I have been using a similar kind of thing on for my iphone 3gs, its a solor powered charger that can be connected to any cell phone with its multi port system. following is the images of that device.
Its a solar powered cell phone charging system with the ability to charge even in low sun light. Following are the features of this device
Multi-device charger
Works with 3200+ devices.
Stores Power
Holds a charge up to one year, so it’s ready anytime, day or night.
Hybrid versatility
Charge it from the sun, via USB port or from an optional wall charger.
Intelligent Charging
No overcharging, just right.
Technical Specification
* Long life internal 1000Amh Lithium-Ion Battery.
* 1 High Efficiency Solar Cell.
* Multi-function LED light/start button.
* Charging cable input (USB / AC).
* Charge output
* 5-6V, 700mA output range.
* 4.2 Watts
* Strong, impact resistant Lexan Shell
The above specs are of
solio solar charger. As compared to a battery powered charging system, the solar power charger is far batter as there is no running cost to charge and is able to charge different brand cell phones.