Battery may not show a full charge in Mac OS X
If the battery icon in the Finder menu bar is set to show the percentage of charge , you might sometimes notice that the battery does not attain a full 100 percent charge when the power adapter is attached. This is normal behavior; battery life is maximized if charging is not continuously cycled on and off when the battery’s charge capacity is between 95 and 100 percent. When the battery level eventually drops below 95 percent, it will charge all the way to 100 percent.
Choose the method for:
"Resetting SMC on portables with a battery you should not remove on your own".