This solar powered battery charger is vastly better that most if not all battery chargers described on the internet, and is much better than any of the cheap solar battery chargers u can buy. I designed this to be used on very long extended backpacking trip, where size and weight are a concern. However I will explain how to modify to fit you needs.
Solar Cell Basics:
Solar cells have a open circuit voltage of about 0.4-0.6 Volts, and can have a short circuit current in upwards of 4 Amps. The voltage is fixed for a solar cell, and different materials give different Voltages. The current of the cell varies with the size of the cell. Realisicly, in normal direct sunlight, and optimal power conditions, the voltage and current supplied from the cell is about 70-80% of the open and short cicuit, Voltage and current. Cell can be combined together in parellel or series to increase the voltage or curent. The Voltage is added when the cells are attached in series and curent are added when the cells are attached in parellel. Cells in series adopts the lowest voltage cell's Voltage. Similarly cells in parellel adopts the lowest current cell's current. So it is benifitial to match similar cells acordingly. Any number of parellel/series combination can be used to achive the desired Voltage and current.
Rechargable Battery Basics:
Batteries current capacity is desinated by the amount of miliamp-hours, or mAh, it posesses. This is defined as "C". This means that the battery can supply a current of C for one hour. Conversly the battery will be fully charged if a current, C, is supplied to the battery for one hour. A slow tricle charger will supply a battery with a current of C/10. A fast charger will supply a current of C/3 or C/4. A Battery should never be charged with a current higher than C/2.
The Solar Battery Charger:
The circuit is based on a switching step-up dc-dc converter, MAX1907. This converter will allow a voltage as low as 0.7 Volts to be steped up to any desired voltage up to 5 volts. The solar cells were chosen because of their high efficeincy and hagh open circuit voltage, however any solar cells can be used.
I chose a solar cell configuration that would almost garrentee the one Volt start up while maintaining a small size and high current. I did this buy attaching two 1/2 cell cells in parellel. Then attached that in series with the two full cells. This gives a 1.8V open circuit Voltage, and a 3A short circuit current in a rectangle of about 200mm x 150mm.
I chose the MAX1709 for its high efficency at high currents. I decided on an output voltage of 2.7V, 0.3 Volts higher than two NiMH batteries in series. The output Voltage is controled by R3 and R4, see datasheet for details. Be sure to keep the wires or traces big and short where there is high current. ie the inductor and ground. Also make sure the inductor is 1uH or very close to it.
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