Q: Can you overcharge your MacBook Pro battery?

NOPE. Lithium based batteries, as used on almost all portable devices today, always include a charge control circuitry to absolutely prevent that under any circumstance. The circuitry strives to allow a max charge of about 95% capacity, leaving a safety margin. And the circuitry is built INTO the battery, so there is no way to sidestep it.

Why? If you overcharge a Lithium cell, the lithium metal precipitates and catches fire. Dell discovered that in a very hot way when they were the first to attempt to use this technology back in the early 90's, without any protection. Flamin'Dells were a very real possibility then. Nowadays battery makers have become very conservative to avoid the awful lawsuits that would ensue, and lithium batteries are very safe.

http://batteryuniversity.com/