Re: reverse biased Ni-Cad cells

From: Dave Platt (dplatt_at_radagast.org)
Date: 12/21/04 

Date: Tue, 21 Dec 2004 05:12:22 -0000

>A quick check on a 6 pack containing D size Ni-Cad
>revealed one with -0.4V, another with -0.1V. I know
>that's not a very good thing, so here are my questions:
>
>a) is there some simple circuit that could be used
>to prevent this? I'm thinking of a parallel Schottky
>diodes, but preferebly with a Vf under 0.1V.

I don't think you can protect individual cells against this with a
simple circuit. Reverse-biasing tends to occur as a result of
overdischarging a battery pack... one cell drains to zero before the
others, and the others continue to push current through the drained
cell, in effect "charging it backwards". You can't prevent this with
a simple diode protector before some amount of damage occurs, I fear.

The best thing to do is stop discharging the pack when the voltage
drops to a certain threshold. I've heard that 1.0 volts per cell is a
really good place to stop... there's very little useful charge
remaining in the pack at this point, and this is usually high enough
to stop the discharge before any one cell reaches 0 volts.

>b) how much damage can reverse voltage cause,
>and how quickly?

I understand that it causes the growth of dendrites (thin fingers of
metal) inside the cell, through the separators. In effect, the cell
develops internal short-circuits, which can cause the cell to
self-discharge quite rapidly each time you recharge it.

>c) what should be terminal voltage when discharing
>Ni-Cads? I plan to build a discharger from a battery
>holder where they would discharge through some
>series diodes and a resistor.

If you *must* discharge NiCd packs for some reason, don't go below 1.0
volts per cell.

My understanding is that full discharge of NiCds is not actually very
useful at all. Doing it is beneficial only under fairly specialized
conditions, and doing it improperly is more likely to damage the
cells that it is to help. 

-- 
Dave Platt <dplatt@radagast.org>                                   AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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