If you read the instruction manual for your cordless tools, there's probably a warning about not over charging your battery packs. While many of the chargers made by the larger tool companies now have a built in timer, many of the old and less expensive new imported tools don't offer this option.
An easy way to get around this problem is to use a home security timer like the ones used to turn on table lamps when you are away on vacation. (available at any hardware store) Simply set the timer to turn off after the desired time period. Make sure you set the timer so it won't turn back on. If you have a couple of battery chargers you can use a power strip with the timer to turn them all off at the same time.