This instructable will show you how to make a simple discharger for your NiCad or NiMH battery packs for Airsoft AEG's or RC cars, boats planes, whatever. Draining your battery packs is an important step in preserving the life of the battery. Charging a depleted battery is better than charging a half charged battery. The design is not mine though, it is borrowed from this one here and here. I didn't see the point in paying $5 plus the shipping for something I could easily make for less that amount. I bought all the supplies(2 12v bulbs, heatshrink tubing and tape) at Wal-Mart for a total of $5.63. You may be able get spend less at other stores or if you already have some or all of the materials.
http://www.instructables.com/id/Make-a-simple-battery-discharger-for-NiCad-or-NiMH/