There comes a time when the battery in your MacBook Pro will become offset and give a faulty percentage or time indicator. There is a way to fix this and keep your battery calibrated correctly. Apple recommends that you do this step the first time you receive your new MacBook Pro or any Mac computer with a battery and continue to do it every couple of months after that to keep your computer in tip top shape.
Calibrating your battery isn’t hard, but will take some time to complete once started.
Make sure your Mac is fully charged — Before you start your test, make sure your computer has a full charge; this will be indicated by a green light on the end of the MagSafe power adapter.
Allow your computer to rest — If you’ve just charged your Mac, allow it to rest for two hours so the battery has time to keep the charge and sustain it.
Unplug and drain the battery — Once the Mac is fully charged for at least two hours, you may unplug it and start to use the computer. The point here is to wear down the battery and deplete it until the computer shuts down by itself.
Continue to use the computer after the warning of low battery — Save work once you see this warning and continue to use the computer and fully drain the battery.
Let the computer sleep — Once the Mac’s battery has been depleted, it will automatically go to sleep. At this point, you want to let the Mac go uncharged for five hours so that the battery will truly drain all the way down.
Charge it back up — Once the battery has been drained for five hours or more, you may then charge it up again. When charging, leave it off and do not turn it back on until the battery indicator is green.
At this point your battery has been recalibrated and should show accurately what your computer percentage is. For more diagnostics of your battery, check out the free tool called coconutBattery, which will show you detailed information on battery statistics and all the fun stuff for the more technical person.