I have a 2015 S550 (w/o 4matic) that I purchased almost one year ago with 21K on it. The wife received an auxiliary battery malfunction error yesterday morning. I drove it yesterday 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 what appears to be 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? The auxiliary battery is a small battery resembling a motorcycle type battery, right? Anyway, where is the auxiliary battery on a 2015 S550?

Thanks.

 

I received the answer on another Benz forum and thought I would provide it here in case the answer may be useful to someone else later. Both the main batter and the auxiliary battery within a 2015 S550 are located within the trunk. The main battery is located under the carpeted panel at the very rear center of the trunk. The auxiliary battery is located behind the passenger side panel of the trunk. Attached are 2 PDF's with instructions.

 

Please read this blog I have found about w222 auxiliary battery https://xenons4u.co.uk/blog/mercedes...ion-explained/ . There is a video on installation guide as well, I am sure it will help you. Further, you can check below mentioned links, I am not sure which one would be compatible but must look at them. I am also using one of their auxiliary batteries (https://xenons4u.co.uk/mercedes-benz...converter.html) and it works pretty fine.