The black bar is the Negative side and prevents power backflow.If we put it in wrong we're going to prevent power from flowing into our circuit!If we use two rechargeable AAs that put out a total of 2.4Vs we're going to need a solar panel that is at least 3 - 4Vs just to meet basic levels of charging.The higher the voltage of our solar cell (or cells) the less light we need to charge up our batteries.You could easily connect two of those in a series to get your 4Vs.
(In the picture it's the black wire.)Then tape it up for protection.(Something that has annoyed me with nearly every Altoids Solar Guide out there.) You could also use a combination of several smaller cells to get your four volts.For example, 2V cells are very cheap and small on ebay.2) You can buy a premade circuit off ebay, or even off my website Brown Dog What I really find annoying is that on all the commercial solar USB chargers I see their internal battery is only 1000 ma. A rechargeable AA battery has between 2000 - 3000 ma of current in it. We need to use rechargeable batteries for this project.They cost around and work with most everything. I prefer Ni Mh AAs over everything else because they're easy to find, cheap, and reliable. Since we're using two AAs in this project our charger will have 2000 - 3000 ma of current.