project12 design review

8/3/2019 Project12 Design Review

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Intelligent Battery Charger

Design Review

Kevin Happ, Sharat Tiruveedhula

TA: Xiangyu Ding

September 28, 2010


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I. Introduction:

Title: Intelligent Battery Charger

The consumer electronics we use in our day-to-day lives utilize a wide range of battery types.

The average person needs a variety of battery chargers to accommodate their personal needs.Instead of having an individual battery charger for an iPod, car battery, and other commonhousehold batteries like NiMH and NiCd, we feel it would be useful to have a single device tocharge each battery. The charger will break the charging cycle for each battery into separatestages, in order to ensure that each battery is charged as efficiently as possible. Chargetermination technique will be implemented to ensure that the batteries are not supplied with over-voltages and stay within their recommended temperature ranges.

Objectives:

Project Goals:

Implement AC to DC conversion Develop constant current and constant voltage sources to supply batteries Supply batteries with current/voltage very close to the ideal input for each stage Digital display of analog parameters Efficiently charge different types of batteries

Customer Benefits: Convenience and portability Prolong battery life due to intelligent battery charging User can monitor charging status, i.e. time left until fully-charged and present voltage

Features: Charges lead-acid car battery, Li-ion iPod battery, AAA Ni-Cd, and AA Ni-Mh Displays status of battery and shuts off automatically when fully charged Adjusts method of charging, intelligently, based on current state of the battery Temperature-triggered charge-termination to protect battery


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II. Design:

1. Block Diagram

2. Block Descriptions

AC-DC Converter: Since the output supply is AC, we need to convert to DC, in order to chargethe battery. We use a transformer to step the voltage down from 120V RMS (supplied from theoutlet) to a voltage more appropriate for charging our batteries. The lowered voltage is then sentthrough a full-wave bridge rectifier to create an (albeit poor) DC voltage source. Additionalcapacitor filtering is performed to smooth the DC signal and give us a source with very smallripple.

Voltage/Current Regulator: Regulate the voltage and current depending on the type of batteryand stage in the charging process. The constant voltage output is forced by the rating of the zenerdiode regulator, which follows the AC-DC conversion. For certain stages of the chargingprocesses, the batteries require a constant current, variable voltage source, which is effectedthrough additional circuitry. This additional circuitry is an Op-Amp/BJT feedback controlledcurrent source, with the current level being forced by the value of the sense resistor.

Control Circuit: Information is relayed to the PIC, which then outputs signals to the display andbattery-charging system. These signals determine which battery should be charged, whether thebattery should be charged with constant current or constant voltage, and the level of thiscurrent/voltage. Based on the information relayed to the PIC through voltage and temperaturemeasurements, it will determine the state of the battery, which will then be displayed in a userfriendly manner to give the user valuable updates on the charging process. Signals will berelayed to terminate the charging process when certain voltage/temperature thresholds arereached.

AC-DC

Converter

Voltage/CurrentRegulator

Control Circuit Battery ChargingCircuit

Display


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Battery Charging Circuit: Information will be received from the control circuit that willdetermine how the battery is charged. Separate circuits will charge each individual battery.Switches will be introduced to ensure that, based on the PIC signals, the correct battery chargingcircuit is selected and the correct sub-circuits are then selected. This will ensure that only one

battery is charged at a time.

Display: Digitally displays the voltage across the battery and indicates the time left needed tofully charge the battery. An LED turns on when the battery is fully charged.

III. Schematics and Calculations

Model for 126Vrms to 30Vrms (single-ended secondary) transformer:

, where N1 and N2 are respectively the number of turns on the primary and

secondary sides of the transformer. V1 and V2 represent the voltages on the primary andsecondary.

In the above model, resistors R1, R2, and R3 have no physical equivalent; their sole purpose is toaid in simulating the effects of the transformer in PSpice. The terminals of R3 will, however be

connected to the rectifying circuit.

, where L is the inductance of each winding.

Voltage is stepped down from 126 Vrms to ~28 Vrms:


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Design for Constant Voltage Source:


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Iz : current through zener diode (D6)

Vz : current across zener diode (D6). This is a characteristic of the diode that will bematched with the voltage demanded across the battery.

C : capacitance of capacitor (C1)

Vd = ~0.7 V : Voltage drop across each diode in the bridge rectifier.

: For each half period of the Vsin input, only two of the fourdiodes will be conducting. Therefore the entire voltage drop across the rectifier will be2Vd.

: Equation to determine the resistance Rs, based on the target

output voltage for the battery (Vz). We determine the minimum and maximum values for theoperating range of the current through the zener diode in order to keep it operating in the linearregion and to stop it from dissipating too much power. While the polarities were reversed, thegraph below illustrates how Iz,min and Iz,max are found for each zener diode.


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Constant Current Source Schematic:


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Ic = ~ Ie : The current through the emitter and collector are essentially the same while thetransistor is conducting.

Vin : Voltage at the positive terminal of the Op-Amp

Vsense : Voltage at the negative terminal of the Op-Amp

The Op-Amp automatically readjusts via feedback so that Vin = Vsense. If thecircuit is supplied with a constant DC voltage for Vin, we can say that

, where Rsense is R4 in the above schematic. With a constant Vin,Rsense will obviously be very important in determining the current Ie, andtherefore Ic. We will hook up the circuit so that Ic is a constant current supplying

the battery.

IV. Charging Algorithms

Lead-Acid:

(Buchmann, 2003)


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As shown by the above diagram, the Lead-Acid car battery will go through three differentcharging stages:

1) Constant current @ 0.1 C. We plan to use a C=70 Ah battery, which would equate to acharging rate of I = 7 A. During this stage, the voltage across each cell will steadily rise,

until reaching a threshold voltage of 2.4V per cell (14.4 V total).2) Constant voltage @ 2.4 V. Using the constant voltage power supply, we supply 2.4 V tothe battery as the current steady decreases. Eventually the current reaches 3% of the ratedcurrent, indicating that the battery is fully charged.

3) Float charge of 2.25 V. The float charge can be applied for an unlimited duration and isused to top off the battery and account for self-discharge.

Lithium-ion iPod battery:

(Apple, Inc.)

The iPod battery has a nominal voltage of 4.2 V. We will be testing on the 3 rd generation nano,which has a capacity of C= 350mAh.

Charging stages:

1) If the cell voltage is below 2.8V, the battery is charged at a constant current of 0.1C =35mA.

2) Once the threshold of 2.8 V per cell has been reached, the current is increased to close to,but not exceeding, 1C = 350mA.

3) Once the 4.2 V per cell threshold has been reached, we switch to a trickle charge atconstant V=4.2 V.


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NiCd:

We will use a C=300mAh battery, most likely a Dantona SS-3P. Ni-Cd batteries are hard to comeby and have been discontinued by most major battery manufacturers.

Charging Stages:

1) Charge at a rate of 1C = 300mA2) Charge at a rate of C/10 = 30mA for a trickle-charge

NiMH:

We will most likely use a Rayovac NM715 with C=2100mAh.

Charging stages:

1) Fast charge at a rate of 1C= 2.1A2) After reaching V=1.0V, start charging at I=C/10= 0.21A for 30 minutes.3) Charge at I= 1/300C = 7mA for unlimited duration

V. Performance Requirement

Fully charge the battery without overcharging and come within 0.1 volts of the followingvoltage limits for each type of battery:Car battery (Lead-Acid) 13.8 14.1V (six cell)iPod battery (Lithium Ion) 3.7 V (single cell)Ni-Mh AA battery 1.4 1.6V [single cell]Ni-Cd AAA battery 1.2V [single cell]

Adjust the method of charging based on the current state of the battery Display time remaining calculated accurately within 5 minutes Intelligently charge batteries to for longer-lasting battery life Keep voltage ripple to less than 6% of nominal voltage for voltage source Accurately use temperature differentials as a charge termination technique

III. Verification

1. Testing Procedures

PSpice simulations: once the input voltage and current are determined, a theoretical AC-DC circuit can be designed in PSpice and simulated before purchasing the physical components.It is much quicker to test the circuit on PSpice than testing with real circuit components, and


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provides us with an estimation of the values of the individual components.

Test to see how circuits respond to a range of inputs. We will vary the input voltage by asmall margin (say that it will be 115 Vrms) to see if the circuit still functions properly.

Battery charge circuit will be assembled and tested. Overcharge prevention capability ofthe circuit will be checked for proper functionality.

The Control Circuits battery state detection can be verified by using a voltmeter betweenthe input terminals to read the voltage, which will determine whether the battery is dead, fullycharged or needs charging.

The quickest charge will not always be the most effective; therefore we should runseveral tests to see how our battery charging method affects the percentage of charge stored.

We will verify through temperature readings that the batteries remain within theirrecommended temperature range throughout the charging process.

2. Tolerance Analysis

The most important part of our project is the Control Circuit. The Control Circuit will determinewhat voltage and current should be supplied to the battery charging circuit depending on whatthe state of the battery is. This is very important because inaccurate detection will lead toinefficient charging or overcharging.

To test the functionality of the Control Circuit under extreme conditions, we will test eachbattery in its fully charged, partially charged (more of a range of possible voltages), and deadbatteries. Basically, we will discharge the given batteries to the appropriate levels to see how ourControl Circuit interprets the current state of the battery and relays this information to the othersub-components. In particular, we could foresee having problems with the Ni-Cd battery, as Ni-Cd batteries have a fairly constant voltage. This would make it difficult to discern the state of thebattery only with the voltage across the battery terminals.


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IV. Cost Analysis

LABOR

Name Hourly Rate Multiplier Total Hours Labor TotalKevin Happ $40.00 2.5 140 $14,000.00

Sharat Tiruveedhula $40.00 2.5 140 $14,000.00

Grand Total $28,000.00

PARTS

Item Quantity Cost

Transformers 4 $30

Battery Charger Parts N/A $40.00

Resistors N/A $5

Zener Diodes N/A $3

General Diodes N/A $5

Lead-Acid Battery 1 $50

AAA NiCd 4 $13

AA NiMH 4 $14

PIC 16F877A 1 $7.66

LEDs 5 $10.00

Filter capacitors 4 $4

Op-Amps N/A $4

Npn BJT N/A $2

TOTAL $227.66

Grand Total = Total Labor + Total Parts = $28,227.66


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2. Schedule

Week Task Responsibility

09/27/10 Order Parts Kevin

Start coding on PIC Sharat

10/04/10 Optimize design Kevin

Figure out implementation oftemperature/voltage sensing and PICinterface

Sharat

10/11/10 Construct Voltage Regulator Circuit Kevin

Construct AC-DC Circuit Sharat

Construct Control Circuit Kevin

Construct Battery Charging Circuit Sharat

10/18/10 Write code for PIC Sharat

Test all circuits individually Kevin

10/25/10 Individual Progress Reports Kevin, Sharat

Review/Enhance circuits or fixproblems

Sharat

Implement digital display Kevin11/01/10 PCB Sharat

11/08/10 Prepare for Demo Kevin, Sharat

11/15/10 Prepare for Demo, Begin FinalReports

Kevin, Sharat

11/29/10 Demo Kevin, Sharat

References:

1) I. Buchmann, " Charging nickel-based batteries," 04/2003.http://www.batteryuniversity.com/partone-11.htm. [Accessed 9/26/2010]

2) http://www.apple.com/batteries/

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