If your MacBook (any model) is not charging to 100%, it may need to be recalibrated. Apple explains how to do this, but basically it comes down to three steps:Fully charge your MacBook (and leave it charged for about 2 hours) Fully drain your MacBook (by using it on battery until it turns off) Fully recharge your MacBook again.
That should help the battery indicator recalibrate so that it shows the correct information. If it doesn't, you might need to reset the System Management Controller (SMC), but in most case I've found that a simple recalibration will work.
When I recalibrate, I prefer to make sure that my MacBook will not be interrupted at all either during the drain or the recharge period, so I will leave it on overnight while running Caffeine set to "Indefinitely" and let it really, fully discharge. Then, the next day, I will plug it in and let it charge through the workday, again running Caffeine to keep it awake. (Note: Caffeine will also keep your screensaver from running. You might prefer to go to System Preferences
Article by TJ Luoma (c) TUAW - The Unofficial Apple Weblog - Read full story here.