Over the past year or so I have been using a low cost solar panel to keep the phone charged while camping or hiking. The solar panels have been sourced from Ebay and are about 15 x 15cm and deliver about 2Watts. The ones I chose cost around $10 delivered and weigh about 70grams.
This panel is the oldest one and probably the best one for output. It will deliver 400mA in full sun at 5V to the USB socket that is glued on the bottom.
The panel simply connects negative panel out to pin 1 on the USB socket and the positive out to pin 4 on the socket via a diode to drop the output a little.
The panel charges most Android phones but wont charge an Iphone. This is because pins 2 and 3 of the USB socket are floating. For an Iphone these pins need a voltage from a resistive divider across pins 1 and 4 (Google it). As I dont have an Iphone any more, I have left it as is.
This photo shows the simple connections to the USB socket that is glued with Selastic.
This panel has been used a lot over the last year or so when hiking and works nicely. The panel has velcro on the back that attaches it to my backpacks.
The second, more recent panel from Ebay was from a local supplier. This panel is rated at 330mA on the sticker, but I have seen it put out a little more. The volts on this one are higher at around 7.5 in full sun, and its a little too much for the phone as it stops charging.
The quick and cheap solution was to put a 5.6 volt 1 Watt zener diode across the panel. This works fine, the diode hardly gets warm. Maybe a larger diode would be better, but I had the 1Watt ones on hand.
I also tried a low dropout 5V regulator on the panel which worked fine as well (although the regulator did get a little hot). The first panel seems to get used the most.
The Android phone has an app called Battery Monitor Widget which reports the charge current in mA and takes a sample every 10 minutes. Its useful for evaluating the performance of the charger. In practice, if the panel is in direct sun, the phone will charge in couple of hours. While the panel is velcro’d to the backpack, it doesnt charge as effectively, but keeps the battery sustained.
Update:_2014-02-08: I noticed that my replacement Samsung Galaxy S2 LTE was charging erratically while camping. The charge current would pulse on and off at a 0.5 second rate rather than stay constant. The shunt zener diode across the solar panel to keep the volts from going to high was not regulating tightly enough for the phone. This did not matter on my old phone or a couple of others that I have.
The solution was to make a shunt regulator with a more tightly controlled regulation voltage. I had some TL431 shunt regulator chips leftover from an old LiPo battery balancer project. These came from Rockby Electronics very cheaply. I also had some PNP TO-220 transistors from Rockby. This is the simple circuit I came up with. It works fine and the transistor doesn’t need heatsinking for a small 2 watt 6 volt panel. The variable resistor is adjusted for about 5.2V when the regulator is shunted across the panel. No more pulsing was noticed and the charge is now much smoother.