I am about to change the battery in my 2007 550i. Is it smart to hook up a spare battery in the engine compartment. Then let the car go to sleep mode. At that time switch the battery in the trunk, being careful not to touch the live cables? Too many stories of bad things I have read about people changing there batteries then getting error messages. I wonder if they are not letting the car sleep first.

 

I dont think so. I just replaced mine straight up without plugging anything in and just got the 4x4 error which went away by turning the steering.

Ofcourse, do not touch the live cables or let them touch each other.