I have recently bought a 1996 MB E320 and need to replace the battery. This may seem like an odd question, but I have been told by a mechanic (admittedly, not an auto-electrician) that when changing the battery on these cars (with alarm and immobiliser) a constant charge should be fed to the terminals via a separate mains power source so as to avoid having a period of "zero power" in the system. "Downtime" has been known to cause problems with the ECU etc (apparently).

I am puzzled. Has anyone else heard of this, or should my mechanic go back to school?!?