Your battery is probably fried. The idea that you have to constantly discharge a nicad to prevent a memory from developing is a myth. A memory develops when you discharge a battery to the same level and recharge over and over again (i.e. run it for 20 min, then recharge, over and over). If you randomly discharge (10 min one day, 5 another, 25 another) your batterys and occasionally fully discharge them, you should not see a memory developing. Eventually the battery will wear out and they won't hold a charge. Just time to get new batteries.

My information on this comes from an article I read on NiCad battery's many years ago. I don't remember the source..