I have an 18 volt battery from a milwaukee cordless drill that is dead. Does any one know how to repair them?
Once dead, they are dead. Time to buy a new battery pack. But, if you are dead set on repair, then take the battery pack apart. You'll notice it is nothing but a bunch of 1.5 volt batteries (probably NiCad) connected in series. You could replace them with store bought. But I would not advise this!
I don't recommend trying to repair the cell. Take it to your local hardware or home improvement store. They have recycling centers there. Do not just throw the battery away! Replacement batteries can be expensive, but if the tool is still in good shape it's cheaper than buying a whole new drill. You might check internet sites for better prices on replacement batteries. If you do decide to replace the tool, you can donate your old one with a dead battery to Habitat For Humanity.
Normally when the rechargeable battery pack won't recharge anymore people just go and buy a whole new battery pack. If you really want to repair the pack, you could take the battery pack apart then take it to a battery store that specializes in selling just batteries. All the pack consists of is several round (cylindrical) batteries (cells) that are hooked together via wiring. Normally the wiring is welded to the battery ends. If you take the opened up battery pack to the battery store they can replace the battery cells and wire the new ones together like they should be. It may cost less than a new battery pack, but I'm not sure. I replaced the batteries in my rechargeable shaver once when I was tight on money and my shaver gave up the ghost. It cost me $6 to replace the rechargeable batteries compared to $40 for a new rechargeable electric shaver.
