My wife & I each have coldless drills, which we use intermitently
for household repairs & projects. The only way to make sure we
have a charged battery handy for emergencies is to leave one in
the chargers. Even though these are reasonably high quality
chargers (Ryobi & DeWalt), I'm sure we are shortening the life of
the batteries.

You probably are. I would not charge the batteries until they began to run
down, then recharge no more than four hours (or whatever the manual
recommends as the maxium charging time).

I own an inexpensive 12V Ryobi drill, which gets similar intermittent use.
The charger /does not/ shut off automatically. Indeed, there is nothing to
indicate that the charge is complete! I am very careful about when and how
long I charge it.

The battery does not quickly self-discharge. I've never had a time it wasn't
ready. So, after each use, I'll recharge it for 15 minutes or so.

A 12V battery pack has eight nicad cells. It's important not to repeatedly
run down the battery, as you're increasing the chance of reversing the
lowest-capacity cell in the pack.


It occured to me that one option might be to put the chargers on a timer,
so that they are on for half an hour a day. That should top them off,
without cooking them.

A half hour a week -- or even a month -- would probably be enough.


The catch is that I'm not sure what sort of load the charger presents
when it is off. I could just end up discharging the battery through (at a
minimum) diode leakage.

That would be several hundred K.