Sounds as devil666x says, the battery is either on its way out or already has a problem.
Batteries get problems because they are not fully discharged after being charged. They have a very good memory and will only charge up to the time they were last charged. If it was not fully discharged it remembers it only needs to charge for say 30mins and the next time it is not fully discharged the charge time gets shorter and shorter and so on until the battery is useless.
You could try fully discharging the battery and then try a recharge and full discharge again, to try to recondition its memory, but it is probably already to late for that.