Greetings! I got the idea from this when I got some small solar cells from a catalog. I had noticed that most chargers for Energizer NiMH (Nickel Metal Hydride) battery cells produced about 3 volts. The cells I used were one volt each, but you could use 1,5 volt cells too, or any individual call voltage, just so long as your total cell voltage was somewhere close to 3 volts. Less power would probably work too, but the cells might take longer to charge. Note that this is my first instructable, so it might be significantly less than perfect.
Step 1: Materials Needed
This is pretty simple, and you don't need much. I haven't tested it too extensively, and it's possible that I might need to add diodes or something to allow the batteries to charge. I'll try and post an update if that turns out to be the case.
1. Battery cases; any battery that is normally 1.5 volts (NiMH cells tend to be 1.2, which is close enough) should charge off this array. Do whatever is most practical for your needs; AAA cells, AA cells, C cells, or D cells, to name a few that would work.
2. Batteries; I used NiMH cells because they're the lowest maintenance. They don't have to be discharged completely, like NiCd (Nickel Cadmium) cells, and they're widely available. You would probably need to modify the circuit if you wanted to charge another type of cell.
3. Solar array; I used three one volt crystalline cells fastened together into one big three volt array. I'm not sure if the three volt amorphous cells from portable calculators would work; You could try.
4. fasteners; I had a problem with the solar array falling apart, as it was made from three separate cells; I used two metal rails and some epoxy to keep them together. I also used foam tape later.
Step 2: Assemble the Solar Cell
Here we put the solar power supply together. If you have a solar cell that doesn't need to be assembled like this, go right ahead and wire it up. If you're using multiple cells, first, wire them in series, positive to negative. You can test them with a voltmeter to make sure the connections are good and the cell produces the required juice (3 volts, or somewhere close). Then make sure the cells stay together; I did that with Epoxy, but you can be creative.
Step 3: Battery Cases
Put the battery cases on the back of the array; as I've said, I attached them with foam adhesive tape. wire the cases together in parallel, or positive to positive, negative to negative. They'll charge backwards and all kinds of funny stuff if you wire them in series. Plus, in parallel, you can charge as many or as few as you like and still complete the circuit. Make sure that the positive wire that connects the positive ends of the battery cases connects to the positive power post on the solar array. The same goes for the ground.
Step 4: CHARGE!
Now you're ready to charge! Get your NiMH cells and put them into the battery cases. If you sired the cases right, they should all be oriented the same way, NOT the positive/negative way you see in most devices. If everything works, you should be able to set the solar array in the sun and charge the batteries. It would be perfect for backpacking trips if you have devices like flashlights and GPS units that run on AA batteries, and you don't want to carry extra alkaline cells. The possibilities are limitless!