Well, it’s taken a while and a lot of tweaking, experimenting, lackluster results and disappointments, but I finally charged my phone from 70% to 100% entirely off my 2-watt solar system. I had been playing with a single 1 watt panel charging a pair of NiMH AA batteries. The results of those tests were mostly successful. With a full day of pretty lousy sun (probably pulling about 1/2 watt, if that) I was able to charge the pair from completely dead to enough to power my portable CD player for over an hour. I was impressed, but underwhelmed, so I went out and picked up a second 1 watt panel and put them together in series. After about a day and a half of very slow charging, I was able to build up the power in my 4 cell NiMH pack. The 4 cell NiMH pack is perfectly suited to charge the phone all by itself. I had been planning on using a 6 cell pack through a 7805, but I noticed that was pretty inefficient by all the waste heat in the IC. The 4 cell puts out just under 5.5V which is absolutely perfect for direct charging. I was going to limit the charge current, just to make sure the phone wasn’t stressed, but I checked, and a direct connection from the pack to the phone caused the phone to only draw about 0.4A which is well within what the batteries can deliver, and well under the 1A the phone normally fast charges with. As I suspected, the phone handles its charging very well. After the charge was complete and the phone’s light turned green, I saw the charge current drop to essentially 0. Awesome.
So from here, while I am impressed and very happy, there are a few improvements I’d like to make. For one, while I have been able to get a few hours of solid 12V out of the array, the current is still far under what would make it 2W. Currently, I’m using bare panels that I don’t think were designed to stand up to the elements like my old panel. The old panel came off of an outdoor motion light, so it was sealed and ready for outdoors. I’m planning on building the panels (possibly with the addition of 6 more) into something like a fat binder and hopefully give them some protection from the elements. Right now, the array sits vertically in my window, which is good, but not great. The angle could certainly be better, even if they don’t actually track the sun. Basically, I’m free to go for as much as I can, since I believe the output current will never be more than ~160mA which is well under the 250mA limit I’ve decided on for my pack (10% of the 2500mAH pack).
Yes, there are improvements, but I think I’ve been at this for quite a while now, so it’s very exciting to get meaningful results. I mean, my cell phone is something I use every day!