Craftsman Cordless Drill Battery Rebuild
I have a 12V Sears cordless drill that has been a much-used tool for the last
three years. The drill came with two batteries but both were about worn
out. 
I
checked Sears to see what a set of replacement batteries would cost.
Imagine my shock when I found they cost $39 each! BOO! The two
batteries would cost almost as much as a brand new drill WITH two
batteries. While that is certainly an option I refuse to throw away a
perfectly good drill. Time to visit the local Harbor
Freight store!
Harbor Freight had a 12V drill and was selling extra
batteries for that the drill for a whopping $5. The battery is the usual
cheap Chinese import but even if it sucks I can buy 8 for what one from Sears
costs. Obviously the two have a different connection scheme but for $5 I
will take a chance.
The Disassembly
I opened up both batteries and found two obvious problems. First of all the two battery packs were laid out differently. The individual cells are spot-welded together with simple steel buss bars. Changing them all around would be very difficult. Second the Sears battery uses one of the cells as the support for the contacts in the protruding part on top of the battery. That means I can't just run wires to the contacts. The good news is that Sears used this same case for a 13.3V battery. That means that there is room for one more cell (occupied by a spacer in my battery). And THAT means the HF battery pack will fit inside the Sears case just as it is. All I had to do was break out the plastic spacer (all the best jobs let me break something).
A
minor issue is the that the Sears batter has a temperature sensor (probably to
keep the charger from overcharging the battery pack). The sensor needs to
be taped to a battery like in the picture. Not a big deal.
After looking at the problem I decided to not try and arrange
the HF battery to match the Sears layout. Instead I figured I could just
cut loose the top cell of the Sears and isolate it so it would no longer be
connected to anything. It would only be in the battery pack to keep the
contacts contacting. So I ran a wire from the positive side of the HF pack and
soldered it to the positive terminal of the top cell
After taping the temperature sensor in place I fitted the new battery pack into the case and insulated to dummy top cell from the pack using some bits from the HF battery pack. When I was done it looked like this:
After that all that was left was replacing the cover. The extra wiring makes the fit very tight and I had to do some tweaking to make it work. The good news is the second battery only took half the time to do. So for around an hour's work I saved at least $70. And when these batteries go bad I will (hopefully) be able to very quickly swap in new packs. Good deal!