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 » Energy Saver and set "Computer Sleep" to "Never". I use an external monitor at my office with the MacBook closed, and simply turn off the power to the external monitor if I'm not going to be using it for awhile.)
Determining the health of your MacBook's battery
If recalibrating doesn't seem to help, you might need to check your battery's overall health. The easiest way to do that is to run the "System Information.app" found in /Applications/Utilities/. Note: "System Information.app" is the name of the app in Lion and Mountain Lion. Previous versions of Mac OS X called it "System Profiler.app."
You can also launch "System Information" (or "System Profiler") by going to the Apple Menu, look for "About This Mac" and then hold down the Option/Alt key and it will change (see image below).
Or, for maximum geek points, paste this into Terminal.app, which will run the "system_profiler" tool at the command line, but limit it to just the information relevant to the battery status.
Whichever method you use, look for the Cycle Count and the Condition as shown in the image at the top of this article.
Like religion, politics, and text editors, battery calibration is a topic which lends itself to vigorous debate and arguments across the Internet. Some of it is based on outdated information, some of it is just incorrect, and some of it may simply not be applicable to your situation. I would encourage you to read Apple's official information and guidance on the subject as your starting point to understand how to best care for and maintain your laptop's battery.