A bit late I know, but...
This happened to me the other day, and after a lot of head scratching I found a soulution.
It seems that the B&D charger does not like dead flat battery packs ( less than 1.5v ).
I put my battery pack onto a workbench DC power supply 1A at 12 volts or so for about an hour.
Ater I got some 12v or so into the pack the B&D charger was quite happily to charge it up to the full 18v.
In future I'll just charge the batery pack a bit more often.
Roy
July 2008