Hook the battery to the power tool and use the tool, or run the tool until the battery goes completely dead. Then leave it attached to the tool overnight.
Run the tool with the battery in the morning. If it will run the tool, use it until it goes completely dead. By doing this, you are essentially erasing the memory in the battery and deleting the false empty cycle of the battery.
Place the battery on the charge only when it will not power the tool at all. Let it charge fully, which should take overnight.
Run the tool with the battery attached until it is again completely dead. Don't charge the battery again until it's completely dead.
Repeat the charge and drain process several times, and the battery will get better at holding a charge and charging completely.