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From: myers@hpfcrlm.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: NiCd behavior
Message-ID: <17660178@hpfcrlm.HP.COM>
Date: 24 Jun 91 18:35:46 GMT
References: <1991Jun15.023737.2673@mccc.edu>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 69

>Okay, I'm a consumer I spent $50 (35 UK Pounds) on a consumer NiCad pack for
>my camcorder. Originally it gave me 40 minutes recording time, but now it's
>down to 15 minutes and falling fast.

You didn't mention how old (either in months/years or charge/discharge cycles)
the pack is, so I'm assuming that you think it's dying before its time.
(These things DO NOT have an infinite life, but I'm guessing that you expected
a lot more life than you got.)

Also, how are you charging the pack?  IS IT definitely a NiCd battery (many
of the camcorder packs are lead-acid gel cells, and these behave differently)?
Has the pack ever been exposed to temperature extremes or very excessive
charging?  Are all the cells mechanically intact (no dents, holes, etc.)?

Without knowing this, it's hard to be specific about this particular case,
but I'll at least post some further general information:

The "voltage depression" phenomenon I mentioned (the one which is mistaken
for "memory) happens if a NiCd cell/battery is overcharged; should overcharing
occur, the cell's output voltage will be decreased on the next discharge.
This occurs solely due to the overcharge, regardless of the previous discharge
cycle, and so is not what we'd call "memory" - however, overcharging IS more
likely after a short discharge, due to the way most people recharge NiCds
(put 'em on the charger and leave 'em until the next morning or whenever, 
regardless of how long they were discharged).  This explains the belief in
"memory" - many people DO see an apparent loss in capacity after a short
discharge cycle - but it wasn't the short discharge that caused it; it was
the excessive charge.  They've just picked the wrong cause.

Unfortunately, most consumer gear and chargers don't treat NiCds very well.
The simplest way for a camcorder, say, to figure out how much battery capacity
remains is to monitor the cell voltage, and trip an indicator when this falls
below a certain level.  Unfortunately, that's a bit simplistic - in the case
of voltage depression, the "LOW BATTERY" indicator will come on fairly early
in the discharge cycle, even though the cell has capacity remaining - it's
just brainlessly looking at an absolute level.  Similarly, the chargers
usually have no means for monitoring the progress of a charge - they don't
watch either the battery voltage or temperature (two important things to 
look out for, the latter especially important if "quick charging" the cells),
and so don't know when to quit.  They will very happily drive the cell into
an overcharged state.

NiCds have a very distinctive discharge characteristic - the discharge starts
with a short, rapid drop from, say, 1.4V or so down to the nominal cell voltage
of around 1.2V.  Here it will stay for most (about 80% or so) of the cycle,
dropping very slowly.  As the cell nears the end of the cycle, a sudden drop
begins again - this time, it'd go damn near all the way to 0V if you let it.
(And in a battery, there is at least one really good reason for not letting
this happen - you don't wanna reverse charge any of the cells.)  These two
"knees" are an important clue to where you are in the curve.  You should stop
charging soon after reaching the upped "knee" (or if a similar rise in cell
temperature is seen), and stop the discharge soon after the lower knee is
reached.  

In your case, I don't know if the cell is really low (reduced capacity), or
if it's just the camcorder SAYING it's low.  Here, I would suggest a forced
discharge, but only because it'll get the pack into a known state.  Discharge
it until you're down to around 0.8-0.9V/cell - for a six cell (nom. 7.2V)
battery, this would be a battery voltage of about 4.8-5.4V.  Then charge it 
until you see no more than 1.4V/cell - again, a six-pack would be then at
8.4V.  Try the pack then, and see if it gives "normal" service.  If you cannot
restore the pack to normal operation after a couple of these cycles, there
is something wrong - you have a shorted cell, one or more cells have vented,
etc..  It would then have to be replaced.


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.