After reading through most of this thread, I guess I should consider myself fortunate to have 80k miles on my '06 E90 and it is just now starting to show signs of a dying battery! I've owned the car since there were only 6 miles on the odo, so I know it's the original battery. Anyway, thanks for the write up!
I would like to ask a few questions, though.
1. Anyone in the Boston/Lowell area with a BT tool willing to help a fellow E9* owner out to avoid dealing with the overpriced dealers in the area? I'd be willing to pay you for your trouble, within reason, or buy you a beer
2. What do you think about connecting jumpers from another vehicle to the jumper posts under the hood while doing the battery swap to keep the computer, radio stations, ect. from being cleared? I've heard you need to reset your radio if power is removed and I have no idea where the code is (however I didn't see anyone mention that in this thread...).