Dewalt drill battery replacement

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Depends. Some cheap drill chargers have no more than a DC source with a
resistor in series, and charge in say 4 hours. If you leave them on longer
it will ruin the batteries. I'd look inside the charger base for signs of
proper electronics.

It's been decades since I've seen a 4-hour charger that didn't include
a mechanism to shut off the current at around full charge, but the
mechanism may be crude, like a thermostat. The last time I did, it was
for a TI calculator with fast-charge nicads designed with a shortage of
chemical on one set of plates to make overcharging impossible (in
theory).

The best basic charger does a 1/10th capacity constant current charge
which requires some 14 hours from flat. This can be achieved by adding one
transistor and a few components. It's the kindest way to charge batteries
and they will last longer. Even the most sophisticated fast charger will
shorten battery life over one of these.

In practice, best battery life is probably achieved with fast chargers
that aren't too fast and that don't overcharge since they contain good
shut-off mechanisms that rely on temperature difference, rate of
temperature rise, rate of voltage change, etc. Also cells last longer
per charge when charged fast since that causes larger crystals to grow
during the charging process.

 

[Dave Plowman (News)]

Also cells last longer per charge when charged fast since that causes
larger crystals to grow during the charging process.

Not my experience with pro use of Ni-Cads. A timed 14 hour charge at
1/10th the nominal capacity after running them down to the point where
they need changing gives the longest life, IMHO. Of course it might matter
if they're used for high discharge use - mine aren't.