Load of bullsh*t IME (not makkan's post - just about the need to recode!).
I replaced the battery on my 57-plate e90 330i 2 weeks ago. Did it myself using a guide I found on here - just unplugged the old one and attached the new one. I ensured the replacement was the same spec (amp rating and capacity) to avoid any problems as I've heard that’s where they can develop.
It's gone - no longer an e90 330i owner