Because I have found NiMH to have far worse 'memory' effect than has been
claimed for NiCad. They also have an utterly woeful self discharge
characteristic which means you can't pop them in a device and expect it to
operate for long in a couple of months time.
I used to use an HP laptop provided by my employer. Just after a year the
NiMh battery would not hold more than a minute or so of charge. When I
contacted HP's tech support they said that with NiMh you should completely
discharge a NiMh battery before recharging it and that once a charging
cycle has started it should be completed and not terminated before the
battery is full. Pity they forgot to mention it in the manual.
Now these limitations make me wonder why on earth they ever used NiMh for
laptops when the realities of use prevent such fussy requirements from
being followed.
I acquired a new battery, my employer paying for it of course, and despite
my best intentions the same thing happened, it was dead within 18 months
and lost a lot of its capacity well before then.
I subsequently acquired an IBM laptop with an NiMh battery, this time I
paid for it. The same thing happened, the battery lost most of its
capacity within a year. The instruction manual clearly states the strict
usage regime required to keep the battery alive. Pity it is completely
impractical or downright impossible to follow as the machine does an
auto-save shutdown routine before the battery is completely flat so it is
not possible to completely discharge it short of removing the battery and
shorting it with a suitable resistor. That is just too much stuffing
about.
The real problem with NiCads seems not to be a memory effect but rather
overcharging. Most NiCad chargers seem to be really dumb and just keep on
trying to stuff the juice in after it is full, they then cook and that
reduces their capacity.
I have put my charger on a plug in timer and that seems to take care of
the problems.
NiCads are good for powering flashes because they can give up their
contents very rapidly and so recycling times are fairly short.
Now the absolute "bees knees" is Li ion (Lithium ion). I power a portable
DAT player with one intended for a Sony Handicam. Absolutely no memory
effect, charge it whenever you like and it has negligible self-discharge
so leave one for a year and it will still be nearly fully charged. They
also have a large capacity for their size. However, I think the chemistry
is 3V so no AAs 8-( they cost an arm and a leg and they are very fussy
about how they are charged.
Giles
Tom Scales wrote:
> I'm curious. Why do you prefer NiCads? The have a much bigger problem
> with memory effect. I have a pretty larger number of the Nexcell NiMH AAs
> that I use in everything, with a nice Maha charger.
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