In article ,
William Sommerwerck wrote:

How do you define "overcharging"?

Pretty much the way you do later, and the way that the manufacturers
seem to. "Overcharging" is when one continues to force charging
current into the cell, once the cell's electrochemistry has reached
the point of saturation and no further useful electrochemical
conversion can be performed.

Switching to a trickle charge at the end
of the charge cycle is, technically, overcharging, but no one considers it
abuse.

Actually, some of the manufacturer data sheets I've read seem to
recommend against it.

NiMH cells can tolerate huge charging currents. MAHA specifically states
that do not recommend charging at _less_ than 1/3 C, and permit charge rates
as high as 1.0 C!

True. That's one (not the only) definite advantage of NiMH cells -
they can be recharged very quickly.

Whether this applies only to their cells, or pretty much everyones, I don't
know. But NiMH cells don't appear to be particularly "delicate".

They're not particularly delicate in terms of their rate of charge
absorbtion _during_ proper charging. As you say, they can eat a lot
of current.

They are, however, more easily damaged than NiCd cells by the
overheating which occurs if you continue to pump energy into them
after their electrochemistry has saturated.

"Overcharging" probably means, as others have suggested, continuing the
charge past "negative delta V" and continuing to charge at a high rate to
the point where the cell badly overheats.

I read "overcharging" as any continued charging past the point of
"full". High-rate and low-rate overcharging does affect NiMH cells
differently, as the latter doesn't heat up the cells very much.

But, as I said in my "moody" missive, this is something you should ask the
manufacturer, as only it knows how its only cells respond to various
charging protocols.

True. Unfortunately, without further information about how the
specific charger operates and behaves, even the cell's manufacturer
probably won't be able to give a useful answer.


Yes and no. NiMH chargers can use either a rise in temperature (which might
be hard to judge when the sensor is not part of the battery pack) or a drop
in voltage to signal "full charge". The latter is supposedly larger and more
distinct at higher charge rates.

I don't believe either of these apply to nicad charging.

Actually, both of them do, although NiMH and NiCd cells differ
somewhat in both of these respects.

During the normal charging cycle (when they're still accepting
charge), NiCd batteries do not heat up very much at all... the
electrochemical process in these batteries is said to be endothermic
during charge acceptance. The cell's terminal voltage rises slowly
during this phase of charging. Once the plates are fully charged up,
the electrochemical reaction changes, and a secondary reaction
develops which releases the energy as heat... and so the NiCd cell
heats up significantly. As a result of the change and the heating,
the cell's terminal voltage stops rising, and actually drops
significantly. This reversal of the voltage curve with time isn't
hard to detect, and most NiCd fast-chargers seem to use a "negative
delta-V" detection circuit to determine that the cell has reached full
charge and to shut off the current (or drop it to a trickle).

NiMH cells behave a bit differently. They do warm up somewhat during
the main phase of charging - the electrochemical reaction is
exothermic. Like a NiCd, their terminal voltage rises slowly during
the charge cycle. Also like a NiCd, when they reach full charge they
start dissipating most of the incoming charge energy as heat, and (in
a fast-charge scenario) they can get quite warm quite quickly.
However, the effect of this on their terminal voltage is a bit
different... it stops rising, but it doesn't begin to fall
significantly until you've gone pretty far past the full-charge point
and gotten them pretty hot... and the manufacturer data sheets I've
read say that this degree of overcharging will shorten their life
appreciably.

So, the manufacturer data sheets I've read recommend using the
temperature rise (absolute and/or delta-temperature-over-time)
directly, using a thermistor, as the primary means of detecting full
charge in a NiMH. Zero-delta-V-over-time makes a good secondary
shutoff mechanism, and a timed shutoff for safety is also recommended.

--
Dave Platt AE6EO
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