Is there an easy way to see if the car has an auxiliary battery? In the e350 under the hvac intake is an H7 size battery, no battery in the trunk, I have not checked under the right dash.

my 2009 e320 bluetac only has the main H8 battery in the truck.

It depends on the year and model. For 2010-2013, the aux battery was in the dash. That was to back up the electronic shifter. Otherwise you wouldn't be able to move the car out of park if the battery was dead. For 2014+ the aux battery was in the trunk and was used for the start/stop feature. I think the E550 might have had it in a different location.