Charging batteries properly is more difficult than you might think, and definitely more difficult than cheap power tool companies engineer for. The simple lightweight charger that comes with a cheap cordless drill is almost assuredly a piece of crap that will kill the batteries in short order.
Smart chargers monitor the state of the battery to cut off charging at the proper time. For NiCd and NiMh cells, this reduces cell damage. For LiIon cells, it reduces explosions and generally highly-exothermic scenarios. Atmel’s got a decent appnote on some of these algorithms:
AVR450: Battery Charger for SLA, NiCd, NiMh and Li-ion Batteries
Now you can build your own, and keep from having to buy a new drill just to get a new battery (that the store you bought it at no longer has / wants 80% of the price of the drill for).
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