February 21, 2009

FAIL: Cordless Drill Battery Modification

Not every DIY project succeeds. Sometimes, there are failures. Sometimes the failures are pretty spectacular. It is ok to fail, however, so long as you learn something. This is why I am sharing these not so successful projects.

A few years ago the battery pack in my Ryobi 9.6 volt cordless drill died, I decided a replacement battery was just too expensive for the value of the drill. I had nothing to lose with the battery at that point, so after disassembling it, I determined that a bunch of NiMH AA cells could replace the existing Ni-Cad 'sub C' cells for the same voltage, but with 30-50% increased capacity (mAh).

Thomas Distributing carries tabbed NiMH cells. The tabs make it easier to solder cells together to form a battery pack. So I ordered up the right amount of cells and set to work.

Not having performed a lot of electrical tinkering at that point in my life, I foolishly used 22 gauge wire to connect the battery tabs together. It's what I had on hand, but sometimes you just have to buy the right material for the job. The battery pack went together pretty easily and it wasn't long before I had it charged up and ready to go.

Unfortunately, this is where the story goes down hill...For a project I was working on, I needed to drive four 6" screws into solid oak blocks. I got through about one and a half before the smoke started pouring out of the battery compartment. I quickly tore it apart and separated the batteries, my heart racing the entire time.

The high torque required to drive the screws into the hard oak (it nearly stalled the drill) drew so much current from the battery pack that the wires heated up to the point of melting the insulation and causing the pack to short out. Several of the batteries got so hot that they vented acid inside the enclosure.

Undaunted, I ordered more batteries and rebuilt the battery pack using heavier gauge wire.

Since I still needed to drive the remaining screws, I again used them as my test subjects. I got through the remaining screws just as the smoke started pouring out of the battery pack, again.

This time, before I could get it torn apart to stop the short, one of the cells exploded with a rather impressive bang!

That marked the end of my attempt at cordless drill battery pack re-building. I never figured out exactly what caused the second meltdown. Obviously something shorted out, but it didn't appear to be the same problem as before.

Under normal use the re-built battery pack would have worked just fine, but the extreme test conditions I put it through were too much. I would rather have an early failure, however, than to be lulled into a false sense of security. Especially considering the potential to have caused a fire.

So my recommendation is, don't try this at home. And by the way, I ended up spending more on the batteries than if I had just bought a new OEM battery pack.

No comments:

Post a Comment