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From: tomk@orca.UUCP (Tom Kloos)
Newsgroups: net.analog
Subject: Re: an observation,a new angle on batteries
Message-ID: <1333@orca.UUCP>
Date: Thu, 31-Jan-85 00:53:41 EST
Article-I.D.: orca.1333
Posted: Thu Jan 31 00:53:41 1985
Date-Received: Sat, 2-Feb-85 10:13:11 EST
References: <215@abnji.UUCP> <597@amdcad.UUCP> <559@mako.UUCP>
Reply-To: tomk@orca.UUCP (Tom Kloos)
Distribution: net
Organization: Tektronix, Wilsonville OR
Lines: 24
Summary: 

Far worse than not discharging a nicad battery completely is reverse
charging it.  This can easily happen in multi-cell packs where one cell
discharges to zero before the rest of the cells do.  As the other cells
continue to supply power, the dead cell is 'charged' backwards.  This
unfortunately seems to contribute to partial or total shorts, thus
leading to much shorter (no pun intended!) cell life.

If your drill has only a couple of cells, it's likely that the motor
won't provide useful torque before the weaker cell can be damaged.
In pieces of equipment with many cells in series (like my 2 meter
ham handheld), the equipment will still function reasonably well as the
battery pack becomes weak, thus easily allowing significant operation
with a cell under reverse bias.

Since nicads develop 'memory' only after many identical
charge/discharge cycles, it's probably safer as a general rule to start
charging before any one cell has a chance to discharge completely.  If
memory becomes a problem, cycling the cells to zero INDIVIDUALLY a
couple of times is the best way to fix them.  That way, the reverse
bias problem can be avoided.

-Tom Kloos, WA7NJK, Tektronix, Wilsonville, Oregon
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