NMIH batteries - was: Re: HP 25 and HP55

From: Dwight K. Elvey <dwight.elvey_at_amd.com>
Date: Fri Oct 22 11:01:43 2004

>From: "Dave Dunfield" <dave04a_at_dunfield.com>
>>>> -- I took apart the 25's battery pack and replaced the nicads with
>>>> nickel metal hydride cells (the same ones I use in my camera)
>>> Not a good idea. Nickle metal hydrides need different types of
>>> charging. The won't last long with a NiCad charging circuit.
>>I won't charge them in the calculator, they are regular AA cells so I
>>can remove them and put them in a proper charger. :^)
>Here's a question for battery guru's ... I have a gadget I built for
>preventing "memory effect" in various nicad powered devices - mainly
>ones like my Digicams which only discharge the battery 1/2 way (or
>less) before complaining that the battery is low.
>It is basically two springy metal bars which lets me put up to 10 AA or
>C cells in parallel to discharge (I've got portable _at_M and 70CM rigs powered
>by a C cell pack), an Ammeter, a low resistance, and a stud rectifier diode.
>Basically, it draws about an amp from the cell when they are near fully
>changed, and trickles down to nothing as they approach .7ish volts, and
>never lets them go to zero.

 Actually, NiCad's can be discharged to zero without problems. It
is tha back charging in stack, as batteries, that causes the problems.
In fact, NiCad's are often shipped with shorting bars across the

>I works REALLY well with nicads - I've got cells that I've been using in
>my radios for many years and they still work very well (Prior to using
>this, they would noticably lose capacity after a while)...
>But: It doesn't seem to be quite as effective with NMIH cells ... they
>still seem to be losing capacity ...

 These cells shouldn't be discharged to zero. Just different technology.
One thing you might try ( as an experiment ). Hit them with a momentary
high current load. This is done on NiCad's to improve their performance
and may have a similar effect on nickle metal hydrides. It does need
to be controlled because like NiCad's, once the burst seal is broken,
expect them to be worthless in a few months.

>Any explaination, comments etc.?
>dave04a (at) Dave Dunfield
>dunfield (dot) Firmware development services & tools: www.dunfield.com
>com Vintage computing equipment collector.
> http://www.parse.com/~ddunfield/museum/index.html
Received on Fri Oct 22 2004 - 11:01:43 BST

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