i believe sony is the maker of the battery?
anyhow, a few yrs back i had a sony laptop, i misinformedly left the battery in the computer and left the computer plugged in all the time, 2 yrs later, the battery holds about half of the charge... then i was told i wasn't supposed to do that. it's a Li-ion battery
i was told that the computer, once become fully charged, start using the battery until it gets down to 99% and then start charging again... which is what hurted the battery
perhaps right now macbook/pros don't go the same route, perhaps right now once it's fully charged, it just leaves the battery there, but i, when i plug it in the wall, i take out the battery
i have a thinkpad that i leave out the battery when i plugged in, it's been...3 yrs or so, it still holds 7 hrs of charge at reg. use