After 4 years, I don't care what the pimple-faced stoner at the B&D
service center says. The battery is shot, especially if it hasn't been
used much.

Most power tool manufacturers use crap cells in their battery packs. I
don't know of one that uses decent cells. You can buy a $20 B&D or a
$400 Makita or Milwaukee. Same cells. All crap.

NiCd memory is a myth. It used to be true, 20 years ago. Technology
has a way of progressing, but old wives' tales have a way of living on
forever.

The charger base has a rectifier bridge in it that changes the AC to
DC. 15VAC is the correct wall wart, but it needs to be big enough to
supply the current. This doesn't preclude a short circuit in the
charger base or battery ruining your wall warts, though.

All in all, for the time and money you're spending, you'd be money
ahead just chucking the old drill and getting a new one. I personally
like the Ryobi OnePlus from Home Depot. Good midrange price point,
good value for the money.