Sounds like the battery is on it last leg and going to need replacing. That is a very common thing for a battery that is going bad. They give false percentage readings. The problem is that over time a battery will hold less and less power. Meaning less and less of the battery is actually usable. So while it will say it has say 100% it really only has say 54% because the other 46% is no longer accessible/usable. Time for a new battery.