One of my favorite makers, Tim, has a good how-to on bringing some old ni-cads back to life using a welder – this is one of those that you shouldn’t do unless you know what you’re doing, k?
“Nicad batteries often die in such a way that they won’t take a charge and have zero voltage. This usually means they’re shorted out by crystal dendrite growth.
Here’s a method of bringing them back to life by zapping those shorted crystal dendrites away with too much current and/or voltage. We’ll use a welder as a power source. You could also use a car battery, a DC powersupply, or almost anything with some voltage. Charged-up capacitors are popular for this because you can get a very fast pulse out of them and still limit the power. it’s a lot safer that way…” – Link.