Someone really did not want the battery in the MacBook Pro to come out of the upper case.
We tried valiantly with our iFixit 6 Inch Metal Ruler to free the battery from its aluminum confines, but to no avail. Rather than risk puncturing a lithium-polymer battery cell, we left the power source in place.
To complicate matters further, the TrackPad cable lies underneath the battery. Attempting to pry the battery off the upper case could easily sever the fragile cable, which would be bad.