Nickel Metal Hydrids

From: Pete Turnbull <>
Date: Mon Jan 17 19:29:43 2005

On Jan 17 2005, 17:06, Paul Koning wrote:
> >>>>> "shoppa" == shoppa classiccmp
<> writes:

> shoppa> This raises a question for refurbishing older equipment: is
> shoppa> it always OK to just drop a set of NiMH's in place of
> shoppa> same-size NiCad's? Most of the "stupid" NiCad battery
> shoppa> chargers were just 0.1C trickle chargers, but the "smart"
> shoppa> ones that looked for the voltage rise at the end of charge
> shoppa> a NiCad may not see this with a NiMH pack. Anyone have any
> shoppa> experience?
> I'm not sure that a trickle charger that works for NiCd is ok for
> NiMH, and I am positive that a fast charger for the one will NOT work
> right for the other. Each battery technology has its own charging
> rules -- lead acid, NiCd, NiMH, and the various flavors of Lithium
> battery are all different in very significant ways.

Lead-acid, lithium, various others, and NiMH/NiCd certainly need very
different charging regimes, but NiMH and NiCd are very similar to each
other. The most important difference is that for a NiMH you should
stop the charging when the voltage flattens out, and before it starts
to fall again; for a NiCd you should stop when the voltage just starts
to fall again. Of course, you shouldn't let them get too hot, and you
shouldn't fast-charge very cold batteries either.

Maxim do a couple of inexpensive chips (MAX712/MAX713) that take the
hard work out of all that; used with a thermistor, they won't
fast-charge cold cells until they warm up, and they won't fast-charge
until the cell voltage rises above some threshold. I've used them but
I've not put enough cycles through enough cells to see what the life is
like. Interestingly, Maxim's data sheet indicates that the difference
between detecting the voltage slope flattening and detecting the slight
fall is less important to NiMH cells, especially when the charge rate
is fairly *high*.

That means for a lot of applications you could safely use an NiCd
charger for NiMH cells, but vice-versa would leave you with slighly
undercharged NiCds.

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Mon Jan 17 2005 - 19:29:43 GMT

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