Dave Platt wrote:


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.

I have two chargers designed to charge AA NiMH in three hours or less.
I wish they sensed temperature, but it seems they work strictly by
voltage changes.

The first one was designed for NiCds as well. I don't recall any
trouble with NiCds, and what you've written may explain it. With NiMH,
each charger has occasionally stayed on longer than expected, and I
removed the cells because they felt hot. I haven't seen any signs of
damage from these incidents.

Before I bought my first NiMH cells, I looked at data published by an
amateur photographer using several brands of cells and more than one
charger. Sometimes when he took pictures he would find that a set of
cells hadn't taken a normal charge. I think that's a drawback in
charging more than one cell in series in a circuit designed to shut off
when a cell is charged. Even when each cell is charged in its own
circuit, I think gas bubbles formed in a cell during charging may cause
a voltage fluctuation that may shut off a charger. I wonder if that
happens more often with new cells.