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,
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
Step 1: Gather Your Materials
Dead Nicad batteries
Nicad battery charger
Voltage source - we'll use a welder
Step 2: Check Your Battery
If you suspect your charger isn't working you can trickle-charge it from a different voltage source.
To make sure you don't overcharge it put some little lightbulbs from christmas lights in series so the current is below 1 amp. I use 1/2 amp usually.
When you're sure your battery has had a fair chance to charge, check the voltage with your multimeter. Since you're reading this, the voltage is probably a lot lower than the label says it should be.
To see how much current your battery can put out, run the drill. Grab the chuck and stall it to get a feel for how much power it has.
That way you can compare the "before" to the "after" and see which is better.
Science depends on rigorous methods like these.