turn off the computer, plug into wall power and then perform an SMC reset, which if you have a unibody is holding shift control option then pressing the power button for a couple seconds, you'll see the battery meter reset on the side of the computer and your LED on your MagSafe will flick.
After that, boot the system and see if the percentage increases/decreases. I would then perform a battery calibration by running the unit until it shuts off and goes into safe sleep. Then plug in power and let it charge. When you see the green light, let it sit plugged in for another 5-6 hours. You can use it while it's charging.
That process performs a battery calibration. Good luck.