I have two Ryobi 18V kits (drill, circular saw, sawzall, dustbuster, flashlight). I got the 2nd kit (which came with 2 batteries & a charger) for the same price as 2 batteries – so now I have spares of all my cordless tools!
Recently, one of the batteries stopped taking a charge. There’s a somewhat famous video out on the interwebs in which a welder is used to zap a dead cordless tool battery back to life. I don’t own a welder but I read elsewhere that any relatively high current DC power source could be used effectively. So I plugged in my 12V (car) battery charger and set it to its highest setting (in this case, 6 amps). Holding the negative clamp against the (-) terminal on the battery, I then very briefly tapped the positive clamp against the (+) terminal on the battery. I then popped it into one of my flashlights. It was supposed to get brighter, but it didn’t. I tried again. Same result. I tried a third time, this time holding the positive clamp to the battery’s (+) terminal for a half a second or so. When I tested it in the flash light, it was clearly brighter. I pulled it out and increased the zap duration to about a second. Even brighter. I then stuck the battery into the charger and left it over night. The next morning, it had a full charge – and a strong one. That battery is now as strong or stronger than any of the other three.
Apparently, the science at work here has to do with the build-up of some sort of crystals inside the individual battery cells. Over the life of the battery, these crystals eventually form a bridge from the positive terminal (of an individual cell) to its negative terminal. This renders the cell incapable of taking a charge. The individual battery cells are 1.2V, so in the case of an 18V battery,there are 15 individual cells. Losing one cell leaves you with 16.8V. Losing 2 or 3, you’re down to 15V or less. Once you lose enough individual cells, the remaining good cells don’t have a path for properly charging. (And in some cases, the charger can sense a problem and refuse to charge.) What the high current DC zap does is liquefy those crystals, restoring the individual cells and making the whole battery good again.
1. This is for NiCD batteries ONLY – DO NOT try this with LiON batteries. Some people say it works with NiMH, others say it doesn’t (or advise against doing it.) If you have a dead NiMH battery, do some googling and use your own judgment.
2. Reportedly, this can make your battery EXPLODE, so wearing protective clothing (and eye protection) is strongly advised. (Personally, I’m inclined to believe that if something is going to blow up, there’s likely to be some heat that will build up as a warning sign. My battery didn’t show any sign of heating up and everything went smoothly. Your mileage may vary.)
3. I am providing this for informational purposes ONLY. DO NOT actually do this. There. If you try this and your battery blows up, I TOLD YOU NOT TO, so don’t blame me.
-- Founding member of the (un)Official LumberJock's Frugal Woodworking Society - http://lumberjocks.com/topics/29451