Recovering a “dead” cordless drill battery

A couple of weeks ago, when starting a project, I discovered that one of my batteries for my 18V Dewalt cordless drill didn’t work. It wouldn’t make the drill even flinch and there was no voltage at all across the terminals. No problem, I’ll charge it. When I put it in the charger, the blinking light reported that the battery pack was “dead” and charger wasn’t going to attempt to charge it. Since a new battery costs $80, I figured I’d investigate.

I took the battery pack apart, hoping that one cell had gone bad and that I could replace it. I discovered that there weren’t any shorted or open cells. After reading a few discussions on usenet and on a hunch, I thought that maybe the charger was just being overly conservative in not charging the battery due to its low voltage.

Ok, first a disclaimer: don’t try this at home. If you blow anything up or catch yourself on fire, it’s your own fault. I took my fully charged good battery and my “dead” battery. I took a couple of short wires and temporarily attached the positive terminals to each other and likewise with the negative terminals. I let it sit that way for about a half a minute, closely monitoring the temperature on both batteries. After disconnecting them, I checked the “dead” battery, and it showed a couple of volts across the terminals! Woo hoo! I put the battery back on the charger- this time it worked! I’ve run through a couple of cycles now, and the battery performs just well as the “good” battery! I theorize that my hunch was true, but all I know is works now!

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