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. Speaking of which,WARNING:
If you get killed by a poisonous explosion it means you did something wrong.
Electrocution is a real possibility also.
Ask your parents how to not electrocute yourself with a welder.
If your tool has a non-battery problem, here's how to fix that.
Watch the video and see how zapping is done.
Excellent stills and video shot by Fungusamungus
Excellent Video editing by Noahw