AS A KEEN HANDYMAN, I have a number of power tools, including a few cordless types that run off nicad battery packs. These bat­tery packs range from 7.2V to 14.4V and almost inevitably contain Sanyo or Panasonic nicad cells, regardless of the brand of the tool itself.

The unit is easy to build since virtually all the parts are on the PC board. Keep the wiring neat and tidy by using cable ties and note that the large metal diecast case is necessary for heatsinking.

Properly treated, these battery packs should be good for hundreds of charges and can potentially last many years. Unfor­tunately, proper nicad chargers are usually expensive and the cheap chargers supplied with the original equipment often incor­rectly charges the cells and dramatically shortens their life.

Recently, I found that my 2-year-old 9.6V cordless drill battery wouldn’t perform to its rated capacity after charging. Unfortunately, battery packs are fairly expensive to replace, sometimes costing almost as much as the entire drill kit – and that’s if you can purchase the battery pack separately at all. Often, you will simply be told to just "buy a new drill".

In fact, it is far cheaper to purchase your own cells and manufacture a "new" battery pack using the old case. This invol­ves soldering leads between the battery tags to connect them in series. Note, however, that you should never solder directly to the cell cases – that can damage them and is quite dangerous.

In selecting replacement cells, I researched the manufactur­er’s specifications on charging and guess what?
– the battery charger that came with the drill didn’t comply with these speci­fications. Instead, the supplied charger is a very simple device that applies a constant current to the battery pack and doesn’t cut out once the pack is fully charged.

As a result, once the cells are fully charged, the battery starts to heat and the internal pressure builds up. This can lead to permanent cell damage and in serious cases, the battery can rupture or vent electrolyte.