From: Scottl@mot.com (Scott Lindsey)
Subject: Re: Can a 'dead' nicad be recharged?
Organization: Motorola Land Mobile Products Sector
Distribution: us
Date: Thu, 10 Sep 1992 16:02:42 GMT


Thanks for the insight below... Apparently this implies that batteries such as
camcorder batteries can not be revived in this manner because they consist of
several cells in series (in light of their voltage of 6 or so volts).

Has anyone successfully taken apart a camcorder battery??? The $50 that it costs
to replace seems more and more reasonable as I contemplate 1)extracting the
bad cell 2) zapping the bad cell or trying to inject some distilled water, 
and 3) attempting to reassemble it...

In article <1992Sep9.212813.7081@ke4zv.uucp>, gary@ke4zv.uucp (Gary Coffman) writes:
> 
> In article <1992Sep8.222410.9895@cs.yale.edu> dave@master.chem.yale.edu (David Maxwell) writes:
> >From article <1992Sep8.171432.1@eis.dofasco.ca>, by fraser_h@eis.dofasco.ca:
> >> I've just picked up a cordless drill at a garage sale. It came
> >> with 2 sets of nicads, one of which will hold a partial charge,
> >> the other will not hold any charge (so I'm told by a local
> >> surplus supply store where I had the batteries tested). The
> >> batteries are about $70 each... is there any way they can be
> >> recharged? What's the explanation for nicad's "memory" I hear about
> >> all the time, and what's the difference between the chemistry in
> >> a nicad and that in a lead-acid battery.
> >> 
> >> -- 
> >> Hugh Fraser					Dofasco Inc.
> >> (fraser_h@eis.dofasco.ca)			1330 Burlington St East
> >> (416)544-3761 X6941				Hamilton, Ont. L8N 3J5 Canada
> >
> >I remember reading about this in some computer magazine which
> >described laptop batteries.  The person suggested that the
> >full charge could be restored if the battery was completely
> >discharged and recharged a couple of times.  The technique was
> >to discharge with decreasing loads levels until the battery is
> >dead.  For example, start with a small electric motor and 
> >work yourself down.  If the battery is going to work at all,
> >this should do it.
> 
> Do NOT do this. Never completely discharge a multi-cell NiCad
> battery. The weakest cell will discharge first and then reverse
> charge. This will damage the cell and eventually cause it to
> fail. If a NiCad pack develops voltage depression, it's ok to
> discharge each cell individually, but never as a complete pack.
> To avoid reversing a cell, never discharge a multi-cell pack
> below 1.0 volt per cell.
> 
> Voltage depression, miscalled "memory", can occur under specific
> charge/discharge conditions. It is a temporary condition, and not
> harmful to the battery. It is caused by doing repeated partial
> discharges to *exactly* the same charge level followed by *over*
> charging of the pack. If you use a *proper* NiCad charger that
> senses the rapid temperature rise, or sharp voltage drop, as a 
> NiCad battery reaches full charge, and switches off charging at
> that time, you won't ever see voltage depression.
> 
> If you do get a pack with voltage depression, fully charge it
> at the 10 hour rate, then *carefully* discharge it with a 
> *constant current* sink at the 10 hour rate for no more than
> 9 hours. A resistor, lamp, or motor is not a constant current
> sink and should not be used. You can make a constant current
> sink with a common 3 terminal regulator for low current draw,
> or a 723 and pass transistors for higher currents. You shouldn't
> make a practice of this, get a proper charger so it won't happen
> again.
> 
> NiCads can fail in three ways. The most common cause of failure
> is over charging which causes the cell to vent and dry out. This
> is usually fatal, but sometimes you can inject distilled water
> into the cell through the vent and rejuvenate it. The second most
> common failure is a cell short due to dendrite growth. This is
> usually fatal too, but sometimes you can temporarily "blow" the
> short away by rapidly discharging a large capacitor through the
> cell. This is risky and can cause cell explosion if you use too
> large a capacitor charged to too high a voltage. The third failure
> mode is a reversed cell. This is almost always fatal, but sometimes
> you can *slowly* trickle charge the cell in the forward direction
> and recover it. All of these recovery tricks must be done on 
> individual cells, NEVER on a multi-cell pack. If you have a failed
> pack, you have to take it apart and deal with the cells individually.
> Otherwise, you will damage *other* cells in the pack while trying
> to salvage the damaged cell.
> 
> Gary
> 
>