NiMH charge controller

Re: NiMH charge controller

What charge rate are you looking for? If C/10 or slower (at least 10 Hours to recharge from dead)--you don't really even need a charge controller (you could use something like this to monitor the charging rate/voltage).

For example, if you where using 2,000 mAH AA NiMH batteries, the C/10 rate would be 200 mAmps. You could put 1-6 (maximum number of batteries in series depends on the exact Vmp of the solar panel and the ambient temperature) or so batteries in series for 200mAmp rate. And put 3-4 of those (equal battery strings) in parallel behind your 12 volt / 700 mAmp panel. It would take 2-3+ days of full summer sun to recharge the "set" from near dead.

You probably want one blocking diode per string to prevent leakage current at night going back into the solar panel or battery strings leaking into other strings...

Batteries in each series string should be matched sets (same brand, same size, ideally discharged the same amount). You can make some very cheap and reliable current sources so that you can "program" the charging current for each string (AAA/AA/C/D etc.).

If you are planning on D cell NiMH batteries which run around 10,000 mAH ratings... C/10 => 1,000 mAmp or 1 amp... Your 700 mA panel is less than C/10--so you could put between 1 to approximately 6 D Cells in series with one Vmp>12 volt 700mA panel. Again, 5 hours per day:

• 10 AH * 1/0.700 amps * 1/0.66 charging eff * 1/5 hours sunny summer day = 4.3 sunny summer days from dead

If you are are looking to "fast charge" (C/2 to C/0.5 or so) so that you can charge in 30 minutes to 2 hours--you probably are better off with your solar panel charging a standard 12 volt battery then using a standard fast charger that can run off a car adapter (12 volt lighter plug)--or bite the bullet and use an efficient 12 volt to 120 VAC inverter.

The the solar panel (and 12 volt holding battery) would be sized to the AH needs of the charger and how may/how large of batteries are are recharging.

If you can't be at the whims of the weather, then a 12 volt lead acid battery and smart charger is probably the best way to go.

The little I have seen from before when trying to find a "nice" solar AA/etc. battery charger--They have all been very small solar panels and take days to weeks to recharge the batteries.

I have not seen solar based MPPT input chargers for NiMH batteries. Would be interesting to see if there are any out there.

There are other ways of controlling charging--timer, AH counters, etc... But they can be hard on the batteries (if fast charging) or if the batteries are still 3/4 full, etc.

-Bill