MacBook Pro Battery – Why MacBook Pro Stops Charging at 99%

See http://www.apple.com/batteries/notebooks.html. This link suggests that you should fully discharge and recharge the battery at least once a month. I did not see this on that page, but I read somewhere that it is ok to leave a mac plugged in for an extended time, so just make sure you exercise the battery and let it completely discharge every once in a while.

Some obvious causes:

1. Need to wait 30 to 60 seconds for the SMC to read the battery and decide what to do
2. check for pencil lead/debris on the mac end of the magsafe
3. check for pins stuck down on the magsafe cord end of things
4. The SMC needs to be reset on the Mac
5. The battery, all adapter or charging circuitry itself is faulty or detecting something that causes a pause and the mac might need service. (This is exactly what your mac will do if it needs an 85 watt adapter but you've somehow gotten your "friend's" 65 watt adapter - if your adapter isn't putting out enough power, the mac can only run and not support the CPU/GPU load as well as the charging circuitry. Similarly, if the battery is drawing more than designed, the mac will prefer to run than charge and tells you this in the not charging message.)

Due to the way batteries work, they can be out of tolerance / spec at only a specific portion of the entire voltage range. The sensors will correctly detect “good battery” until you reach that one point where the voltage drops too fast and shows aging or a problem with the cells.

Do a calibration run at your earliest convenience if your battery doesn’t continuously self regulate and calibrate. Next, think about a visit to the genius bar or other Apple Support / service desk if the problem continues. Take your wall adapter with you when you go as it can be checked with the mac together.