Here is an easy way to discharge.
a) First you absolutely need to go to Settings ---> Energy Saver --> Battery and check "Prevent computer from sleeping automatically".
b) Start a movie, either on iTunes or DVD or similar, and have it loop when the laptop is not plugged in. This will drain the battery (normally it will play the movie around 2 times depending the length of the movie, and the laptop will turn itself off either a hard shutdown from lack of power or sleep due to low power.
c) Then once this is done, charge fully and repeat 2-3 times. This should then properly calibrate the battery.