I have taken a 6 volt DC transformer and recharged watch batteries, had one blow up but with gloves on there was no damage, I think one of those would do quite a bit more damage though!
Ni-cad batteries develop a "memory" after a while and won't charge up to their full capacity. They need to be fully discharged in order to get their full capacity back.
To do this, go to radio shack and get an approx. 50 ohm resistor of any wattage, hook one lead of the resistor to one end of the battery, hook the other lead of the resistor to the other end of the battery and let it sit overnight, then hook a piece of wire across the battery the same way so that the battery is shorted out, and let it sit for an hour. The purpose of this is to completely drain the battery without shorting it out while it has a charge, if you short it out with the wire while it has a partial charge, you could damage the battery or the wire will get so hot it will melt or start a fire.
After the battery is completely discharged, it's ready to charge up again and it should have it's full capacity back. If it doesn't, it's time for a new battery.
This also may work with some types of batteries other than ni-cad.