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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: NiCd recharging
Message-ID: <17660114@hpfcdj.HP.COM>
Date: 17 Sep 90 19:02:47 GMT
References: <1990Sep7.132221.6202@ux1.cso.uiuc.edu>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 36

>The conditions that cause the memory effect are just *not* present in
>typical use, even highly cyclical use.  Probably the best way to avoid
>the memory effect is to take the batteries into *overcharge* when recharging.
>This is the typical recharging method, by the way.

That's one way, but unfortunately, repeated high levels of overcharging
(over-overcharging? :-)) can also reduce cell capacity and shorten service
life (esp. if operating/charging at elevated temperatures).

The *best* way is to employ a charging system which, through monitoring
cell voltage, temperature, or both, knows when to quit.  Unfortunately,
this is an added expense that few manufacturers of NiCd-using consumer
equipment are willing to put up with.

There's another design problem involved, one which I was reminded of by
some e-mail on this subject.  NiCds have a lower nominal cell voltage than
carbon-zincs or "alkaline" cells to begin with, but a fairly flat discharge
curve.  Some designers, though, don't correctly account for the somewhat
lower voltage, and the equipment winds up with too high a "cutoff voltage"
(that voltage at which the product stops working properly, or signals
"low battery") - meaning that the user will think the battery is low even when
plenty of capacity remains.  This has the added "benefit" of making it more
likely that the battery will be put back in the charger, encouraging
further overcharging and further shortening the battery's life.  A vicious
circle. :-( (And of course, after just a few times around this loop, the
user winds up with a dead pack, curses yet another "example" of "nicad
memory", and starts the process of killing yet another battery.)

NiCds *are* an extremely reliable power source, but you can't just slap 'em
into a design in place of carbon-zincs, add a cheap charger, and expect 'em
to be happy.


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers@fc.hp.com                          |  sentient life-form on this planet.