A Mac portable is just that: a portable computer. It was designed to be plugged and unplugged to allow the battery to discharge and recharge on a normal cycle. As such, if you use your machine plugged in all the time, then it is important that you discharge (or calibrate) your battery every so often. If you rarely use the battery, then Apple recommends completely discharging the battery and charging it again at least once a month. If you use your notebook frequently on the battery and plug it in to "top off" the battery, then a full discharge cycle is required less often.
Follow these steps to calibrate the battery:
1. Plug in the power adapter and fully charge your MacBooks's battery until the light ring or LED on the power adapter plug changes to green and the onscreen meter in the menu bar indicates that the battery is fully charged.
2. Allow the battery to rest in the fully charged state for at least two hours. You may use your computer during this time as long as the adapter is plugged in.
3. Disconnect the power adapter while the computer still on and start running the computer off battery power. You may use your computer during this time. When your battery gets low, the low battery warning dialog appears on the screen.
4. At this point, save your work. Continue to use your computer; when the battery gets very low, the computer will automatically go to sleep.
5. Turn off the computer or allow it to sleep for five hours or more.
6. Connect the power adapter and leave it connected until the battery is fully charged again.
Checking Battery Condition
If you hold down the Option (alt) key on your keyboard while clicking the menu bar battery icon, then you'll see a Condition option. The battery condition can be in either one of four states: Normal, Replace Soon, Replace Now, or Service Battery.
Normal means that the battery is operating properly.
Replace Soon means that the battery is functioning normally, but is currently holding less charge than it did when new, and may need replacing.
Replace Now means that the battery is functioning normally, but holds significantly less charge than the battery when new, and needs replacing before it could possibly do harm to your hardware.
Service Battery means that the battery isn't functioning properly or your computer has noticed a change in its behavior. You should take your battery and/or computer in for servicing if this problem continues after a few restarts.
If you don't plan on using your Mac portable for more than a six-month time period, then you'll want to properly store your Mac's battery with a 50% charge. This will ensure that it won't fall into a deep-discharge state, or lose capacity and therefore have a shorter life.
Temperature is also a huge factor when storing your computer. You'll want to ensure your Mac and its battery are stored in an environment where the temperature is between 50ºF and 95ºF (10ºC and 35ºC). Anything more or less, and you could seriously damage your battery over a long period of time.