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From: ftpam1@acad3.fai.alaska.edu (MUNTS PHILLIP A)
Newsgroups: sci.electronics
Subject: Re: rechargable battery "memory"
Message-ID: <1990Aug9.223441.19242@hayes.fai.alaska.edu>
Date: 9 Aug 90 22:34:41 GMT
References: <105@javelina.cs.arizona.edu>
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In article <105@javelina.cs.arizona.edu>, cliff@cs.arizona.edu (Cliff Hathaway) writes...
>With the plethora of cordless, rechargable appliances and tools in our
>machine room tool cabinet, the question of the which charging schedule 
>will prolong the life of the batteries and provide a reasonable useful
>charge has come up.  The best information we have is that the batteries
>should not be recharged until they have been drained to avoid giving
>them a memory (e.g., if a fully-charged flashlight is consistently used
>for 5 minutes, and then recharged, it will soon develop a memory, and will
>only shine for 5 minutes before it is discharged).

     There was an article in "Power Electronics" magazine a couple of years ago
about this very topic.  The consensus of the manufacturers was that there ain't
no such thing, or least they can't duplicate it under lab conditions.

     The problem is with overcharging:  Nickel Cadmium batteries (the kind
used for virtually all small rechargeable battery packs) are just like car
batteries in that the electrolyte is mostly water.  If you overcharge, the
water evaporates, just like in your car.  But NiCads are sealed so you can't
replace the water.  (I have seen wet-cell NiCads from the phone company.)
When you discharge a battery for 5 minutes and then run it through a full
charge cycle, you inevitably overcharge it.  If you always run your batteries
down before charge, they don't overcharge.

     This is what I read and it fits my own small experience.  Incidentally,
I hear rumors about a "millenium" cell from Gates that comes with a lifetime
guarantee: if it ever fails to hold a charge, they will replace it.  Perhaps
they have found a way to lick the electolyte loss problem.

Philip Munts N7AHL
NRA Extremist, etc.
University of Alaska, Fairbanks