As other posters have said.
Battery dud, especially if Ni-Cd, over 2 years old. See if you can borrow another battery to prove. Chargers do fail but B&D chargers do not seem to give many problems.
I repair power tools as part of my living and around 90% of faults in cordless tools prove to be battery related.
I agree with answer 1
the battery can show 18v on an open circuit but as soon as you turn it on the voltage drops to an unserviceable voltage. it is time for a new battery.
i had the same problem with a 12v drill. finally figured out my charger was kaput. seems like this is a little more common than one would think.
Time for a new battery.
Duff or worn out rechargeable battery or a faulty charger. Your battery should show 18 + volts on a test meter, dc range, when fully charged, less than that is kaput! The charger should be putting out a slightly higher voltage.If you do not have a test meter you can pick these up for under a fiver, albeit very limited functions. If you have a B &B service point near by they will test it for you. If it is a gonner, shop around before you buy a new one as there is some rubbish out there!