On Sun, 24 Jul 2005 10:46:35 +0100 "Highland Ham"
wrote:

I am sure this has been posted before, but I cant recall the response.
I have several 12V gel cells out of UPS's which are dead.
Apparently they can be revived by desulfating them - any ideas on the way
to do this?

Apply briefly a high current to break the sulfate by charging a high
capacity electrolytic capacitor (10000microFarad) to say 40 -50 V ( or even
higher) and discharge via battery.

That's the "trick" for possibly fixing a shorted NiCad, but it won't
fix a sulfated lead acid cell. Sulfated cells can only be recharged
extremely slowly. They will exhibit a high cell voltage while doing
this, but trying to charge them faster with higher current will only
electrolyze the water in them and damage the plates.

The reason for having a 'dead' sealed battery can also be that the battery
has been overcharged resulting in lack of electrolyte.

Absolutely right! There's no way to charge a lead acid battery that is
dry. You can add water to some, but some are so well sealed that you
have to destroy them to get in there.

Some people have been successful to 'revive' an electrolyte starved battery
by getting access to the rubber vent caps ,removing these temporarily and
injecting a minor amount of distilled or de-ionised water with a syringe.

A sealed battery is NOT necessarily a 'gel' type.

Right, but you can tell a lot by the voltage across the cell while
it's trickle charging. Gel cells, starved lead acid cells, and regular
lead acid cells all share the same chemistry and will show 2-2.4 volts
per cell, NiCads and NiMH are 1.2-1.4V, and Lithium cells run about
3V. Of course none of this works if the cell is dry.

A LOT of these die just by drying out. Injecting a bit of water is a
good idea if you can get in there.

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Jim Adney
Madison, WI 53711 USA
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