Solar panels can be used to charge cell phones, tablets, ebook readers, and netbooks. Please see the post about building a solar panel here.
USB connector has 4 wires in it. The two wires close to the center carries data. If you are facing the USB connector, the wire on the left is +5 DC Volts, and the wire on the right is the Ground. Most cell phones will draw current up to 500 mA, unless the source is designed as a fast charge.
The solar panel I built charges up a 12 Volts car battery. We need to convert 12 DC Volts to 5 DC Volts to charge from the USB.
I tried to use 7805 Linear voltage regulator to convert 12 DCV to 5 DCV. This works, but the problem is that the regulator generates a lot of heat. If we assume that the input is 12 Volts, 5 volts is used and 7 volts is wasted. (5/12 –> 41.6% used, 7/12 –> 58.3% wasted as heat). In reality the input voltage is a little higher and therefore the efficiency is even lower.
There may be a way to connect two 7805 regulators in series so that only 2 volts is wasted, however I could not find a way to make this work. If somebody knows how to do that, I would be very interested in hearing about it.
First, I found a dual USB connector from an old circuit board and de-soldered it. This is not an easy thing to do because each USB connector has four pins plus the four corners of the case need to be de-soldered, but it is do-able.
I have ordered several DC to DC converters from proDCtoDC website from this link (item #090033).
If you look at the specs, this small unit provides 12 DCV, 5 DCV, and 3.3 DCV at 92% efficiency. When I ordered it, the price was closer to $2 per unit with free shipping.
The unit below can charge two cell phones at the same time. So far it is working without any problems (no heat is generated).
The picture above shows the front and the back side of the SMPS power supply.
The pictures below shows that I used plastic spacers to connect a second board with the USB connector on it to the power supply with two wires. Super simple.