Saving / Upgrading an Old Cordless Screwdriver to NiMH

I've got a handy little cordless screwdriver which didn't last long with the stock NiCd (Nickel-Cadmium) batteries. Seems like the factory batteries we're really sorry. Worked good for the first six months - then, as usual, wouldn't hold a charge. Such a shame to toss such a handy little device just because the factory went cheap on the batteries.

Well, I have a bunch of NiMH (Nickel Metal Hydride) batteries laying around from another project (don't ask). These AA batteries I have came from Amazon.com at $2 bucks, each. I think I bought three dozen to get that price.

Anyway, the four junk NiCd batteries are C size, but only 2/3 height, rated at 1400 mA-h. So, my 2500 mA-h AA batteries are going to be much more capable. Plus, the NiMH have a much better discharge curve that Ni-Cd.

Step 1: Opening Space

There were a bunch of ribs molded into the plastic to hold the odd shaped Ni-Cd batteries. Using a pneumatic die grinder with a cut-off wheel, plus a grinding stone wheel, it was just a matter of determination to get rid of all the plastic that was in the way. The hard part was preserving the mounting posts for the screws which hold the cover together.

Step 2: Solding Together a New Battery Pack & Repacking

My batteries don't have mounting tabs. No worries, I snapped off the tabs off the original batteries and soldered them onto the new batteries. To make a connection between batteries, I used some copper solder wick - some nice 3/16" wick worked great.

Step 3: Short Circuit!

Wow, do those batteries ever get hot when short circuited!

I had just packed the case together - was just driving down the last screw in the case when I noticed it. The handle was warm. In fact, it was starting to get hot. Maybe it was just the left over heat from the glue gun. I had put a little lump of glue inside to hold one of the batteries.

But no, it was getting hotter!

Time to reopen the case again and see what the problem was. Boy, everything looked OK. But, the batteries were smokin hot! Gosh, I hope I didn't hurt them. Eight bucks in batteries down the tubes?

My guess is the solder wick in the middle section got touching. It was just the bottom two batteries that were really hot.

A little anti-static bag cut up made for a good plastic shield. I guess I was lucky the short happened right away. Better to find the short now!

With the plastic shield in-place, I reassembled the case once again.

Step 4: Finished

All done, and back on charge. The stock charger is rated at 6V / 160mA. I think I can just leave the screwdriver on charge continuously without any worries.

Thanks,
Jim