Almost all rechargeable batteries you buy in the store are NiMh batteries. Usually people use a wall adapter to quick charge them in an hour or so, but we're not going to be that fancy. We're going to use the "trickle" method to slowly charge them up over time. This also prevents them from overcharging or getting destroyed.



To be safe we need to be sure that we're not throwing more than 10% of their capacity at them when charging. For example my AA batteries have a capacity of 2,000 mah. The max I can charge them at is 200 ma.



When you decide what solar cell to use make sure you're not going over the 10%.



At the same time be sure that you're getting at least 3.5 volts out of your solar cell otherwise the batteries won't charge. Remember, the more voltage your cells put out the less sunlight you'll need to reach your minimum voltage requirement. (Higher voltage cells = more charge time, even in low light.)



For this project I've chosen some 4V cells that put out 50 ma. I picked it because it's just the right size to fit into the Aloids tin. If you pick a bigger sized cell you can always glue it onto the outside of the tin.



You could also use a combination of smaller cells. I've used combinations of little 1.5V and 2V cells in series to get my desired voltage levels, while still fitting inside the tin.