From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: sci.chem,sci.energy
Subject: Re: Can a 'dead' nicad be recharged?
Date: 9 Sep 92 21:28:13 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group

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
Xref: sparky sci.chem:3457 sci.med:17046 sci.engr.chem:472 sci.engr.civil:437