From: dogbowl@dogbox.acme.gen.nz (dogbowl)
Newsgroups: sci.chem,sci.energy
Subject: Re: Can a 'dead' nicad be recharged?
Keywords: high current
Date: Thu, 10 Sep 92 00:32:54 Y
Organization: The Dawghaus BBS

cescript@mtu.edu (Charles Scripter) 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
> > 
> > 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.

This works if the batteries are suffering from 'memory effect'
( actually memory effect is very rare these days. )
Note that when you are cycling the batteries, you will achieve best results
if each battery is discharged individually. The reason for this is that
if the batteries are in series when they are discharged, one will
invariably discahrge before the other(s) and then be reverse charged.
This is extremely detrimental to battery life, and is the usual reason for
battery pack failure.

( Nicads are best discharged in parallel, and charged in series )

> 
> I think I also heard that "shock them" with high voltage (need to
> be able to hit with a few thousand volts for short times).  This
> allegedly burns out the internal shorts.  I've never tried it, so
> you're on your own.  If the batteries are completely dead, you've
> got little to lose....
> 

Close, but not quite. The voltage required is not high, but the CURRENT
is. This is a technique to use when you find that a battery will not
hold its charge for more than a few days ( note that Nicads have an
inherent 1% per day self discharge rate ).

The reason for Nicads not holding their charge is usually a crystalline
growth inside the battery. The high current 'zap' is an attempt to burn
it out. It is best to 'pulse' the current. Only 'zap' individual batteries.
Do not leave a battery connected to a high current source for more than
a 2-5 seconds at a time or it may explode or vent.
If it starts to feel warm, STOP.

( I usually zap batteries with a current of 20 times their mAh rating )

Note also that if a battery has been reverse charged, or developed a high
internal discharge rate, that these procedures will only prolong the
inevitable - sooner or later you will have to get a new pack, so start saving.

As for the prices, look around. Nicad packs tend to be expensive, wheras
if you can obtain the batteries that go inside them from your local
electronics store, you usually will get them substantially cheaper, although
you must take care to get batteries with the appropriate charge/discharge
characteristics. Batteries designed for calcuator usage, for instance
will not last very long if used in a high discharge current situation like
a cordless drill.




dogbowl at The Dawghaus BBS  ( +64 6 357-9245 )