Solar Powered Battery Charger

1. Christine Placek Philip Gonski Group 4 ECE 445 – Spring 2007 Solar Powered Battery Charger

2. Objectives Operate in various sunlight conditions Sunny and cloudy days Operate for a wide variety of input voltages 6.8V to 13.5V Charge both lithium and nickel chemistries

3. Lithium Ion Battery Common Usage: Cell phones, MP3 players, other portable electronics Energy/weight 160 Wh/kg Energy/size 270 Wh/L Power/weight 1800 W/kg Charge/discharge efficiency 99.9% Self-discharge rate 5%-10%/month Cycle durability 1200 cycles Nominal Cell Voltage 3.6 V Li1/2CoO2 + Li1/2C6 ↔ C6 + LiCoO2 Source: Wikipedia

4. Nickel-Cadmium Batteries Common Usage: widely used in small electronic devices Energy/weight 40-60 Wh/kg Energy/size 50-150 Wh/L Power/weight 150W/kg Charge/discharge efficiency 70%-90% Self-discharge rate 10%/month Cycle durability 2000 cycles Nominal Cell Voltage 1.2 V 2 NiO(OH) + Cd + 2 H2O ↔ 2 Ni(OH)2 + Cd(OH)2 Source: Wikipedia

5. Charging Lithium Batteries Very Strict Regulation of Current vs. Voltage Charge Voltage at 4.2V or 4.1V most common Our batteries charge at 4.1V to prolong lifetime Many Built-in safety precautions Temperature, low voltage Charge time ~3 hours at 500ma Fast chargers skip stage 2 in next slide 70% charged

6. GLED ON http://www.batteryuniversity.com/partone-12.htm

7. Charging NiCd Batteries Three types of charging Slow(.1C->14 to 16hrs) 70% efficient Quick(.5C->3 to 6hrs) Fast(Full charge with topping->1hr) 90% efficient Pressure and Temperature increase Full Charge detected by: Voltage Drop Rate of temperature increase Timeout timers

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

9. Circuitry

10. Block Diagram

11. Solar Cell Our solar cell: 8in x 12in

12. Buck Converter • Accepts input between 6.8V to 48V • Low Current Drain ~2.5mA • Regulates voltage at 5V and 500mA to MAX1501 as determined by main inductor

13. Charging 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

14. Charger 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

15. Charging Unit

16. Voltage Regulator • MAX6129 • 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

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

18. Testing/Results

19. Circuit Performance GLED on for fully charged battery Vbatt=4.1 V RLED on for discharged battery Steady voltage increase across battery Drew about 450mA from supply at 7V Within boundary of solar cell operation Low output ripple voltage~20mVpp

20. Output Ripple

21. Charging Time

22. Power Considerations Our solar panel gives 8.06W (peak sunlight) Charging efficiency: 23.7%

23. Solar Panel Conclusions • Although only 2.27W are used, we need a solar panel that gives: • 6.8V (min to turn buck converter on) • 450mA (max current draw) • -> 3.06W • Most smaller solar panels do not give enough current

24. Challenges Inductor coupling The LT1777 buck converter sometimes produced sporadic voltage/current Appeared to be load-dependent Unable to charge NiCd batteries – buck converter does not give high enough output

25. Future Recommendations Both Voltage Reference and Buck Converter are temperature sensitive! Only in very limited proportions, but heat sinking should be considered Choose different buck converter -Need output more reliable, higher than 4.5V http://datasheets.maxim-ic.com/en/ds/MAX6129.pdf www.linear.com

26. Power Recommendations • Use buck converter that needs less current • Then, smaller solar panel can be used • Decrease power used by LEDs • Find lower-power PIC, or implement in different fashion

27. Questions?