Followings are instructions from Apple regarding battery calibration. The computer can be used as normal during Step 1 to 4.
1. Charge the battery to full
2. Keep the adapter plugged in for at least two hours
3. Disconnect the adapter while the computer is on
4. Drain the battery until the computer turns off automatically
5. Leave the computer in sleep for at least five hours
6. Don’t turn on the computer right after reconnecting the power source
7. Only turn on the computer after it is fully charged