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From: crandell@ut-sally.UUCP (Jim Crandell)
Newsgroups: net.physics
Subject: Re: lifetime of rechargeable batteries
Message-ID: <302@ut-sally.UUCP>
Date: Wed, 21-Nov-84 20:26:14 EST
Article-I.D.: ut-sally.302
Posted: Wed Nov 21 20:26:14 1984
Date-Received: Sat, 24-Nov-84 21:04:39 EST
References: <1789@garfield.UUCP> <9900002@acf4.UUCP>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 21

> Now the rumor that I hear is that if you let a nicad discharge totally, you
> get to buy a new one.

If you mean a battery of several cells connected in series rather than
a single-cell ``battery'', then you're right.  Problem is that it's
generally impossible to get two cells with exactly the same amount of
charge in them, so if you wire them in series and, say, put a load
resistor across the combination, all goes as expected until the one
with the lesser charge completely discharges.  After that, the one
with the greater charge continues to discharge by ``charging'' the
discharged cell.  You'll notice, however, that the polarity of the
voltage that the discharging cell applies to the discharged cell is
incorrect for charging.  The damage caused by allowing this condition
to persist for several minutes (to say nothing of hours) is usually
not reversible by electro-chemical means; hence you're out the cost
of at least one cell, if you're lucky enough to be able to replace
them separately.
-- 

    Jim Crandell, C. S. Dept., The University of Texas at Austin
               {ihnp4,seismo,ctvax}!ut-sally!crandell