William Sommerwerck wrote:
According to the manufacturer web pages I've read, overcharging *any*
NiMH cell significantly can shorten its life... they're somewhat less
tolerant to this than nickel-cadmium cells.

How do you define "overcharging"? Switching to a trickle charge at the end
of the charge cycle is, technically, overcharging, but no one considers it
abuse.

In 1975 my station got a hand-held communications radio with a NiCad
battery. The radio hung in a charger that would switch to a trickle
charge when the battery was charged. That would have been ideal for a
lead-acid battery, but in a matter of weeks the NiCad would no longer
take a charge. The boss contacted the battery manufacturer and learned
that NiCads should not be left on a trickle charge. Nobody considers it
abuse because it's not intentional misuse, but trickle charging is harmful.

I began using aa NiCads in 1981. They were supposed to be good for 500
charge cycles. Within about 50 cycles, their self-discharge was so high
that they would go flat if not used within a few days. Eventually they
wouldn't charge at all. I bought a lot of them for 17 years because I
found them more practical than alkalines even if I got only a fraction
of the advertised service life.

Then I tried a set of NiMH cells along with a microprocessor-controlled
500ma charger. They performed so well that I bought many more. After
ten years, all my NiMH cells will hold a charge for months. That
charger would also work for NiCads, and I discovered that they, too, had
outstanding service lives if fast-charged.

A big killer of nickel cells is internal salts. They allow electrical
leakage so a cell won't hold a charge long and eventually can't be
charged at all. These deposits increase during trickle or C/10 charging
but apparently not with fast charging.