In article ,
Butch wrote:

Never run into this problem before, I have a nicad charger that Ive used
for years and it charges AAA, AA, C, D, and 9 volt. Great little box.
Now I just received a pack of Ni-MH AA size and need to know if my nicad
charger will be ok to charge them? Will I do damage to the NiMHs?

As I understand it: NiMH batteries do "prefer" a somewhat different
charging regime than NiCd batteries - they're less tolerant of being
overcharged.

A NiCd "smart" charger will usually charge the batteries with a
constant current, and monitor the voltage across the battery. When
the batteries hit full charge, they begin to heat up, and the voltage
across them decreases somewhat. The charger will detect the voltage
drop (a "negative delta V") and shut off the charge. Fast chargers
usually include a temperature sensor, as well, and a failsafe timer.

NiMH batteries can be charged with a similar regime, but it's
apparently not a good idea to wait until the delta-V goes negative...
by that point the NiMH battery has heated up quite a bit, and that's
not good for its lifetime. A NiMH smart charger will cut off the
charge current when the voltage across the battery stops rising ("zero
delta V"), before the battery heats up very much.

NiCd batteries can be trickle-charged at a low rate, and I believe
I've read that this is not a good approach to use for NiMH batteries
(or, if you do it, the trickle current must be a good deal lower).

See if you can find the data sheet for the Maxim MAX712 and MAX713
charge controller chips - they have a lot of the details.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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