The 2011 Mercedes E350 auxiliary battery location is either:
Many vehicles in the E Series have two batteries to help support the high electricity demand.
The smaller, auxiliary battery keeps your car’s electrical components (GPS, instrument cluster, Blind Spot Assist, etc.) turned on. Location of the auxiliary battery may depend on your specific model. It may be under the hood or in the driver’s side panel cluster.
Check your owner’s manual for confirmation of location before you start taking anything apart.