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.

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

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.

There is a lot of useful info on sealed batteries on the Web. Google is
the usual tool for that.
A good site is : http://www.buchmann.ca/

Frank GM0CSZ / KN6WH