Ni-Cad

Is it possible to rejuvenate a dead Ni-Cad battery?

Offered by Tian.

Lead-acid yes somewhat; Ni-Cad, not really. You can try overcharging for a couple of hours and might get some results, but not 100%.

Offered by Steve.

Rejuvenating Ni-Cad batteries is possible depending on what is wrong with it in the first place. If the cell is not chargeable due to too many recharging cycles then "no" is the answer (electrically warn out). If it received a short reverse charge (typical of single cells when a battery is discharged too much) or has formed a bridge of conducting salts between the plates and is discharging rapidly (not holding a charge for very long), then the answer can sometimes be yes. The way this is done is to zap it with a short burst of voltage and current many times the normal charging rate. Typically, what is done is to charge a large electrolytic DC capacitor to say 40-150 volts then discharge it through a small 1.5 volt cell in the charging direction. This zaps and sometimes burns out the conducting bridge. sometimes it doesn't work if the bridge is too big or too spread out. If done properly the result is the battery will hold a charge longer than it did before zapping it. The cell really goes back to near new leakage. Another way is to quickly arc it across a 12 volt car battery. One caution do all this rapidly if too much heat builds up too fast the cell could theoretically explode before the bridge burns out. I have done both of the above approaches many times and have never had a problem but anything is possible, so be cautious. Wear gloves and safety glasses and stand back. There are times when the bridge is not conductive enough to break the link and no change happens to the battery. I estimate about a 50% success rate.

Offered by Mike.

I have two Motorola Ni-Cd batteries (standard 8 hr.) for my cell phone and one is being used as a spare and used occasionally. This spare one was "dead". I charged several times but it didn't hold the charge. Last night, I followed exactly Mike’s instructions and zapped this dead Ni-Cd with a 12V car battery for about 10 seconds. There was an arch on the first touch, hence I believe there must be a short inside the Ni-Cd, and after that I tested this Ni-Cd and I got the full charge back and it is as good as "new". I think this is important, because after the pole shift, dry cell batteries and Ni-Cd will be scarce, and Ni-Cd is more practical to use for emergency lights like flashlights and so on. Thanks very much Mike, it was a very good suggestion.

Offered by Tian.

You discovered something I forgot to mention. If it is going to work with the battery at all it will arc heavy current flow at first, then each arching attempt thereafter will show a slow current flow as the cell goes into a charging mode. Another thing to note is it works better to do it on each cell of the battery separately. If one tries it on the whole battery one may get varied results. For the full battery (made of more than one cell) there may be one shorted and the rest are OK. The good cells will limit the flow to the shorted one if one attempts this on the whole battery.

Offered by Mike.