When the battery is fully charged, charging stops entirely. Even with the AC adapter connected and the computer shut down, the battery will drain very slowly (as all batteries do). When its charge level falls below 95%, if the AC adapter is still connected, charging will automatically resume and top the battery back up. This charging regime absolutely prevents overcharging, and also allows the battery to remain cool most of the time even if the AC adapter is connected. Charging heats the battery — an inescapable consequence of the laws of physics — and heat is an enemy to batteries, so it's highly desirable to avoid unnecessary heating.
Although your battery can't be overcharged, it
can be underused. Leaving it connected to the AC adapter ALL the time denies it the regular light exercise it needs. For all the information you might ever need about your battery, read this excellent User Tip:
http://discussions.apple.com/thread.jspa?threadID=1764220
