The problem with modern MBs is that it drains battery sitting. My 97 E320 has that flashing red LED on the radio and the anti-theft device. Both are draining the battery. So I use the automatic trickle charge once in a while just to make sure the battery is fully charged.

I have a friend who works for Johnson Control and he said that if a battery is kept full, it can have a service life greater than 10 years. He said premature failure of batteries is because they are not kept fully charged.