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From: rees@apollo.uucp (Jim Rees)
Newsgroups: sci.electronics
Subject: Re: NICADs
Message-ID: <36f19f36.b8ab@apollo.uucp>
Date: Fri, 28-Aug-87 15:46:00 EDT
Article-I.D.: apollo.36f19f36.b8ab
Posted: Fri Aug 28 15:46:00 1987
Date-Received: Sun, 30-Aug-87 02:11:48 EDT
References: <8708040222.AA14785@ucbvax.Berkeley.EDU> <646@gec-mi-at.co.uk>
Organization: Apollo Computer, Chelmsford, Mass.
Lines: 30


    Is there any truth in the rumour that using Nicads infrequently shortens their
    life, but using them frequently extends it?

Using them infrequently damages them, but not permanently.  You can recover
the full capacity by putting them through a couple of discharge-charge
cycles.

The worst thing you can do to nicads is reverse-charge them, as happens at
the very end of the discharge cycle when one cell has completely discharged
but the rest of them keep forcing current through the dead one.  For this
reason, you should stop using nicads at the first sign of a drop in voltage.
A cell that has been damaged in this way will appear to have a dead short
internally.  Often you can "fix" these by putting a very large (~10C)
charge current through them briefly (~1 second) to burn off the internal
short.  This fix often only lasts until the next discharge cycle.

The second worst thing is to overcharge them at greater than a 1C rate
(for example, >1 amp for a 1 amp-hour cell).  Up to 1C, the extra power
is harmlessly dissipated as heat, but over this rate, and the cell starts
to dry out.  Old HP calculators came with chargers that way overcharged.
I suspect this was a way of making money on replacement batteries, which
cost $20 for $4 worth of cells (a different kind of overcharge), but
maybe I'm just paranoid.

The best way to treat nicads is to fully discharge them once a month,
stop the discharge immediately when the weakest cell drops below a
volt, then charge them completely at a 1C rate for 16 hours.

This is all based on personal experience and manufacturer's literature.