Thursday, October 18, 2007

Rebuilding Drill Batteries.

Rebuilding Drill Battery Packs

I’ve had my Craftsman 12 volt cordless drill for about 8 years, and I’ve rebuilt the batteries 3 times. The original sub-C cells were 1200 mAh, but I have been using 1600 mAh cells from Radio Shack. I’ve had fairly good results with them, but typically 2 or 3 cells would go bad long before the rest. A few have even leaked. Another problem has been the cells were just a bit oversize, and trying to get them to fit into the battery case has been a nightmare.

It was time to replace the batteries once again in one of the packs, and I discovered Radio Shack no longer carries NiCad sub-C’s. I naturally turned to the Internet, and found a number of outlets for NiCad batteries. One supplier, had the best deal: 10 1800 mAh cells for about $18, plus shipping. didn’t list the manufacturer, but at that price I figured I’d give them a try. I place my order, and in a week my batteries arrived. The were well packed, in what appeared to be the orginal Japanese manufacturer’s box. There were no markings on the cells, but they did have long tabs, and each positive tab had a piece of heat shrink tubing, presumably to prevent short circuits during shipping.

Opening the battery case only required removing 4 screws. I made a sketch of the visible battery connections before removing them from the case, then I carefully removed the batteries part way, taped them together and then removed them completely. I made another sketch of the interconnections on the bottom, and I also noted where the temperature sensor was positioned. I unsoldered the leads from the two batteries at the end of the string and replace the cells with the new ones from I positioned the tabs so I could solder them together without having to use any wire, then removed them from the case and wired the cells together on the bottom. A small piece of insulating plastic that was in the case was carefully repositioned, as was the temperature sensor (using a bit of ordinary masking tape to secure it). Because the batteries were the correct size, I had no problem reassembling the case. I popped the battery into the charger, and in about 2 hours my battery was ready to test. I drilled about 50 holes in hard maple with a 3/8” bit, each about 2” deep, and much to my surprise the drill was still going strong. I guess I’ll be replacing the cells in my 2nd battery pack, which I thought was OK, because the new batteries really outperform the ones in that pack.

Rebuilding a battery pack with cells from cost me less than half the cost of a new pack from Sears, and I expect better performance than from a replacement, given the new cells have a much higher rating than the originals. The total time to replace the cells was well under an hour, and only required decent soldering skills.

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