Revitalising Ni-cads
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I have spent several years involved in varous applications for all types of cells, both primary and secondary, as well as keeping their useful service life as long as possible for both economic and maintenance reasons.

Recently, a local amateur requested suggestions on how to re-vitalise ailing ni-cad cells, and the following is based on my knowledge and experience.

With Ni-Cads, what eventually happens is that microscopic conducting crystals grow across the two plates inside the cell, probably spirally wound if a "cylindrical" cell. Initially they are so tiny that they only conduct a tolerable amount of current and generally the user doesn't even notice their effect, which, over a period of time, slowly increases the apparent rate of self-discharge. Eventually, there are so many of these crystals across the plates, in parallel electrically speaking, that the cell appears to the charger (and user) that it is a short circuit.

Also the outside of the cell, which is never quite completely sealed, suffers similiar "growths" which contribute to the self-discharging external to the cell.

There is a suggested method of effecting a cure, at least to some cells not too far gone. This only applies to cylindrical cells of C size or larger. DO NOT USE ON BUTTON / DE-AC CELLS / SEALED CELLS.

Initially, remove the heatshrink from the outside to get down to the bare metal can. This can be replaced later, with new heatshrink.

Remove the wires - if cells connected with tagging strip, cut in middle so you can later reconnect using wire if necessary. Wash and scrub the cell vigorously, initially in hot water with a dash of washing up liquid, to remove all traces of the crystals and greasy deposits. Rinse very thoroughly in many changes of water,(or under a tap) if possible the last rinse in distilled water. Dry completely, with special care around the top sealing area, and leave on a warm tank / radiator / somewhere warm overnight, to ensure it is completely dry. Solder the wires back on, or reconnect the tagging strips.

Now, the clever bit that should work with most cells.

The aim is to blow the little crystals across the plates to bits!
This will stop the undesirable discharge! To do this needs a very high peak pulse of current, and one way of doing this is as follows:

Obtain a high value capacitor(s) - say up to 50,000uF at up to 40 volts, with a high ripple current rating and a method of charging them up, power supply or batteries. If you use several, hook them up with heavy wire - we want many amps to flow. Charge caps to approx. 30 V.

Discharge the capacitor into each dead cell individually, several times.

CAUTION - THERE IS GOING TO BE A BIT OF A SPARK, HEAT AND PUFFS OF SMOKE. USE GOGGLES AND INSULATED PROBES AND DON'T LET YOUR FINGERS "GET IN THE WAY".

If possible also charge at around 10-20 amps for a few seconds, to "burn off" any remaining bits. A car battery and two headlamp bulbs in parallel, will provide a high peak current, settling down to about 10 amps after about 1 second. Use a fuse in line - 35 amp type.

Give the cells a spray of WD 40 and re-heatshrink them up.

Hopefully most will have been revitalised for more service. It is good for them to have the occasional high rate charge, though bad to overcharge them - i.e. get them too hot. Also, shallow cycling and low rates of discharge result in them not giving of their best.

I hope this helps at least some cells live longer before they are committed to the bin prematurely.

No responsibility will be taken for the above suggestions - If you have any doubts, don't do it.

73, Nick G0HLX @ GB7WOK

PORTED AND REALIGNED FROM PC TO AMIGA FORMAT BY BOB G0LBQ - N6XDI.