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

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

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 Li 1/2 CoO 2 + Li 1/2 C 6 C 6 + LiCoO 2 Lithium Ion Battery Source: Wikipedia

Common Usage: widely used in small electronic devices Energy/weight Wh/kg Energy/size 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 Nickel-Cadmium Batteries 2 NiO(OH) + Cd + 2 H 2 O 2 Ni(OH) 2 + Cd(OH) 2 Source: Wikipedia

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 Our batteries charge at 4.1V to prolong lifetime Many Built-in safety precautions Temperature, low voltage Temperature, low voltage Charge time ~3 hours at 500ma Fast chargers skip stage 2 in next slide Fast chargers skip stage 2 in next slide 70% charged

GLED ON

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

Circuitry

Block Diagram

Solar Cell Open Circuit Voltage (V) Short Circuit Current (mA) Power (W) Fluorescent light (1ft distance) Window in 445 lab, sunny day Outdoors, direct sunlight Outdoors, cloudy day Our solar cell: 8in x 12in

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

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

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

Charging Unit

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 ic.com/en/ds/MAX6129.pdf

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

Testing/Results

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

Output Ripple

Charging Time

Power Considerations Output to battery 1.913W Buck converter 51.6mW Charger41.7mW 5V regulator 82uW LEDs197mW PIC66.5mW TOTAL2.27W Our solar panel gives 8.06W (peak sunlight) Charging efficiency: 23.7%

Solar Panel Conclusions Although only 2.27W are used, we 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 Most smaller solar panels do not give enough current

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

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

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

Questions?