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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Batteries!
Message-ID: <17660131@hpfcdj.HP.COM>
Date: 10 Dec 90 19:51:20 GMT
References: <5664@uafhp.uark.edu>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 47

>NiCads -- Can be overcharged all you want.  For the space, you get less a*H's
>	then with Gel Cells.  Major problem:  They form memories.  In an
>	application like this, where I charge for awhile, drain (probably)
>	a very small amount of the capacity of the cell, and they recharge,


Nope.  Exactly backwards, I'm sorry to say, but this is the common 
misunderstanding regarding NiCds.  I've posted some articles with more detail
(and will re-post or e-mail if there's enough demand), but the basic story
is that the problems of NiCd "memory" are vastly exaggerated; a true
"memory" problem requires some very specific and unusual circumstances, and
very, very, rarely happens in real-world applications.  (So seldom that one is
tempted to use the word "never.")

The real problem with NiCds, and what usually gets labelled "memory", is 
actually called "voltage depression" and occurs when the cell has been
overcharged.  When overcharged, the cell voltage will drop sharply fairly
early in the next discharge cycle, causing the *appearance* of a loss in cell
capacity (especially in equipment which monitors cell voltage to trigger a
"low battery" light or some such).  The cell will recover undamaged if it
is properly charged in the next cycle.

Unfortunately, the vast majority of NiCd chargers, such as for consumer
rechargeable equipment, are too simple to detect the completion of a charge
cycle, and will happily continue to fill the cell well into overcharge.  Thus,
if the user puts the cell in a charger after only a partial discharge, the
cell is very likely to go into overcharge and exhibit voltage depression on the
next discharge cycle - giving rise to the notion that it has developed a
"memory", even though the depth of discharge is NOT really the problem.  (This
has, in turn, led to people constructing "deep dischargers" and other silly
things to attempt to combat a non-existant problem, when in reality all they're
doing is wasting a charge.)

What you really want to say about NiCds is that they can be left on "trickle
charge" forever, but only if the charger is smart enough (or the trickle
current low enough) so as to keep the cell from overcharging.

Sealed lead-acids also trickle-charge nicely, and aren't quite as finicky about
a slight overcharge.  (You do not, of course, want to overcharge them
severely either.)  Gel-cells would probably be my choice for such an
application as the one described, but in EITHER case I'd definitely want to
invest a little effort on the charging system.


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.