There are dozens of YouTube videos on how to do this, using everything from a Mig welder to another battery. My dead batteries were 14.4 volt, and I used a 19.2 volt battery that my father-in-law had laying around doing nothing.
The first thing I had to do was determine the positive and negative contacts on the battery, since they weren't marked. I used a multi-tester for that (YouTube is your friend on that one, too!)
I took the multitester and checked the voltage being delivered from the dead battery. It was about 7 volts.
Once that was done I hooked the negative terminals from both batteries together. Then I put the wire onto the positive terminal of the 19.2 battery, then used the other end an just touched the positive terminal of the dead battery 10 or 12 times. Each touch was about a half-second each.
I then tested the voltage delivery again, and it was now around 15 volts. That was a very positive sign.
Then I put it in the charger. Before when I tried to charge it, the charger told me the battery was faulty. Now, it began charging. Another very positive sign. So I repeated the process on the other three batteries I had and got similar results. So it really does work. For how long I don't know, but for a little while at least I don't have to buy new batteries for two otherwise functional drills.
A couple of things I found while doing this project:
- Don't let the ends of the wires from the source battery touch. All kinds of heat and smoke will result. Ask me how I know.
- Apparently, the source battery has to be of a higher voltage than the dead battery. I did see one video where someone used a car battery for zapping and it seemed to work, but most of the videos said you need a higher voltage.
- Two of my batteries had two terminals and two of them had four. As long as you find positive and negative it doesn't seem to matter which ones you use.
- For the wires, I used some test leads I found, which had alligator clips on the ends and they worked nicely because the terminals on the batteries were blade-types. If you don't have blade type terminals you will have to get creative with your wiring. The test leads weren't very heavy duty. If I were using the welder I probably would have wanted to use heavier wire.
- One video had some separate wires he connected to the dead batteries terminals rather than touching them with the wires directly connected to the source battery. Sometimes this will generate sparks and it can damage the terminals, which means you will have some touch-up work to do after zapping. Touching the wires rather than directly touching the battery terminals makes sure this doesn't happen.