWh) 1.38 1.39
Charge to battery (Ah) 4.19 4.20
PV to electricity efficiency 13.1% 12.7%
Boost converter efficiency 93% 92%
Solar to battery charge efficiency
12.2% 11.7%
The PV arrays were operating at a voltage above their maximum power point voltage,
Vmpp, therefore the PV to electricity efficiency is lower than the previous paper where the
PV to electricity efficiency of the same type of panel was 14.5% at 52°C. The efficiency of the
boost converter is 92-93%.
Overall the system is still an efficient and simple option for charging an EV battery as it has
no inverter or rectifier losses and the charge control is relatively simple.
3.3. Direct solar photovoltaic charging of a high voltage nickel
metal hydride traction battery [3]
This paper builds on the results of the last paper by charging the same high voltage NiMH
battery using solar PV energy. However this time the Sanyo 190W modules are connected in
series to give an array output voltage capable of directly charging the battery without the
need for a DC-DC converter to boost the array voltage to the battery voltage. Test charges
were carried out with 5, 6, 7, and 8 modules in series. The open circuit voltage of the 5
module array was less than the final battery charge state of 340V so this configuration could
7
not charge the battery to its final voltage. Table 3.3 shows the results of 5 test charges
carried out with between 6 and 8 modules in series.
Table 3.3 – Test charge results
No. modules in series
6 6 7 7 8
Experiment duration (minutes)
151.4 125.8 64.7 122.5 95.1
Avg. solar irradiation (W/m^2)
707 972 930 642 720
Avg. PV voltage (V)
334.6 332.8 336.2 335.7 335.0
PV Vmpp (V)
306.4 299.5 360.1 367.4 420.2
Avg. charge rate
0.31C 0.38C 0.52C 0.36C 0.42C
Energy to battery (kWh)
1.68 1.72 1.23 1.58 1.44
Charge to battery (Ah)
5.05 5.21 3.66 4.75 4.32
Solar to battery charge efficiency
13.3% 12.2% 15.0% 14.7% 13.3%
The most efficient configuration was when 7 modules were connected in series, ranging
from 14.7%-15%. This is because the average charging voltage most closely matched the PV
Vmpp for 7 modules in series (PV Vmpp was calculated for each test based on the
temperature of the modules during the test). The Vmpp for the 6 module configuration was
just as close to the charging voltage however the efficiency was much lower because PV
efficiency drops much more quickly when the PV voltage is above the Vmpp than when the
PV voltage is below the Vmpp.
It is interesting to note that although the 7 module configuration was the most efficient in
terms of solar to charge efficiency, it was not the optimum configuration for delivering the
maximum charge to the battery. For the test with 15% solar to charge efficiency, only
3.66Ah were delivered to the battery before it reached its charge cut-off voltage of 340V.
8
This is significantly below the 5.21Ah delivered using the 6 module array during the test with
the lowest solar to charge efficiency of 12.2%. This is most likely because of the charging
current profile of the different configurations. For the 7 and 8 module arrays, the Vmpp of
the array is above the charge cut-off voltage of 340V and therefore the charge current is at
its maximum just before cut-off. With the 6 module array, the current is decreasing as the
charge voltage approaches 340V because the array is moving away from its Vmpp. This
illustrates the generally accepted consensus, [4, 5] that for the optimum charging strategy
(i.e. maximum battery charge), current should be decreased as the battery approaches its
float voltage.
This paper shows that the direct solar PV charging of a NiMH EV battery is viable, however,
achieving both efficient charging and fully charging the battery is difficult.
9
4. Electric Vehicles
4.1. Electric Vehicle Technology
An electric vehicle (EV) is a vehicle that uses an electric motor to provide the power
required for propulsion. There are several variations in the EV family, the ones described
below are the main technologies currently in use:
Hybrid Electric Vehicle (HEV)
Hybrid electric vehicles use an electric motor driven by a small electric battery in
conjunction with an internal combustion engine (ICE) to provide power to the wheels. The
electric motor can take the vehicle from idle up to about 30-40mph, but for higher speeds
the ICE runs the wheels directly. The battery is charged by the ICE and so fossil fuels are still
the source of energy, however, the small battery allows for improved efficiency over
conventional vehicles. The ICE can be shut off completely when the battery is charged and
the vehicle is running at low speeds, e.g. city driving. The electric battery also uses
regenerative braking which recovers energy when the vehicle brakes to charge the battery.
In conventional vehicles this energy is normally wasted as heat.
Plug-in Hybrid Electric Vehicle (PHEV)
Plug-in hybrid electric vehicles are much like hybrid electric vehicles except that the battery
is bigger and the ICE is smaller. The larger battery can be charged by plugging in to an
electricity source or by the ICE. Therefore if PHEVs are plugged in every night as
recommended, they can run for up to 40 miles on electricity from the battery before the ICE
starts up [6]. This means significant displacement of petroleum fuel with multi-source
electricity. In some PHEVs such as the Chevrolet Volt, the electric motor is the only system
that directly drives the wheels, the ICE is only there to charge the battery. This means that
when the ICE is running it can be geared to run at its max. efficiency.
Battery Electric Vehicle (BEV)
Battery electric vehicles run entirely on electricity and have no ICE. They rely completely on
plugging into an electricity source to charge. This means that they have the potential to run
from completely clean sources of energy, however, their range is a major drawback. The
10
Nissan Leaf for example has a maximum range of about 175km [7]. This range also depends
on driving habits, for example, driving at higher speeds will reduce the range considerably.
Battery electric vehicles are the focus of this thesis.
4.2. Electric Vehicles in Ireland
As of October 2012, 192 electric cars have been sold in the Republic of Ireland [8]. Several
car manufacturers, such as Renault, Peugeot, Citroen and Mitsubishi, have electric car
models, however, the Nissan Leaf is the most popular commercial model. Official
government policy is to encourage the uptake of electric vehicles over the next decade and
Electric Ireland are rolling out charging infrastructure across the country to support uptake.
There are 3 different ways to charge EVs in Ireland [9]:
(i) Home charge points – these are installed in peoples’ homes and are the primary
source of electricity for EVs as the EV can be plugged in to charge overnight. An
EV will draw single phase 16A (3.6kW) when connected to a home charge point
meaning that a full charge takes 6 – 8 hours.
(ii) Public charge points - Public charge points are being installed on-street and in
locations such as shopping centres and car parks. They are connected to a local
3-phase electricity supply and depending on the car type and battery size,
charging takes between 1 and 6 hours. As of October 2012, there are 860 public
charge points [8].
(iii) Fast charge points – Fast charging is done with either a 3-phase, 63A AC (44kW)
supply or a 120A, 400V DC (50kW) supply. As of October 2012, there are 30 fast
chargers in ROI [8] and all of these are DC fast chargers. The Nissan Leaf,
Mitsubishi iMiEV, Peugeot iOn and Citroen C-ZERO support DC fast charging. A
50kW DC fast charge point can charge a compatible electric car up to 80% in 20-
30 minutes. The AC fast charging will cater for other models, the next available
being Renault Zoe.
The government has standardised charge points with 2 socket types. An IEC Type 2 socket
and plug (Mennekes) is used for home, public and AC fast charging while a CHAdeMO socket
and plug is used for DC fast charging.
11
The author decided to take the Nissan Leaf as the typical electric car as it is the most
popular in Ireland.
4.3. Nissan Leaf
The Nissan Leaf is a 5 door family car and has roughly the same performance as a 1.6 litre
petrol engine. In place of an internal combustion engine, the Leaf is run by an 80kW AC
motor that gives the Leaf a maximum speed of more than 140km/h [7]. The motor gets its
energy from a 24kWh lithium-ion battery. The battery is composed of 48 modules in series.
Each module has 2 lines in parallel with 2 cells in series in each line. Each cell has a charge
capacity of 33.1Ah and an average voltage of 3.8V [10]. Therefore the energy capacity of
each module is
and the total energy capacity is
Figure 4.1 shows the layout of a single Leaf battery module
Figure 4.1 – Leaf Battery Module
The Leaf has two connectors for charging the battery, a Mennekes connector for home and
public charging, and a CHAdeMO connector for DC fast charging. With home and public
charging, an onboard charger in the Leaf converts the AC electricity to DC electricity and
charges the battery. With DC fast charging, the charging station communicates with the
battery via the CHAdeMO connector and the DC current goes straight from the charging
station to the battery.
12
5. Solar Photovoltaic Panels
5.1. Photovoltaic Theory [11]
Solar photovoltaic cells are capable of converting light directly into electricity. They normally
consist of N-type silicon and P-type silicon. Silicon doped with phosphorous is N-type silicon
and has free electrons. P-type silicon is silicon doped with boron and has free holes. When
the two layers are put in contact with each other, some of the free electrons from the N-
type silicon fill up some of the holes in the P-type silicon creating a “barrier” at the junction
between the two materials. This barrier is really an electric field that is trying to keep
electrons on the N-side and holes on the P-side. When a photon of light with enough energy
strikes a PV cell, it knocks an electron out of its hole and the electric field will push the
electron to the N-side. If an external current path exists, the electron will flow through it
back to the P-side. This electron flow creates an electric current while the electric field at
the junction creates a voltage, and current multiplied by voltage is power. Figure 5.1 shows
the process.
Figure 5.1 – PV cell [12]
Efficiencies just above 40% have been achieved with PV cells, however typical commercial
PV panels have efficiencies from 6-20%. That is, 6-20% of the solar irradiation incident on
the PV panel is converted into electricity.
13
5.2. Galway Solar Irradiation
To calculate how many PV panels would be required to charge a Nissan Leaf parked at the
Engineering building, it was first necessary to get data on the solar irradiation in Galway.
There is a weather station run by Dr. Marcus Keane and IRUSE (Informatics Research Unit
for Sustainable Engineering) at the engineering building in NUI Galway equipped with
various sensors, including sensors for detecting solar irradiation in W/m^2. Solar irradiation
measurements taken every minute were obtained from the weather station for the whole of
2012. This data was then analysed in MATLAB to work out average solar irradiations for
each month. The data was extracted as follows:
Firstly hourly irradiance figures were extracted from the data. This was done by
getting the average of all the measurements for one hour.
Next the average daily instantaneous irradiance figures were calculated by getting
the average of all the hourly irradiance figures in each day.
Daily instantaneous irradiation figures (W/m^2) were converted to Wh/m^2/day by
multiplying by 24 (hours).
Finally average daily irradiation (Wh/m^2/day) for each month was calculated by
getting the average of all the days in each month. E.g. for January this was the sum
of the daily irradiation each day in January divided by 31, while for February it was
the sum of the daily irradiation each day in February divided by 29 (2012 was a leap
year).
Figure 5.2 shows the average daily irradiation (kWh/m^2/day) for each month of 2012.
Figure 5.3 shows the solar irradiation for Galway predicted from a model developed by S.
Armstrong & W.G. Hurley. Their methodology combines hourly observations of cloud
conditions with monthly sunshine hours data in order to determine the frequency of clear,
partly cloudy and overcast skies [13].
14
Figure 5.2 – Daily Solar Irradiation for Galway
Figure 5.3 – Predicted Solar Irradiation in Galway from model by S. Armstrong and W.G. Hurley
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Daily
Sola
r Ir
radia
tion (
kW
h/m
2/d
ay)
Actual Data
Least Squares Fit
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Daily
Sola
r Ir
radia
tion (
kW
h/m
2/d
ay)
15
As can be seen the observed solar irradiation from IRUSE agrees well with available data on
solar irradiation in Galway. The biggest discrepancy is for June and July, however this may
be because Galway had a bad summer in 2012. May is the month with the highest levels of
solar irradiation with an average irradiation of 4.79kWh/m^2/day while December is the
lowest with only 0.52kWh/m^2/day.
It was decided to use the May levels of sunshine for the design of the charging system.
MATLAB was again used to calculate the instantaneous irradiation each hour from 9am -
5pm for a typical day in May.
Firstly the May data was extracted from the raw data.
For each hour, average irradiance was calculated by averaging all the values for each
minute over that hour.
A daily instantaneous irradiation curve was obtained by getting the average 9am-
10am irradiation, 10am-11am irradiation etc. i.e the hourly irradiation values from
9am – 10am each day were added up and divided by 31 to get the irradiation
between 9am-10am for a typical day in May.
The average instantaneous irradiation from 9am-5pm was then calculated,
multiplied by 8hr and divided by 1000 to get the cumulative daily irradiation from
9am-5pm (kWh/m^2/day).
Figure 4.2.3 shows the instantaneous irradiation curve from 9am-5pm for a typical day in
May.
Figure 5.4 – 9am-5pm Instantaneous Solar Irradiation for May
9am-10am 10am-11am 11am-12pm 12pm-1pm 1pm-2pm 2pm-3pm 3pm-4pm 4pm-5pm300
350
400
450
500
550
600
Insta
nta
neous S
ola
r Ir
radia
tion (
W/m
2)
16
The instantaneous irradiation peaks between 12pm and 1pm at 573W/m^2. The average
instantaneous irradiation is 462.7W/m^2. The total cumulative solar irradiation from 9am-
5pm is 3.7kWh/m^2/day. This represents 77% of the total irradiation for a full day in May.
5.3. Solar Array Sizing
The solar panel that is being modelled for the charging system is the Sanyo HIP-210NH1-BO-
1 [14]. It has a max. power of 210W, an efficiency of 16.5%, open circuit voltage (Voc) of
50.9V and short circuit current (Isc) of 5.57A. Firstly the author investigated how many
panels would be needed to fully charge a Nissan Leaf every day. First the area of panels was
calculated using the equation:
( )
Where is the energy capacity of the EV battery in kWh, is the solar irradiation in
kWh/m^2 and is the efficiency of the PV panels. Then to determine how many PV panels
were required, the author simply divided by the area of one PV panel. Table 5.1 and figure
5.5 show the results of the above calculations for the 12 months of the year.
Table 5.1 – No. Sanyo 210W panels required to charge Nissan Leaf each day
Month Solar Irradiation (kWh/m^2/day)
Area Required to Charge EV (m^2)
No. Panels Required to Charge EV (m^2)
Jan 0.61 238 188
Feb 0.98 149 117
Mar 2.33 63 50
Apr 3.89 37 30
May 4.79 30 24
Jun 4.20 35 28
Jul 3.94 37 29
Aug 3.99 36 29
Sep 2.76 53 42
Oct 1.72 84 67
Nov 0.91 159 125
Dec 0.52 281 221
Average 2.55 57 45
Apr-Aug Average 4.16 35 28
17
Figure 5.5 – No. Sanyo 210W panels required to fully charge a Nissan Leaf each day
As would be expected, there is a huge variation between how many panels are required in
summer compared to winter (24 panels in May, 221 panels in December). The average
number of panels needed is 45. If we only consider the 5 months from April to August, the
average number of panels required drops to 28, covering about 35m^2. Multiplying the
maximum output of one panel by 28, this translates into approximately a 6kW array to
power a Nissan Leaf from April to August.
Rather than sizing an array that would deliver a full 24kWh charge to the Leaf each day, it
was decided to size an array that would provide enough charge for a daily commute to the
engineering building from Galway city or one of the surrounding towns or villages.
Therefore the PV array was sized to deliver enough charge for the month of May for a
Nissan Leaf to travel 25km from the engineering building. This is enough to get to towns
such as Oughterard or Headford. Figure 5.6 shows this range on a map of Galway. The red
circle represents the range.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec0
50
100
150
200
250N
o.
Panels
Required
18
Figure 5.6 – Desired travel range from PV charge
Next the author determined how much energy was required for the Leaf to travel 25km. A
chart that shows how far a Leaf can travel based on remaining battery charge was used to
do this [15]. As mentioned previously the distance possible varies with speed so a speed of
50mph (80km/hr) was assumed for the daily commute. At 50mph and 100% charge, the Leaf
is capable of 97 miles. (note: the chart assumes only 21kWh of useable charge in the
battery). Therefore the distance per unit charge is simply
Therefore if 25km of range is required, the charge required from the PV array is
It was then a simple matter of using the previous equations to work out the number of
panels needed for May.
19
( )
Where is the energy required from the array in kWh/day, is the total irradiation from
9am-5pm in kWh/m^2/day, is the PV efficiency of the panels, and is the area per panel
in m^2. This gives
( )( )
5.4. Solar Array Characteristics
Solar modules can operate over a range of different voltages, and as the voltage varies, the
current also varies. Every PV module has its optimum operating voltage under specific
conditions. At this voltage, the maximum power point (MPP), the module is delivering the
maximum available power. Moving the voltage away from the MPP will decrease the power
being produced by the module and will therefore reduce the efficiency of the system.
It was decided to connect the 5 Sanyo 210W panels in series to have the output voltage of
the array as close as possible to the charging voltage of the leaf, because in general, the
closer the input and output voltages of a dc-dc converter, the better the efficiency.
To determine the MPP of the array, it was necessary to model it in excel under different
conditions to determine its I-V and P-V curves. The solar array was modelled using the
simplified single diode model shown in figure 5.7. This model is commonly used
because of its practicality and the fact that it represents a reasonable compromise
between accuracy and simplicity.
Figure 5.7 - Simplified Single diode model
20
For the purposes of the model, the array was assumed to be one big solar panel so the
open circuit voltage and Open circuit voltage temperature coefficient were multiplied by
5 as the panels are to be connected in series.
Shown below are the equations used to plot the I-V and P-V.
Shown below are the values needed to plot the output current.
q Charge on an electron (q = 1.6022 x 10-19 C) K Boltzmann constant (K=1.38*10-23 m².kg.s-2 °K) n Ideality factor (n=1.5) I Output current Iph Photo-generated current Isat Saturation current V Output Voltage (0 to open circuit voltage) Ns Number of cells in series (509) Rs Series resistance (Rs= 0.004Ω) Tcell Solar Panel temperature Iscref Short circuit current at standard conditions (Iscref = 5.57A)
S Solar radiation
Sref Reference solar irradiance (S=1000W/m²)
isc Short circuit current temperature coefficient (isc = 1.67mA/ )
Tref Reference temperature at standard conditions (Tref = 25 )
Voc Open circuit voltage
Vocref Open circuit voltage at standard operating conditions (Vocref = 254.5V)
voc Open circuit voltage temperature coefficient (voc = -0.635V/ )
1. ...
).(
NsTcellKn
RsIVq
satph eIII
)( refcellvococrefoc TTVV
1...
.
scell
oc
ph
sat
NTKn
qVe
II
refcellisc
ref
screfph TTS
SII 1..
21
A voltage of 0.5V/cell was assumed so the number of cells, Ns, was calculated by
dividing the open circuit voltage at standard operating conditions by 0.5V.
The equation for the output current cannot be solved analytically as I appears on both
sides of the equation. However I-V and P-V curves were obtained as follows:
In the equation for the current, I, replace the (V + I.Rs) term with Vd and plot I
from Vd = 0 to Vd = Voc
Calculate V from Vd = 0 to Vd = Voc using V = Vd – I.Rs
Plot I versus V and P versus V where P = VI
The I-V and P-V curves for the array vary with both cell temperature, Tcell, and solar
irradiation, S. Figures 5.8 and 5.9 show how the curves vary with constant solar
irradiation but varying temperature. The solar irradiation shown is the average solar
irradiation from 9am-5pm for May.
Figure 5.8 – Array I-V curve at varying temperatures
0 50 100 150 200 250 3000
0.5
1
1.5
2
2.5
3I-V Curves @ S = 462.7W/m2
Voltage (V)
Curr
ent
(A)
T=0°C
T=25°C
T=50°C
T=75°C
22
Figure 5.9 – Array P-V curve at varying temperatures
It can clearly be seen from figure 5.9 that the temperature has a big influence on the PV
array. The MPP is the voltage at which the array produces the most electricity. It can be
seen that as the temperature of the array rises, the MPP voltage drops as does the value of
the max. power. The power and efficiency respectively vary from 511W and 17.3% at 0°C, to
428W and 14.5% at 75°C.
For this array it is assumed that the cell temperature will always be 25°C. Figures 5.10 and
5.11 show the how the I-V and P-V curves for the array vary throughout a typical day in May
as the sun moves across the sky assuming a constant cell temperature of 25°C. The MPP
voltage does not vary as it does when the temperature changes. The power being produced
by the array varies but this is because the solar energy falling on it varies. The MPP voltage
and the efficiency remain constant at 207V and 16.5% respectively.
0 50 100 150 200 250 3000
100
200
300
400
500
600P-V Curves @ S = 462.7W/m2
Voltage (V)
Pow
er
(P)
T=0°C
T=25°C
T=50°C
T=75°C
23
Figure 5.10 – Array I-V curve at varying irradiations
Figure 5.11 – Array P-V curve at varying irradiations
0 50 100 150 200 250 3000
0.5
1
1.5
2
2.5
3
3.5I-V Curves @ T=25°C
Voltage (V)
Curr
ent
(A)
9am-10am
10am-11am
11am-12pm
12pm-1pm
1pm-2pm
2pm-3pm
3pm-4pm
4pm-5pm
0 50 100 150 200 250 3000
100
200
300
400
500
600
700P-V Curves @ T=25°C
Voltage (V)
Pow
er
(P)
9am-10am
10am-11am
11am-12pm
12pm-1pm
1pm-2pm
2pm-3pm
3pm-4pm
4pm-5pm
24
As can clearly be seen, there are significant power reductions when the voltage is not at the
MPP, especially as the voltage moves towards the open circuit voltage. If the array was
connected directly to a load, the module’s voltage would be pulled to the load’s operating
voltage. Therefore, unless the load voltage matches the MPP voltage of the module, there
will be efficiency losses.
Clearly it is desirable to keep the PV module operating at its MPP at all times. This is
achieved by using maximum power point tracking (MPPT). MPPT keeps the array voltage at
the MPP voltage all the times. This is usually achieved by having a DC-DC converter between
the array and the load and varying the duty cycle to match the load impedance with the
MPP impedance.
25
6. DC-DC Converters [8], [9]
A DC-DC converter is a device that takes in one voltage at the input and converts it to a
different voltage at the output. Linear converters can sometimes be useful as they are
relatively simple and cheap to build. They generally consist of some kind of voltage divider
to obtain the desired output voltage. However, the output voltage can only be lower than
the input voltage meaning that they are unsuitable for boost applications. Because the
“extra” voltage is simply dropped across a resistor there can be large heat losses which
means lower efficiency. Thermal management can also be an issue because of this.
Switch mode converters on the other hand use power switching techniques to achieve the
DC-DC conversion. They are more costly and complex to build than linear converters but
have much greater efficiencies and can be used in boost applications. They use inductors
and capacitors to store energy for small periods of time. They also have switches to regulate
the output voltage. There are three main types of switch mode converters, boost
converters, buck converters, and buck-boost converters. Buck and Boost converters are
looked at in more detail below.
6.1. Basic Switching Converter
Consider the diagram in figure 6.1 of a basic switching converter.
Figure 6.1 – Basic Switching Converter
When the switch is closed, . When the switch is open, . The duty cycle, D,
represents the percentage of the period, T, that the switch is closed. Figure 6.2 shows the
output voltage, , as a function of time.
26
Figure 6.2 – Output voltage as function of time
It is obvious that is a function of and D. In the case of this simple switching converter,
.
6.2. Pulse Width Modulation (PWM)
The switch in a switching converter is normally controlled by pulse width modulation
(PWM). Figure 6.3 shows how PWM generates the duty cycle.
Figure 6.3 – Pulse Width Modulation (PWM)
A triangular sawtooth wave, Vsaw, is compared with a reference voltage, Vref. If Vref is
bigger than Vsaw then the switch is closed, while if Vref is smaller than Vsaw then the
switch is open. Therefore the duty cycle of a DC-DC converter can be varied by varying Vref.
27
6.3. Buck Converter
In a buck converter, or step-down converter, the input voltage is stepped down to a lower
output voltage where
and
It is similar to the basic switching converter discussed previously but an LC low pass filter is
inserted after the switch. This produces a more steady DC voltage and current at the output.
The converter operates in mode 1 when the switch is closed and mode 2 when the switch is
open. Figure 6.4 shows (a) the circuit diagram of a Buck Converter, (b) the equivalent circuit
for mode 1 (switch closed) and (c) the equivalent circuit for mode 2 (switch open).
Figure 6.4 – Buck Converter
28
In mode 1 the switch is a short circuit and the diode is an open circuit. The inductor current
equals the input current. This current increases linearly but is prevented from changing very
quickly by the inductor. When the current through any inductor is changing, a voltage is
induced across the inductor that opposes this change in current. The inductor “wants” the
current through it to remain constant. This can be seen from the equation
Where L is the inductance of the inductor and
is the rate of change of the inductor
current. So in simple terms, the more the current tries to change, the more the inductor will
try to resist it. The output voltage is .
In mode 2 the switch is an open circuit and the diode is a short circuit. The inductor now
supplies the current to the output. The inductor current decreases linearly but again the
inductor prevents it from changing very quickly as it “wants” a constant current. The polarity
of the inductor’s voltage reverses as it now attempts to stop the current decreasing. The
output voltage is .
Figure 6.5 shows (a) the inductor voltage and (b) the inductor current.
Figure 6.5 – (a) Inductor voltage and (b) inductor current
29
6.4. Boost Converter
In a boost converter the input voltage is boosted up to a higher output voltage, where
( )
and
( )
For charging EVs from the PV array sized in the previous section, this is the converter that
would be needed as the output voltage from the PV array will always be less than the
voltage required to charge an EV. The same components are used in a boost converter as in
a buck converter but they are arranged differently. Figure 6.6 shows (a) the circuit for a
boost converter, (b) the equivalent circuit for mode 1 (switch closed) and (c) the equivalent
circuit for mode 2 (switch open).
Figure 6.6 – Boost Converter
30
In mode 1 the switch is a short circuit and the diode is an open circuit. The circuit is
essentially split into two separate circuits. On the left, and the inductor current
equals the input current. The current increases linearly. On the right, the load is sustained
by the capacitor which discharges keeping the output current constant.
In mode 2 the switch is an open circuit and the diode is a short circuit. The inductor current
decreases linearly. . However, , will be negative as it is acting against the
current decreasing and therefore will be greater than .
Figure 6.7 shows (a) the inductor voltage, (b) inductor current, (c) diode current, and (d)
capacitor current. In (d), R is the equivalent resistance of the load.
Figure 6.7 – Boost converter waveforms
31
7. Lithium-Ion Batteries
7.1. Lithium Ion Technology
The cathode in this kind of battery is a lithiated metal oxide (LiCoO2, LiMO2, LiNiO2 etc.)
and the anode is made of graphitic carbon with a layering structure. The electrolyte is made
up of lithium salts (such as LiPF6) dissolved in organic carbonates. When the battery is
charged, the lithium atoms in the cathode become ions and migrate through the electrolyte
toward the carbon anode where they combine with external electrons and are deposited
between the carbon layers as lithium atoms. This process is reversed during the discharge
process [16]. Figure 7.1 shows this process [17].
Figure 7.1 – Common li-ion battery [17]
Lithium Ion (li-ion) batteries were first proposed in the 1960’s but it was not until 1990 that
the first commercial li-ion batteries were produced by Sony. Since then, improved material
developments have led to vast improvements in terms of the energy density (increasing
from 75 to 200 Wh/kg) and cycle life (increased to as high as 10,000 cycles). The coulombic
efficiency of Li-ion batteries is almost 100% (i.e charge in/charge out) – another important
advantage over other batteries [18]. They are now seen as the power source of choice for
sustainable transport [17].
7.2. Charging
Ideally Li-ion cells should be charged with a specific charging profile. Chargers that can
control the battery voltage and current during charging are commonly used to do this. A li-
32
ion cell is fully discharged when it reaches 3V. It is fully charged when it reaches 4.2V. It is
possible to discharge to approx. 2.5V however this is not recommended as it can reduce
lifetime and capacity [19]. Figure 7.2 shows a Constant-Current Constant-Voltage (CCCV)
charge profile for a li-ion battery [4].
Figure 7.2 – Li-ion battery charge profile [4]
If the cell voltage is below a threshold voltage, usually 3V, it should be trickle charged at a
constant current, typically 0.1C, until it reaches the threshold voltage. When the cell reaches
the threshold voltage, constant current charging commences. This is typically at a rate of
0.5C-1C. Charging above 1C can be detrimental to the cell. A charge rate of 1C means that if
the battery was supplied with that current for 1 hour, it would be fully charged. So for a
2.2Ah li-ion cell, a charge rate of 1C is equal to 2.2A while a charge rate of 0.5C is equal to
1.1A.
Once the cell reaches its maximum voltage, usually 4.2V, it enters constant voltage charging.
The voltage is held constant while the current tapers off. Once the current drops below a
specified value, usually 0.02C-0.05C, the charge terminates. Li-ion cells are very sensitive to
overcharging and therefore it is imperative that the voltage does not exceed 4.2V. There is a
substantial fire hazard if li-ion cells are overcharged. Typically voltage regulation of 1% is
used [19].
7.3. Demonstration System Li-ion Cell
For the demonstration system a 1Ah 3.8V li-ion cell will take the place of the Nissan Leaf
Battery. A test charge was carried out on this cell to measure the actual charge into it. For
33
the test charge, a charger from Sparkfun that uses the MCP73843 linear IC from Microchip
[20] was used to charge the cell. The charge controller charges the cell at a constant current
determined by the sense resistor (1A in this case) and then at a constant voltage of 4.2V.
Figure 7.3 shows the charger circuit.
Figure 7.3 – Li-ion charger
To monitor the test charge, an arduino was used to read voltage and current values. The
voltage into the charger and the cell voltage were measured on the analog input pins of the
arduino while the current was measured using the ACS712 current sensor from Allegro. The
current sensor outputs a voltage that is proportional to the current going through it and this
voltage was then measured on one of the analog input pins of the arduino. The author used
an open-source software called gobetwino to log the voltage and current measurements
from the arduino to an excel file every 20 seconds. The cell was discharged to 3V using an
8.2Ω power resistor before the test charge. Figure 7.4 shows a block diagram of the set up.
The current drawn by the MCP73843 is usually less than 4mA so this current was neglected.
Therefore it was assumed that the current into the charger was the same as the current into
the cell so only one current sensor was needed.
34
Figure 7.4 – Li-ion cell test charge
Figure 7.5 shows the cell voltage and current measurements throughout the discharge.
Figure 7.5 – Li-ion cell charge voltage and current
The constant voltage stage of the charge isn’t strictly constant voltage as the cell current
starts to decrease at 4.1V and the voltage continues to rise gradually to 4.2V. The two
current drops are most likely interference on the current sensor output.
0 20 40 60 80 100 1200
0.2
0.4
0.6
0.8
1
1.2
1.4
Time (Minutes)
Curr
ent
(A)
0 20 40 60 80 100 1203.6
3.7
3.8
3.9
4
4.1
4.2
4.3
Voltage (
V)
35
The total time taken was 1 hour and 58 minutes. 1,044mAh was delivered to the cell. The
constant current mode current was 0.96A and the cut-off current was 0.05A. The total
power delivered to the cell was 4.24Wh while 4.76Wh was delivered to the charger. This
represents an efficiency of 89%. The battery was then discharged to 3V using the 8.2Ω
power resistor using the arduino to measure voltage and current again. 4.01Wh was the
energy discharged. This represents a charge/discharge efficiency of 95% and an overall
efficiency of 84%. During discharge 1,080mAh was discharged from the cell. It is impossible
to get more charge out during discharge than was put during charging so the discrepancy is
probably down to the battery not having been fully empty before charging or perhaps the
accuracy of the current sensor. However this does show that the coulombic efficiency of li-
ion cells is about 100% as reported by others [2].
7.3.1. Average Charge Voltage
The average charge voltage was calculated based on charge delivered rather than time
spent at that voltage. E.g. the cell spent a long time at 4.1-4.2V but very little charge was
being delivered compared to during the constant current mode phase. For each 20 second
interval, the voltage during that interval was multiplied by the charge added during that
interval. These values were then added together and divided by the total charge added to
the battery, to give an average charge voltage of 4.06V. Figure 7.6 shows a graph of battery
voltage against cumulative charge.
Figure 7.6 – Cell voltage vs cell charge
0 200 400 600 800 1000 12003.2
3.4
3.6
3.8
4
4.2
4.4
Cumulative Charge (mAh)
Cell
Voltage (
V)
36
8. Charging System Design
8.1. Converter Design
Appropriate components had to be chosen for the DC-DC converter that would boost the
array voltage to the battery voltage. The battery voltage range is based on a single li-ion cell.
A li-ion cell voltage can vary from 2.5V when deeply discharged to 4.2V when fully charged.
The Leaf battery pack has 2 rows of cells in parallel, with each row having 96 cells in series.
This means that the minimum battery voltage is
and the maximum battery voltage is
A suitable inductor, MOSFET, diode, input capacitor, and output capacitor had to be chosen.
These components were chosen for high efficiency and to be able to withstand the current
and voltage stresses of the system operating at maximum power. The inductor and
capacitors also had to be chosen to limit current and voltage ripples to specified values. For
a factor of safety, the max. Vin,mas was taken as the open circuit voltage, the max. Iin,max
was taken as the short circuit current and Vout,max was taken as just higher than the fully
charged battery voltage.
Vin,max = 255V
Iin,max = 5.57A
Vout,max = 410V
Iout,max = 3.47A
8.1.1. MOSFET
The MOSFET chosen was the FCA76N60N N-Channel MOSFET from Fairchild [21]. It can
withstand drain to source voltages up to 600V and can handle currents as high as 76A. It
was chosen because it has a low on-resistance, typically only 28mΩ. It has a typical total
turn-on time of 58ns (td(on) + tr) and a typical total turn-off time of 267ns (td(off) + tf). Initially a
synchronous converter (two MOSFETS) was designed in PSPICE and the relatively large turn-
37
off time limited the switching frequency. However the converter was later changed to a
non-synchronous converter (one MOSFET, one diode) so the turn-off time was not a big
issue. This is discussed again later.
8.1.2. Inductor
The inductor is responsible for keeping the current ripple within a pre-defined limit. To
determine the size of the inductor required, the max. allowable current ripple first had to be
chosen. For DC-DC converters, a value of 40% is normal [22]. For a boost converter, this
means that the current ripple should never exceed 40% of the input current. The inductor
should also have as low a DC resistance as possible to minimise losses. The equation for the
inductance required is [22]
where L is the inductance in henries, is the input voltage in volts, D is the duty cycle,
is the current ripple in amps and is equal to 40% of the input current, and f is the frequency
in Hz. is virtually fixed at 207V, the max. duty cycle is 0.486, and the frequency is fixed at
200kHz (justification for this value is in a chapter further on). The lowest current ripple
required is when the input current is lowest, 1.6A (4pm – 5pm). This corresponds to a
current ripple of 0.64A. Subbing these in gives a max. inductance value of
( )( )
( )( )
The author then began searching for a suitable inductor but no inductor could be found that
had a fixed inductance of 786uH over the full current range of the system. However, the
author did find a variable inductor that met the specification of not letting the current ripple
rise above 40% of the input current. The inductance of a variable inductor drops as the
current going through it increases. This is suitable for this application as the required
inductance also drops as the current increases. The inductor chosen was a 1mH TJ9 inductor
from Vishay [23]. It has a DC resistance of 59mΩ. The spec sheet gives the value of
inductance at specific currents, see table 8.1. By assuming a linear inductance curve
between each current value, a curve of Current vs Inductance was calculated. Using these
38
inductance values the % current ripple was then calculated over the entire operating current
range. Figure 8.1 shows the inductance and current ripple plotted against inductor current.
Table 8.1 – Variable Inductor
Current (A) Inductance (uH)
0 998.56
1.91 763.90
3.09 679.02
4.41 594.14
5.74 509.27
Figure 8.1 – Variable Inductance and Current ripple
At 1.6A, the current ripple is 39.2%. The converter remains in continuous conduction mode
(CCM) until the current drops below 0.6A.
8.1.3. Diode
The diode chosen was the UG8JT ultrafast rectifier from Vishay [24]. It can handle a reverse
voltage of 600V and take up to 8A of current.
0 1 2 3 4 5 6400
600
800
1000
Inductor Current (A)
Inducta
nce (
uH
)
0 1 2 3 4 5 60
50
100
150
Curr
ent
Rip
ple
(%
)
39
8.1.4. Capacitors
The output capacitor was chosen so that the output ripple voltage would not exceed ±1% of
the final battery float voltage. The final battery float voltage is 403V so this corresponds to a
max. voltage ripple of 4V. The voltage ripple has two parts, the ripple due to capacitor
charging and discharging, , and the ripple due to the voltage dropped across the
equivalent series resistance (ESR) of the capacitor, . The equations for the two are
[22]
( )
where is the output current, C is the capacitance, is the maximum current in the
inductor, and is the ESR. The EMKP 950-2.2 capacitor from Vishay [25] was chosen as the
output capacitor. It has a capacitance of 2.2uF, an ESR of only 3.1mΩ and it can handle 14A
of current. For a worst case scenario, D is taken as 0.5 and is taken as the short circuit
current, 5.57A. At 5.57A, the current ripple is 17.3%, therefore
( )
The max. voltage ripple is then
which is safely within the 4V limit.
Conditions at the input capacitor will be less stressful than at the output capacitor,
therefore the EMKP 950-2.2 will also be suitable as the input capacitor.
8.2. PSPICE Model
8.2.1. Synchronous Boost Converter Model
Originally a synchronous boost converter was designed in PSPICE. The author believed that
replacing the diode with another MOSFET would avoid the diode losses as MOSFETS have a
much lower on-resistance than diodes. The PSPICE circuit is shown in figure 8.2.
40
Figure 8.2 – Synchronous boost converter
However it quickly became apparent that the second MOSFET was not a good idea.
The efficiency of the system was only about 89%-90%, lower than what was hoped
for.
There were large current spikes (300-400A) around the loop containing the two
MOSFETS and the output capacitor, see figure 8.3. Even leaving the high side mosfet
off so that it was essentially just a diode did not get rid of them.
Because of the turn-off times of the MOSFETs, 40kHz was the highest frequency that
the system could be operated at. The efficiency of the system started to drop
dramatically when this frequency was exceeded. This also meant that a very large
inductor was required.
Figure 8.3 – Large current spikes around high frequency loop
Because of these issues it was decided to replace the high side MOSFET with a diode and
this solved the issues. This also makes things easier for the charge controller as it now only
Time
3.570ms 3.580ms 3.590ms 3.600ms 3.610ms 3.620ms 3.630ms 3.640ms 3.650ms 3.660ms
I(Cout) I(HIGHSWITCH:2)
-250A
0A
250A
-416A
429A
41
has to switch the low side MOSFET. In the synchronous circuit, voltages above 400V would
have had to be applied to switch the high side MOSFET. The switching frequency was then
increased to 200kHz so that a smaller inductor could be used. Increasing the frequency
above 200kHz resulted in significant efficiency losses.
8.2.2. Final PSPICE Model
PSPICE was used to model the PV charging system under varying solar irradiation and load
conditions. Figure 8.4 shows the PSPICE circuit of the final charging system.
Figure 8.4 – Charging System circuit diagram
To model the solar panel, the model uses the Maximum Power Transfer Theorem. This
states that the maximum amount of power is transferred from the source to the load when
the internal impedance (Thevenin/Norton impedance) matches the load impedance seen by
the source. Figure 8.5 shows this [14].
Figure 8.5 - Maximum power transfer for solar cell unit [14]
42
The solar cell is a type of power, and its power transfer changes with solar intensity. Any
changes of solar cell power transfer, also change its voltage and internal impedance [14]. To
determine the Thevenin voltage and impedance of the array for different solar irradiations,
the MPP voltage and current must be known for each solar irradiation condition. and
for every hour from 9am-5pm for a day in May were calculated in chapter 4. According
to the maximum power transfer theorem, when the maximum power is being delivered to
the load, the internal (Thevenin) impedance of the array, , is equal to the load
impedance seen by the array. The load impedance seen by the array at the MPP, , is
and by definition, the array is transferring the maximum power to the load at the MPP,
therefore
If the load impedance seen by the array equals the internal impedance, then the Thevenin
voltage will be twice the MPP voltage,
The duty cycle, D, is determined by the array output voltage and the battery voltage, .
Assuming that the array is operating at the MPP, then re-arranging
( )
gives
As mentioned earlier, is determined by the array temperature while is determined
by the state of charge of the battery.
The output impedance seen by the array, , is related to the actual output impedance of
the load, .
43
( )
but recognising that and the equation becomes
( )
( )
and recognising that
( )
gives
( )
As mentioned earlier, a charge controller is required when charging li-ion batteries. The
charge controller can control the battery voltage, , by varying the duty cycle and it can
also control the charge current, . Because
this means it also has control over the load impedance, . Therefore can be set so that
. Therefore
( )
The equations above all assume ideal components in the boost converter. However in
reality this is not the case. Manufacturer PSPICE models for the MOSFET and DIODE were
used to make the overall model as accurate as possible. To account for the non-ideal
behaviour of the model, a duty cycle coefficient and an output impedance coefficient were
introduced into the model. These can be seen in the duty and Zo expressions in figure 8.4.
The value of the coefficients varied depending on the simulation conditions. For each of the
simulations run, the coefficients were obtained using a method of trial and error until the
44
desired panel and battery voltages were obtained. The duty cycle coefficient varied from
0.825-0.972 while the output impedance coefficient varied from 1.04-1.135. In a real system
the charge controller would do this by constantly monitoring the input and output voltages
and changing the duty cycle or load impedance as required.
For detailed measurements such as voltage ripple, current ripple, power losses and where
they occur etc. the model was run at average conditions. That is, the average solar
irradiation from 9am-5pm and the average battery charge voltage. As with the min. and
max. battery voltages, the average charge voltage was based on the test charge of the single
li-ion cell in chapter 7.3. It was calculated by multiplying the average charge voltage of the
the single li-ion cell by the number of li-ion cells in series in the LEAF battery.
45
8.3. Model Results
8.3.1. System Efficiency and Power Losses
The PSPICE model was run under 40 different test conditions. There are 5 values that must
be input by the user
switching frequency, freq
array voltage, Vmpp
array current, Impp
battery voltage, Vo
inductance of the inductor, L
However, the switching frequency is fixed at 200kHz, and because the array is assumed to
be at a constant 25°C, the array voltage is also fixed at 207V. Also, the inductance of the
inductor is dependant on the array current and the array current is dependant on the solar
irradiation. The excel model described in chapter 5.4 works out the array current for a given
solar irradiation. So essentially there are only two variables, the solar irradiation and the
battery voltage. The model was run under 8 different values of solar irradiation (one for
every hour from 9am-5pm) and 5 different values of battery voltage (240V-400V). Table 8.2
shows the overall system efficiency for each simulation and figures 8.6 and 8.7 display the
information graphically. The curves in figure 8.7 are 2nd order polynomial fits to the data
points.
Table 8.2 – Model Results
Time\Vbat 240V 280V 320V 360V 400V
9am-10am 98.07% 97.92% 97.72% 97.55% 97.33%
10am-11am 98.16% 98.04% 97.90% 97.74% 97.58%
11am-12pm 98.20% 98.10% 97.98% 97.82% 97.64%
12pm-1pm 98.25% 98.14% 98.04% 97.87% 97.70%
1pm-2pm 98.23% 98.11% 98.01% 97.83% 97.69%
2pm-3pm 98.18% 98.04% 97.92% 97.75% 97.57%
3pm-4pm 98.05% 97.91% 97.72% 97.54% 97.33%
4pm-5pm 97.88% 97.76% 97.57% 97.31% 97.06%
46
The efficiency is highest when the battery voltage is closest to the PV voltage and the
system is operating high power, 98.25% from 12pm-1pm with Vbat = 240V. The efficiency
decreases as the battery voltage moves away from the PV voltage and as the power of the
system drops. The lowest efficiency is 97.06% from 4pm-5pm when the battery is
approaching its float voltage. Under average conditions, (S = 462.7W/m^2, Vbat = 390V) the
PV to battery efficiency was 97.65%. The overall solar to battery efficiency is the PV
efficiency multiplied by the PV to battery efficiency. For average conditions it is 16.1%.
The PV to battery efficiency is considerably higher than a standard grid tied system where
the inverter efficiency is 93-97% and the charge controller/rectifier efficiency is 97% [2].
Combining these two efficiencies gives a PV to battery charge efficiency of 88-94% for a
DC/AC/DC system compared to 97-98% for the DC/DC system above.
Figure 8.6 – System Efficiency throughout a typical day in May
9am-10am 10am-11am 11am-12pm 12pm-1pm 1pm-2pm 2pm-3pm 3pm-4pm 4pm-5pm97
97.5
98
98.5
Time of Day
Charg
ing S
yste
m E
ffic
iency (
%)
Vbat = 240V
Vbat = 280V
Vbat = 320V
Vbat = 360V
Vbat = 400V
47
Figure 8.7 – System efficiency vs the output power of the PV array
A breakdown of the converter losses were measured from the average conditions
simulation (S = 462.7W/m^2, Vbat = 390V). Table 8.3 and figure 8.8 show where the losses
occur. The total loss is 11.54W, with the MOSFET and switch accounting for over 97% of this
figure. The other 3% is mostly the inductor DC resistance while the capacitor ESR losses
don’t even register on the graph.
Table 8.3 – Converter Losses
Component Losses (W)
Switch 6.39
Diode 4.82
inductor 0.33
Cout 0.005
Cin 0.0001
Total 11.54
300 350 400 450 500 550 600 65097
97.5
98
98.5
PV Output Power (W)
Syste
m E
ffic
iency (
%)
Vbat = 240V
Vbat = 280V
Vbat = 320V
Vbat = 360V
Vbat = 400V
48
Figure 8.8 – Breakdown of converter losses
8.3.2. Boost Converter Waveforms
The voltage and current waveforms from various parts of the boost converter are shown in
figure 8.9. The waveforms are very similar to the ideal boost converter waveforms in
chapter 6.4. The PV voltage and battery voltage ripples respectively are 0.2V and 1.35V. The
voltage across the inductor ranges from 207V (PV voltage) to -183V (PV voltage – battery
voltage). The inductor current ripple is 0.693A (29.5%). Every time the diode switches off
there is a reverse current spike of approx. 8A through it. This current goes around the high
frequency loop containing the diode, the MOSFET and the output capacitor. This is
acceptable as all the components are rated to handle that size current spikes. It is
significantly less than the current spikes of 300-400A that were occurring in the synchronous
boost converter.
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
Switch Diode inductor Cout Cin Total
Po
we
r (W
) Charging System Losses
49
PV voltage (Input voltage) Battery voltage (Output voltage)
Inductor voltage Diode current
Inductor current Output capacitor current
Figure 8.9 – Boost Converter Waveforms from PSPICE
5 6 7 8 9 10 11 12 13 14206
206.2
206.4
206.6
206.8
207
207.2
207.4
207.6
207.8
208
Time (us)
PV
Voltage (
V)
5 6 7 8 9 10 11 12 13 14386
387
388
389
390
391
392
393
394
Time (us)
Batt
ery
Voltage (
V)
5 6 7 8 9 10 11 12 13 14-200
-150
-100
-50
0
50
100
150
200
250
Time (us)
Inducto
r V
oltage (
V)
5 6 7 8 9 10 11 12 13 14-8
-6
-4
-2
0
2
4
Time (us)
Dio
de C
urr
ent
(A)
5 6 7 8 9 10 11 12 13 140
0.5
1
1.5
2
2.5
3
3.5
4
Time (us)
Inducto
r C
urr
ent
(A)
5 6 7 8 9 10 11 12 13 14-10
-8
-6
-4
-2
0
2
4
Time (us)
Outp
ut
Capacitor
Curr
ent
(A)
50
9. Demonstration System
The demonstration system is essentially a scaled down version of the full scale charging
system. A 10W 16V Spectra solar panel from Radionics [26] was used to charge a single 3.7V
1Ah li-ion cell from sparkfun [27]. At first the system was going to use the li-polymer
charger that was used for the charge tests. A buck converter controlled by an arduino was
going to convert the 16V panel voltage to the 5V input required for the charger. Figure 9.1
shows the original system. However this system essentially consists of two DC-DC
conversion stages when it would be possible to have a more efficient system by having the
DC-DC converter as the charge controller. Therefore that design was abandoned and the
author began searching for a way to combine the buck converter and charge controller into
one.
10W PV Panel
Buck Converter
Li-ion Charger
Li-ion Cell
Voltage & Current Sensing
Arduino
Voltage & Current Sensing
Voltage & Current Sensing
≈16V ≈5V
PWM
Li-ion Charger
Li-ion Cell
Voltage & Current Sensing
Switch
3V - 4.2V
Figure 9.1 – Original System Layout
The bq24650 chip from Texas Instruments [28] was identified as an ideal candidate. This
chip is a synchronous switch-mode battery charge controller for solar powered applications.
It controls an external buck converter by controlling the duty cycle of the two external
mosfets. It operates at a fixed frequency of 600kHz. It implements a constant-current
constant-voltage charging algorithm. If the battery voltage is below 3V, it provides it with a
trickle charge until it reaches 3V. The battery float voltage is set with a resistor divider at the
output and the charge current is set with a sense resistor. The chip also provides a feature
51
for tracking the maximum power point (MPP) of a solar panel. The chip reads in the input
voltage through a resistor divider at the input. If the circuit is trying to draw more power
than the panel can deliver and the voltage in drops below a certain voltage (the MPP
voltage), the chip will reduce the charge current to keep the panel operating at the MPP
voltage.
Components were then chosen for the system. Two Si7288 N-Channel MOSFETS from Vishay
[29] were chosen as the external switches. They have a low on-resistance ( 20mΩ) and
were recommended in the bq24650 datasheet. The max. charge current of the system was
chosen as 1A, therefore a 40mΩ sense resistor was required. This corresponds to a 1C
charge rate for the li-ion cell. For 1A charging the datasheet recommends using a 15uH
inductor and a 10uF capacitor. Using the equations for a buck converter [22] the current
ripple, , and voltage ripple, , were calculated for those values.
( )
where is the output voltage, D is the duty cycle, L is the inductance, and f is the
frequency. Subbing in 4.2V for and (4.2/16)=0.263 for D gives a current ripple of
This is a 34% current ripple at 1A which is acceptable [22]. The chip operates in
discontinuous conduction mode (DCM) towards the end of the charge when current drops
as low as 0.1A.
( )
where C is the output capacitance. Using the same and D values as above, this gives
which represents a ±0.1% voltage ripple at 4.2V. In general li-ion batteries should be
charged to ±1% of their full charge voltage [19]. Even allowing for the voltage ripple caused
by the equivalent series resistance (ESR) of the capacitor, the voltage ripple will be well
within the ±1% limit. A 15uH inductor from Wurth [30] with a low DC resistance ( 9mΩ)
52
was ordered, see figure 9.2, and a low ESR output capacitor was gotten in the Engineering
building project lab.
Figure 9.2 – 15uH Wurth Inductor
Figure 9.3 shows a typical application of the chip. The chip was only available in a 16 pin,
3.5×3.5 mm2 thin QFN package, see figure 9.4. This is too small to solder in the lab so a 16
pin QFN test was also gotten. This would make the 16 pins accessible. However, the chip
would not connect properly to the test socket and the whole design had to be abandoned.
Figure 9.3 – bq24650 typical application
Figure 9.4 – QFN package
In place of the bq24650, the author instead used the LT3652 chip from Linear Technology
[31]. The LT3652 is available in a 12 pin MSOP package. It was possible to solder this type of
package onto a PCB. This is also a buck converter and charge controller but there are a few
key differences. It is not a synchronous buck converter (i.e. a schottky diode is used in place
of the high side mosfet) and it has a switched frequency of 1MHz. An internal transistor is
53
used as the switch with an on-resistance of approx. 0.175Ω. The battery voltage and current
is controlled the same way except that a 100mΩ sense resistor is required for 1A charging.
The PV voltage is also monitored the same as the bq24650. It reduces the charging current if
the panel is not producing enough power to charge the battery at 1A. A smaller inductor
would have been sufficient for the LT3652 because it switches at a higher frequency,
however the author already had the 15uH Wurth inductor so this was used. The LT3652
datasheet specifies that a 10uF ceramic output capacitor should be used for all applications.
The SB320 3A schottky diode from Vishay [32] was chosen as the diode. Figure 9.5 shows
the circuit diagram of the final design.
Figure 9.5 – Demonstration System Circuit Diagram
The exposed thermal pad on the back of the chip is also the ground connection for it. The
section of the circuit enclosed by the red line is the high frequency part of the circuit. This
was kept separate from the rest of the circuit with its own ground to minimise interference
with the rest of the circuit. The two grounds were only connected in one place. Figure 9.6
shows the initial circuit board layout.
54
Figure 9.6 - Initial circuit board layout
A DC power source from the lab was used as the power source. The green and yellow LEDs
are charge indicators. When neither is on, the charger is in sleep mode. When the green one
is on by itself the battery is charging normally. The yellow LED indicates faults. When the
power was switched on, the green LED came on, indicating that the battery was charging. A
hall effect current probe connected to an oscilloscope was used to measure the current into
the battery. Only about 200mA was going into the battery instead of the 1A that was meant
to. The input voltage was varied from 7V up to 20V but it did not make any difference. The
author suspected that there was noise coming from the high frequency part of the circuit
and that it was interfering with the readings on one or all of the pins that control charging.
Therefore a ground plane between the high frequency part and the rest of the circuit was
introduced to try and isolate the high frequency part. This was done by having the high
frequency part on one piece of circuit board, the rest of the circuit on another circuit board,
and another circuit board between them with every line grounded. This was meant to shield
the rest of the circuit from the high frequency noise. Figure 9.7 shows the set up.
The ground plane by itself made no noticeable difference in the performance of the charger.
It was then decided to fully surround the high frequency part with something metal that was
grounded. A small metallic box in the project lab was ideal. Figure 9.8 shows the circuit with
the high frequency part completely encased.
55
Figure 9.7 – High frequency part isolated from rest of circuit using a ground plane
Figure 9.8 – High frequency part encased by grounded metallic box and ground plane
The metallic box make a big difference, although it did not solve every problem. With the
input voltage at 10V or below, approx. 0.95A was being delivered to the battery. At 9V there
was 500mA being drawn from the power source. However as the input voltage was
increased above 10V, the charge current dropped. At 16V in (the operating voltage of the PV
panel), the charger was only drawing 120mA from the power source and delivering about
400mA to the battery. Table 9.1 shows the two scenarios.
Table 9.1 - Charger operating at different input voltages
Vin Iin Vout Iout Pin Pout efficiency
9V 500mA 4V 950mA 4.5W 3.8W 84.4%
16V 120mA 3.9V 370mA 1.92W 1.44W 75.2%
56
As can be seen, both the charging current and the efficiency of the charger decrease as the
voltage goes above 10V.
The PV panel was tested with two halogen lights and different configurations of resistors
but the panel did not output any significant power. Therefore the only way to test it was in
sunlight. The module was tested outside on the 27/03/13. An oscilloscope with an internal
battery pack was used to measure the voltages and currents. Figure 9.9 shows the charging
system being set up outside the Engineering building.
Figure 9.9 – Solar charging outside using 10W Spectra PV panel
As mentioned previously, the LT3652 can be programmed with a resistor divider to decrease
the charge current if the PV panel voltage drops below the programmed value. The
operating voltage of the panel is 16V so therefore the resistor divider was set to not let the
PV voltage drop below 16V.
Table 9.2 – PV Charging of Li-ion Cell
Vin Iin Vout Iout Pin Pout efficiency
16.5V 120mA 4V 370mA 1.98W 1.48W 74.7%
The demonstration system showed the concept of charging li-ion batteries with solar power
although the efficiency of the system and the charge rate with the PV panel as the power
source were lower than what was hoped for. However, when supplied with an input voltage
of 10V or below, the system was capable of charging a li-ion cell at 0.95A with an efficiency
of almost 85%.
57
10. Conclusion
Solar energy can provide a clean, renewable source of electrical energy to charge electric
vehicle batteries. Moreover, because solar electricity and battery electricity are both DC,
there is no need for any AC electricity stages between them. This means that the financial
costs and efficiency losses associated with inverters and rectifiers are avoided.
This project details the design of a MPPT DC/DC Converter capable of charging a Nissan Leaf
from a PV array. The system provides enough of a charge for a daily commute from the
Engineering building in NUI Galway to one of the towns or villages outside Galway city. The
converter keeps the PV array operating at its MPP by matching the battery impedance to
the internal impedance of the PV array. The system was modelled in PSPICE and the PV to
battery efficiency varied from 97.1% - 98.3%. This is significantly better than a grid tied
DC/AC/DC system where the PV to battery efficiency varies from 88% - 94%.
A small scale demonstration system was built that successfully charges a single li-ion cell
using a 10W PV panel.
With petroleum prices continuously rising and world reserves of fossil fuels running out, an
alternative form of energy is needed for the transportation industry. At the moment electric
vehicle technology is best placed to replace petroleum. Therefore efficient charging
strategies will be required to meet demand. This project suggests that a DC/DC system
connecting a PV array to a Nissan Leaf is a viable and efficient way of charging a Nissan Leaf.
58
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