I'd check the power saving settings and see if the profile is on the correct settings for your preferences, as there are settings in there to auto power down on set percentages. Also some batteries can loose power capacity if they aren't charged fully on the first use, although i'd expect most people would do that.
i would also be checking the charger is properly connected as my lass's laptop had a similar issue when the power lead became stretched due to endless wire tidying after every use and caused the power charge to fluctuate till wire was replaced.