Donald_S

Does the auxiliary battery on the 2015 S550 get charged by the alternator (like the main battery does) when the engine is running? How about when using a battery maintainer to keep the main battery charged during long periods of non-use of the car? Does the auxiliary battery get charged too?

superangrypenguin

Yes.

Doberman_Fan

Donald,

Have you located the auxiliary battery on your 2015 S550? I have the same year car (w/o 4matic) that I purchased almost one year ago with 21K on it. The wife received the malfunction error this morning. I drove it this afternoon and did not receive a fault message. Anyway, I figure I will replace the auxiliary battery myself to be on the safe side. I found the main battery in the trunk and did not see the auxiliary one. So, I removed all engine panels after starting in the passenger side corner where I figured it would be; I did not find it under any panel. Is it hidden under something else or is it maybe a very small battery stashed possibly in the cabin within the dash near the driver's door like I've seen on a 2009?

Thanks.

MikeAtLincoln

The auxillary battery is the one in your trunk.

Doberman_Fan

Mike, if you are correct the Auxiliary is the battery in the trunk, then where is the main battery? The battery in the trunk is a very large battery that looks like what one would expect the starter-battery to look like. I thought the auxiliary battery is supposed to be a small battery resembling what one finds on a motorcycle, ATV, etc.

konigstiger

See attachments.

Doberman_Fan

Thank you, Konigstiger.
So, the main battery is the one I found within the trunk located at the rear dead center after removing the panels. But, the auxiliary battery is in the trunk as well except it is well hidden behind the passenger side trunk panels.