Solar AA (NiMH) Battery Charger





Introduction: Solar AA (NiMH) Battery Charger

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!



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    12 Discussions

    I build it using a small solar panel and it's working with my NIMH 3.6v 600mah pack. Question: can you overcharge the NIMH battery with this or is this not an issue?

    hellp you could make it better dont take this offensivly but you could maby make a stand or put it in an altoid tin it would be very cool that way you would charge less batterys but it would be portable plz reply

    A diode in series between the solar panel and batteries, prevents the batteries from discharging back through the solar panel at night. The charge current shouldn't be more than one tenth the capacity of the batteries, or they get hot quick, which will reduce battery life. Your charger will charge batteries, but needs to be watched to prevent overcharging, and batteries need to be removed when no sun is on the panels

    1 reply

    Hi Harvey,

    I know its late to ask this, but would you give me the idea on what particular diode should I use that would serve as my Low Voltage Disconnect and over charge protection for this? Do I need like a circuit board or something. I have a solar panel that have an output of 9v, .283amp and 2wp. It interest me to conduct the same but the question is how would I be able to do the system. I'm afraid that if I would connect it directly I might end up melting the battery or totally damaging it.

    Your assistance would be a big help for my experiment


    what do u mean by funny stuff when u charge the batteries in series? tks

    1 reply

    The one time I tried it, one of the batteries ended up with its polarity reversed. I probably did something wrong, but you need more voltage to charge them in series anyway. If you know more about electricity than I do, you could probably get it to work. I'm just saying, It didn't work when I tried it.

    on most cameras to focus you can half press the button and hold until it focuses befor you fully press to take the picture

    1 reply

    On my camera, that is the case, but auto focus must first be enabled from a drop down menu.  I have since figured out how to do that.  Thanks anyway!

    I'm sure you could scale this up to charge as many batteries as you wanted if you knew what you were doing.  NiMH batteries are pretty tolerant; they don't explode nearly as easily as exotic batteries like LiPos.

    "I'm not sure if the three volt amorphous cells from portable calculators would work; You could try." They will indeed work, but you might have to parallel a few together. Those cells are usually pretty darn low voltage or amperage(and sometimes BOTH). Given enough of them... you could set up a serious solar array, but unless you have an endless supply of old calculators, it'd probably be prohibitively expensive.

    1 reply

    I have about three of the amorphous arrays from old broken calculators I've found; I'll have to do some experiments. Thanks for the tip about parallel wiring them.

    I thought I might run into problems like that; I plan to at least add a diode. Thanks for the heads up on the overheating problem, though! I haven't done much testing, as I said, but I did notice a little heat on the one test I did do. I might add a voltage limiter or something as well if I can find one. This is very much a work in progress, and getting advice from fellow builders is partly why I posted it.