Apple - Should I disconnect my MacBook Pro's power cord when the battery is fully charged?
You do not need to disconnect your MacBook Pro's battery. Your battery will stop charging once it is full. Apple's modern batteries are much smarter than previous designs.
To get the most out of your MacBook Pro's battery, follow the Notebook Battery advice from Apple: unplug and use your battery until empty about once a month, then charge back up to full.
At the time of answering, Apple's advice read:
For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her notebook on the train, then plugs it in at the office to charge. This keeps the battery juices flowing.
If you need help following Apple's advice, use Battery Guardian; it is free and will remind you when to deplete your battery.
No. The nature of the lithium battery chemistry does not require periodic cycling to maintain capacity.
Furthermore, the periodic cycling described in the apple documents listed above (@I-M-JM's Post) does not have an effect on the battery chemistry. It only serves to allow the battery capacity meter to accurately track the battery capacity.
Periodically draining the battery only serves to allow the Battery metering systems to accurately estimate the remaining battery capacity. It is not needed to maintain the battery itself, and actually wastes a battery cycle.
Basically, if you don't run your battery down for a long time, the next time you disconnect the laptop from AC, the estimated time remaining value will be incorrect. However, the actual battery capacity and runtime will be unchanged from normal.
The statement "For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally." from Apple's site is actually incorrect regarding the lithium battery chemistry at worst, and an oversimplification at best.
Furthermore, The battery metering and battery charging systems are separate, so inaccuracies in the battery metering system will not cause errors in charging the battery (before you ask). Lithium cells are actually quite simple to charge.
Ref:
http://en.wikipedia.org/wiki/Lithium-ion_battery
http://batteryuniversity.com/learn/article/lithium_based_batteries
http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
I keep my macbook pro plugged in all the time with no adverse effects. And if you want to use your mac laptop in clam-shell mode (ie lid closed, connected to an external monitor), then you will need the power supply plugged in anyway.
Still, I (and Apple) recommend to unplug it every month or so and use the battery a bit, just to remind it that its a battery ;) Keeps it healthy.