Solar+Wall NIMH Charger
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Solar+Wall NIMH Charger

by robodude666 on Tue Nov 09, 2010 10:37 am

Hey guys,

A classmate of mine is working on a project that requires batteries and the ability to charge them either via solar panels or a wallwart. Based on Ladyada's suggestion in the Ask an Engineer session, Lithium Ion batteries are out of question. The servos will draw up to ~750mA and most boost converters I've seen support up to 500mA. When you take all of that into account, and the inefficiency of the circuit you'll end up pulling close to ~2 Amp from the Lithium Ion battery. Not too good, as that's about the limit on most Lithum batteries. NIMH batteries are the next best thing. They're cheap, available in large Amp-Hour ratings, and relatively easy to charge.

Maxim makes the MAX713 NiCd/NiMH Fast-Charge controller, which is perfect for the job. There are many example schematics online, including one Maxium provides. There is also a great page about it here. In either case, it will be used to charge a 7.2v NIMH battery (6 cell). This will require an input voltage of around ~13v (based on equation on the letsmakerobots site). Getting a ~13v wallwart isn't too difficult, and getting 13v from solar panels requires 2 or 3 of them in series. Not a big deal. In order to auto-switch between the sources a diode OR bridge could be used, as per Ladyada's recommendations. Would using the comparator method used in the Arduino be a better option to auto-switch between sources, as it won't drop the voltage by ~0.7v? Also, 3 solar panels in series would be about 18v. Should that be regulated down to 13v or would it not matter too much?

Does this seem like a solid plan, or is there an alternative to a cheap & simple NiMH charger? I ran across this thing, but it looks really cheap and possibly too simple. I know Ladyada did research on NiMH charging for the MintyBoost Rechargable, but I can't seem to find any related pages (though I assume it's not published yet).

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Please be positive and constructive with your questions and comments.