From: knapp@spot.Colorado.EDU (David Knapp)
Subject: Re: Can a 'dead' nicad be recharged?
Organization: University of Colorado, Boulder
Date: Wed, 9 Sep 1992 05:13:17 GMT

In article <1992Sep9.050836.5806@ucsu.Colorado.EDU> knapp@spot.Colorado.EDU (David Knapp) writes:
>In article <1992Sep8.171432.1@eis.dofasco.ca> fraser_h@eis.dofasco.ca writes:
>>I've just picked up a cordless drill at a garage sale. It came
>>with 2 sets of nicads, one of which will hold a partial charge,
>>the other will not hold any charge (so I'm told by a local
>>surplus supply store where I had the batteries tested). The
>>batteries are about $70 each... is there any way they can be
>>recharged? What's the explanation for nicad's "memory" I hear about
>>all the time, and what's the difference between the chemistry in
>>a nicad and that in a lead-acid battery.
>
>
>Try this. Charge it fully. Discharge it fully. Wait a day. It will now be
>sort of recharged again by itself. Disharge that too. Recharge it again.
>Discharge it again. Do this several times. If no improvement is seen,
>all those little ions have decided to relocate permanently.
>
>I have seen a trick for this, although I have no idea why it works.  I think
>it only works for special cases where 'whiskers' of metal paths form and
>kill the battery. I have no idea how common that is but here goes...
>
>Pulse high voltage, 'high' current through it. The rig I saw used half-
>rectified AC and put one cycle through the battery. If you decided to do this
>make sure the battery is behind plexiglass or something because there is a very
>real chance it could explode. It would also send Cd all over hell, so maybe
>forget about doing that.
>
>Buy new generic NiCads and solder them into the configuration the drill
>requires. There is no need to buy their stupid, overcharged pack.
>
>As far as the difference between NiCad and Pb/H2SO4 batteries, correct me
>if I'm wrong out there but I believe it has to do with ion mobility. As 
>the 'memory' is set, the paths of the ions for charge exchange is established
>as relatively 'free' because of the long distances the ion travel. If you 
>only discharge them partially, the ions are only going a short distance
>in the battery begin to favor short paths as the stucture of the electrolyte
>is 'decided'. If you simply discharge your batteries all the way every
>couple months, you will be able to use your batteries *much* longer. If you
>don't, you will get marginal performance out of them.

Fine, this is bad etiquette, sue me. I forgot to mention the (what I think)
difference is between battery types. NiCads have a pasty-almost solid
electrolyte.  Lead acid batteries have a liquid with a *fantastic* ability
to transfer ions through it in just about any way those ions damn well please.
Ions are not as free to move through a solid or paste as they are in a liquid
(depending, of course, on the liquid). Ions moving in that paste are heavily
constricted in direction or path so they build up channels, literally, to 
move through.  Those channels are the memory.


-- 
David Knapp                                      University of Colorado, Boulder
Perpetual Student                                   knapp@spot.colorado.edu
Newsgroups: sci.chem