Here is the schematic of a simple DIY solar cell phone or USB charger circuit. This solar USB battery charger circuit can be used to charge any device, which can be charge from computer USB port. For example MP3 players, cell phone, iphone etc.
The circuit is using a solar panel of 7 to 7.2 volts, one diode and four Ni-MH rechargeable batteries of 1.25 volt and 2000 mAh and a USB type "B" female port. The fully charge batteries will show 5 volt on the digital millimeter which is an ideal voltage to for a USB charger.

panel of 7 to 7.2 volts, one diode and four Ni-MH rechargeable batteries of 1.25 volt and 2000 mAh and a USB type "B" female port. The fully charge batteries will show 5 volt on the digital millimeter which is an ideal voltage to for a USB charger.

How to charge and use:
After completing the circuit check the voltage of your new Ni-MH batteries with your digital multimeter, new batteries are often 50 or 70 percent charged by the manufacturers. if your batteries are not showing 5 or 5.1 volt then charge them for 3 to 4 hours by placing the solar panel in direct sunlight. If you batteries are fully discharged then charge them 8 hours if your solar panel output current is 250mA in direct sunlight or 10 hours if your solar panel output current is 200mA in direct sunlight.
After charging the batteries in sunlight again check your batteries voltage with multimeter now if your batteries are showing 5 or 5.1 volt then your DIY solar cell phone or USB charger is ready to charge your devices. Next time when your solar charger batteries require charging charge them only 3 to 4 hours cycle.

Important Instructions:
Do not charge your devices at the same time when you are charging the batteries of your solar USB charger. First charge your USB charger batteries and then remove it from the sunlight and then connect your devices to charge. Do not charge your batteries more than 8 to 10 hours. 
Use solar panel, which provides 7 or 7.2 volt and 200 or 250 mA in direct sunlight.





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DIY Solar Cell Phone Or USB Charger Circuit