Just before throwing out the drill (the battery has not been holding a charge for over a year) I tried a suggestion that I read about on the internet.
I made up a lead connected to a 24V battery and touched the drill + and - terminals with it several times. Then for good measure I swapped over the polarity and touched the terminals once.
The battery still appeared to be dead, but after charging for two hours in the drill's own charger the battery took a full charge and is now as good as new.
Batteries for the NLEPP144HD are no longer available so this is the answer. Apparently acid builds up inside the cells preventing the battery from charging and the application of excess voltage clears it.
After several days and deliberately discharging it completely and recharging once more the battery is still holding a full charge.
This may work for any power drill with a flat battery, 12V or 14.4V. Just touch the terminals a few times and then place the battery on charge.
Ian, May 2011
