Well I won't pretend to know your exact habits, but running the MBP off mains (plugged in) all the time is a sure way to destroy your battery well before its time. This goes for *any* laptop/notebook.
Unsuspecting users think they are doing themselves a favour by having it plugged in all the time, but the reality is just the opposite. If you know that you are going to be plugged in for a long period of time (i.e., all day), then remove your battery and give it a rest. This will maintain the health of your laptop battery and save you money.
Again, I'm not sure what your exact habits are, but I get the sneaking suspicion that you killed your own batter(ies) through poor battery maintenance.
I'm saying this because it's also happened to me and I had to learn the hard way.
I would do this but Apple decided it was a good idea to seal the battery inside and not let me remove it. Do you think this was the right move?