Solar Powered Battery Solar Powered Battery ChargerCharger

Christine PlacekChristine Placek

Philip GonskiPhilip GonskiGroup 4Group 4

ECE 445 – Spring 2007ECE 445 – Spring 2007

ObjectivesObjectives

Operate in various sunlight conditionsOperate in various sunlight conditions Sunny and cloudy daysSunny and cloudy days

Operate for a wide variety of input Operate for a wide variety of input voltagesvoltages 6.8V to 13.5V6.8V to 13.5V

Charge both lithium and nickel chemistriesCharge both lithium and nickel chemistries

Common Usage: Cell phones, MP3 players, other portable electronics

Energy/weight 160 Wh/kgEnergy/size 270 Wh/LPower/weight 1800 W/kgCharge/discharge efficiency 99.9%Self-discharge rate 5%-10%/monthCycle durability 1200 cyclesNominal Cell Voltage 3.6 V

Li1/2CoO2 + Li1/2C6 ↔ C6 + LiCoO2

Lithium Ion Battery

Source: Wikipedia

Common Usage: widely used in small electronic devices

Energy/weight 40-60 Wh/kgEnergy/size 50-150 Wh/LPower/weight 150W/kgCharge/discharge efficiency 70%-90%Self-discharge rate 10%/monthCycle durability 2000 cyclesNominal Cell Voltage 1.2 V

Nickel-Cadmium Batteries

2 NiO(OH) + Cd + 2 H2O ↔ 2 Ni(OH)2 + Cd(OH)2

Source: Wikipedia

Charging Lithium BatteriesCharging Lithium BatteriesVery Strict Regulation of Current vs. Very Strict Regulation of Current vs. VoltageVoltageCharge Voltage at 4.2V or 4.1V most Charge Voltage at 4.2V or 4.1V most commoncommon Our batteries charge at 4.1V to prolong lifetimeOur batteries charge at 4.1V to prolong lifetime

Many Built-in safety precautionsMany Built-in safety precautions Temperature, low voltageTemperature, low voltage

Charge time ~3 hours at 500maCharge time ~3 hours at 500ma Fast chargers skip stage 2 in next slideFast chargers skip stage 2 in next slide

70% charged70% charged

http://www.batteryuniversity.com/partone-12.htm

GLED ON

Charging NiCd BatteriesCharging NiCd Batteries

Three types of chargingThree types of charging Slow(.1C->14 to 16hrs)Slow(.1C->14 to 16hrs)

70% efficient70% efficient Quick(.5C->3 to 6hrs)Quick(.5C->3 to 6hrs) Fast(Full charge with topping->1hr)Fast(Full charge with topping->1hr)

90% efficient90% efficient

Pressure and Temperature increasePressure and Temperature increaseFull Charge detected by:Full Charge detected by: Voltage DropVoltage Drop Rate of temperature increaseRate of temperature increase Timeout timersTimeout timers

http://www.batteryuniversity.com/partone-11.htm

Circuitry

Block DiagramBlock Diagram

BuckConverter

Charger

5VRegulator PIC

SolarPanel

Solar CellSolar CellBuck

ConverterCharger

5VRegulator PIC

SolarPanel

Open Circuit Voltage

(V)

Short Circuit Current (mA)

Power (W)

Fluorescent light (1ft distance)

7.5 10 0.075

Window in 445 lab, sunny day

8.65 24 0.028

Outdoors, direct sunlight

10.6 760 8.06

Outdoors, cloudy day

9.65 145 1.4

Our solar cell: 8in x 12in

Buck ConverterBuck Converter

Accepts input between 6.8V to 48V

– Low Current Drain ~2.5mARegulates voltage at 5V and 500mA to MAX1501 as determined by main inductor

BuckConverter

Charger

5VRegulator PIC

SolarPanel

LT1777

Vin (10)Vcc (4)

(6) Vsw

(5) Vd

To MAX1501

and 5V reg

(13) FB

GND (1,8,9,16)

Solar panel

Vc (14)

SHDN (3)

SYNC (12)SGND (7)

220uH

1N5819

39kΩ

12.1kΩ 110uF

100pF

12kΩ

100pF2200pF

10uFout

Charging UnitCharging Unit

MAX1501 linear charger

Charges both Ni and Li chemistries

Low minimum input voltage– 4.5 Volts Li– 5.25 Volts NiCd

High maximum input voltage– 13.5 Volts

Low current drain (5-8 mA)

Safety Features

BuckConverter

Charger

5VRegulator PIC

SolarPanel

Charger SpecsCharger Specs

Battery mode can be selected (done w/ PIC)

Can choose charge voltage– We use 4.1V for Li

Current set by external circuitry– Our circuit set for 467mA

BuckConverter

Charger

5VRegulator PIC

SolarPanel

1uF

PIC

10uF

3k

IN (2)

INP (1)

(8) TEMP

GND (3, 13)

(4) SETI

(5) VL

(6) TMAX

(7) FULLI

(12) BATT

(14) SELV

RLED (15)

GLED (16)

MODE (9)

CHGEN (10)

ACOK (11)

MAX1501

Charging battery

1N5819Output of LT1777

Charging UnitCharging UnitBuck

ConverterCharger

5VRegulator PIC

SolarPanel

Voltage RegulatorVoltage RegulatorMAX6129

Can receive 5.2V to 12.6V from solar cell– Draws only ~5.25uA from supply– 200mV Output voltage dropout

Outputs to the PIC which needs low current and around 5V to remain in operation

http://datasheets.maxim-ic.com/en/ds/MAX6129.pdf

BuckConverter

Charger

5VRegulator PIC

SolarPanel

PICPIC

Battery type is selected by switch, PIC tells charging chip which battery has been selected

5V Reg(MAX 6129)

PIC(16F877A)

IN (1) (5) OUT

GND (2)

Vdd (11, 32)

Vss (12, 31)

MCLR (1)

OSC1 (9)

OSC2 (10)

B7 (28)

15k

20 MHz

20pF 20pF

B1 (34)

B0 (33)

MAX1501(charger)

MODE (9)

CHGEN (10)

Solarpanel

BuckConverter

Charger

5VRegulator PIC

SolarPanel

Testing/ResultsTesting/Results

Circuit PerformanceCircuit Performance

GLED on for fully charged batteryGLED on for fully charged battery Vbatt=4.1 VVbatt=4.1 V

RLED on for discharged batteryRLED on for discharged battery Steady voltage increase across batterySteady voltage increase across battery

Drew about 450mA from supply at 7VDrew about 450mA from supply at 7V Within boundary of solar cell operationWithin boundary of solar cell operation

Low output ripple voltage~20mVppLow output ripple voltage~20mVpp

Output RippleOutput Ripple

Charging TimeCharging TimeBattery voltage vs Time

3.85

3.9

3.95

4

4.05

4.1

0 20 40 60 80 100 120 140 160

Time (min)

Bat

tery

vo

ltag

e (V

)

Power ConsiderationsPower Considerations

Output to battery

1.913W

Buck converter 51.6mW

Charger 41.7mW

5V regulator 82uW

LEDs 197mW

PIC 66.5mW

TOTAL 2.27W

Our solar panel gives

8.06W(peak sunlight)

Charging efficiency:

23.7%

Solar Panel ConclusionsSolar Panel Conclusions

Although only 2.27W are used, we Although only 2.27W are used, we need a solar panel that gives:need a solar panel that gives: 6.8V (min to turn buck converter on)6.8V (min to turn buck converter on) 450mA (max current draw)450mA (max current draw)

-> 3.06W-> 3.06W

Most smaller solar panels do not give Most smaller solar panels do not give enough currentenough current

ChallengesChallenges

Inductor couplingInductor coupling

The LT1777 buck converter sometimes The LT1777 buck converter sometimes produced sporadic voltage/currentproduced sporadic voltage/current Appeared to be load-dependentAppeared to be load-dependent

Unable to charge NiCd batteries – buck Unable to charge NiCd batteries – buck converter does not give high enough outputconverter does not give high enough output

Future RecommendationsFuture RecommendationsBoth Voltage Reference and Buck Converter are Both Voltage Reference and Buck Converter are temperature sensitive!temperature sensitive! Only in very limited proportions, but heat sinking Only in very limited proportions, but heat sinking

should be consideredshould be considered

Choose different buck converterChoose different buck converter--Need output more reliable, higher than 4.5VNeed output more reliable, higher than 4.5V

http://datasheets.maxim-ic.com/en/ds/MAX6129.pdf

www.linear.com

Power RecommendationsPower Recommendations

Use buck converter that needs less Use buck converter that needs less currentcurrent Then, smaller solar panel can be usedThen, smaller solar panel can be used

Decrease power used by LEDsDecrease power used by LEDs

Find lower-power PIC, or implement in Find lower-power PIC, or implement in different fashiondifferent fashion

Questions?Questions?