I have a cordless drill with rechargeable batteries. The batteries charge completely in about 20 minutes. They are not supposed to stay on the charger for longer than that. However, unless I am standing right there after the charging time, I forget to take the batteries off of the charger.
To fix this problem, I made a timed duplex power outlet. I plug the timed outlet into a 120v outlet. Then, plug the battery charger into the duplex outlet attached to the timer. Finally, I set the timer to 20 minutes and walk away.
The parts list to make this is simple:
Making this is also simple.