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.
Step 3: Check Your Welder
Turn the knobs to see what voltage range the welder puts out when no current is flowing.
This one puts out about thirty volts at the max setting.
Step 4: Zap the Hell Out of the Frickin' Battery!!
Tap the positive end of your welder to the "plus" terminal of your battery
while holding the negative end to the battery's "minus" terminal.
You should see some sparks and nothing should get welded to anything.
No welding please. If you get killed by a poisonous explosion it means you did something wrong.
It should feel like something good is happening.
Step 5: Try out the battery and See if it Worked
Try out your battery. It ought to be much better almost immediately.
Step 6: Zapping Individual Cells
Pete Lynn dropped this battery pack in salt water. It shorted out the cells and it has been fully dead for a year or so. We peeled it apart to get at the individual cells. We scraped the salty cardboard off them and zapped them with a car battery. After that it worked fine.
It's easier to zap an individual cell than the whole pack at once.
Sometimes you can't revive a cell. You can cut or unsolder it from the others and replace it with a good one.