Hansen Tsang , Mar 23, 2009; 02:13 a.m.
The battery pack is made up of multiple cells. One of the cell in your pack is probably dead. The way to check is to put a volt meter on the positive and negative pole of the battery after you charge it fully. It will probably give you a voltage that is lower than what is stated at the back of the battery pack case. I forgot off hand what that voltage is but it is probably 7.2 volt. I think the battery pack is made up of 2 lithium cells at 3.6 volt each. So if one of your cell died you will only get something like 3.6 volt left after your battery is "fully" charged by the charger. The voltage may be a little higher when you first pull it off the charger. It will read something like 4.1 or 4.2 volts.
You only have one option and that is to buy a new battery pack.