Cordless Drill Advice

[ezdays]

I built my own replacement pack using batteries from Newark Electronics for a fraction of the price of a replacement pack.

It's not likely that you can "overcharge" a battery, as their internal resistance increases with charge to the point where it won't take any additional charge once it is fully charged.
Do NOT use a charger that is rated higher than the battery pack...Bad things can (will) happen....

Some chargers, like a few of the "quick charge" ones that I have, will use a higher charge voltage than the battery's rating. An 18 volt battery has a nominal rating of 20.7 volts, but I've had charges that start off at over 22 volts. That, as you say, will kill a ni-cad battery if you leave it on too long. The better chargers will sense the voltage and not allow it to get overcharged. I've heard that it's best to fully discharge a ni-cad battery before charging it because it has memory and may never fully take a charge. I've tried that with little success.

We made small one-amp and five-amp power supplies for the security industry, and we'd put a series resistor in the charging circuit to keep a float charge in the backup battery, but they used sealed lead-acid batteries which act differently than ni-cads. Besides, the output was set to match the battery voltage, and that goes to what you are saying, overcharging means too high a voltage, not too much current.