Sounds like you are over charging the battery. It is getting too hot and venting. If it's a NiMH battery the delta peak (or threshold voltage) on the charger should be set at 15 mV or close to this for a 6 cell pack. If you don't have this delta peak type of setting, the charger may need to be replaced.
Also see the battery explosion thread
to see what happens when this vent plugs up from repetitive over heating.