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Re: [OM] AA Batteries - sort of OT

Subject: Re: [OM] AA Batteries - sort of OT
From: "Giles" <cnocbui@xxxxxxxxxxxxx>
Date: Mon, 13 Nov 2000 00:14:21 +0000
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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