Help! (Battery capacity when leaving plugged in)

No.

There are lots of threads on here (in the Macbook and Macbook Pro sub-forums) about battery life.

Leaving it on the charger will not materially affect your battery's life.

The 89% battery capacity is just a guess by the computer, it will vary up and down each time you charge your Mac. In particular it will vary with the battery temperature.

 

Apple recommends that you completely drain the battery until the computer sleeps, let it sleep for 5 hours, then allow the battery to fully recharge. It allows the microchip inside the battery to take a more accurate reading of battery life. It also cycles the battery which, when done periodically, increases battery capacity. But yes, the battery reading does vary depending on the temperature and what you are doing on the computer at the time.

 

If you left it plugged in from now until eternity your load-cycle count would stay the same. It's like total mileage on a car- it never goes down.