Auxiliary batter, '08 S550 - PeachParts Mercedes-Benz Forum

cmac2012 · #1 05-06-2025, 08:56 PM

This is my client's car. They've had it for about 10 or 12 years, since then they've acquired two Teslas and this cherry looking ride sits for long periods.

The other day he couldn't start it, no surprise, it had sat for about two months. Oddly enough the main battery reads 12.5v. He had called AAA, they said there were three batteries in the car, I don't know about that, but he ordered an auxiliary battery for this model on the web, we found instructions that it's behind a little panel to the left of the instrument cluster, facing out towards the door.

We took that off and the battery's not there. Web searches and his owners manual (it refers to it as the "consumer battery") indicate it's in the trunk somewhere. One hit I found says it's on the right side, just behind the butt of a guy sitting in the right rear seat.

Not sure if this is the best forum for this question or not. I'm sort of a diesel head so I don't get over here much.

tbomachines · #2 05-06-2025, 09:19 PM

Quote:

Originally Posted by cmac2012

This is my client's car. They've had it for about 10 or 12 years, since then they've acquired two Teslas and this cheery looking ride sits for long periods.

The other day he couldn't start it, no surprise, it had sat for about two months. Oddly enough the main battery reads 12.5v. He had called AAA, they said there were three batteries in the car, I don't know about that, but he ordered an auxiliary battery for this model on the web, we found instructions that it's behind a little panel to the left of the instrument cluster, facing out towards the door.

We took that off and the battery's not there. Web searches and his owners manual (it refers to it as the "consumer battery") indicate it's in the trunk somewhere. One hit I found says it's on the right side, just behind the butt of a guy sitting in the right rear seat.

Not sure if this is the best for him for this question or not. I'm sort of a diesel head so I don't get over here much.

I don't know why they changed it around so much. The same thing in the w212...location depends on your year, engine and drivetrain. Some are trunk, some tucked next to main battery, some in dash. IIRC the w221 was same thing -- changed mid run. In the trunk, check behind the rear passemger seat, youll have to remove the side trunk trim. If you have a SAM/body module or airmatic pump back there you may have a removable panel you can peer into towards the front of the car.

On my w212 it is up under the instrument cluster. Do you have the battery on hand (i.e. do you know what to look for)? In pictures they tend to look like a regular car battery but when I got mine I was surprised at how small it is, about the size of a can of soda (but rectangular), maybe not even. Other designs have a full sized battery in the trunk and a rather small battery on the firewall.

Reason I ask if you have it in hand is if I didnt know what I was looking for, along with a youtube video I think I would have missed it.

Skid Row Joe · #3 05-07-2025, 12:04 AM

There's numerous information points. Pretty simple Rx procedure

cmac2012 · #4 05-07-2025, 10:38 AM

Quote:

Originally Posted by tbomachines

I don't know why they changed it around so much. The same thing in the w212...location depends on your year, engine and drivetrain. Some are trunk, some tucked next to main battery, some in dash. IIRC the w221 was same thing -- changed mid run. In the trunk, check behind the rear passemger seat, youll have to remove the side trunk trim. If you have a SAM/body module or airmatic pump back there you may have a removable panel you can peer into towards the front of the car.

On my w212 it is up under the instrument cluster. Do you have the battery on hand (i.e. do you know what to look for)? In pictures they tend to look like a regular car battery but when I got mine I was surprised at how small it is, about the size of a can of soda (but rectangular), maybe not even. Other designs have a full sized battery in the trunk and a rather small battery on the firewall.

Reason I ask if you have it in hand is if I didnt know what I was looking for, along with a youtube video I think I would have missed it.

Thanks for that, your information was timely and spot on. Looking at the timestamp, right about when you wrote it he found the owners manual which was really helpful. In addition to that, he actually got someone on the phone at the dealership (6 pm no less) who told him it was in the spot accessed through the trunk behind the backseat that you refer to.

He did have a small auxiliary battery on hand, he found info in a search that pointed that direction, 4 or 5 inches long, maybe 3" square, and a video showing how to remove it from behind the pop off panel between the door and the instrument cluster. I removed that, there was no battery in there.

When we pulled off the panel in the trunk behind the backseat, OMG, there was a large battery on the right side, maybe 16 inches wide, and the voltage was zip, nada.

I wrestled it out, and another miracle, he took it down to O'Reilly's about 10 minutes away, and they actually had the exact unit on the shelf! $275 large.

I put it in and the car started up sweet as you please. It only has 30k on the odo, it's a nice car.

tbomachines · #5 05-07-2025, 07:38 PM

Quote:

Originally Posted by cmac2012

Thanks for that, your information was timely and spot on. Looking at the timestamp, right about when you wrote it he found the owners manual which was really helpful. In addition to that, he actually got someone on the phone at the dealership (6 pm no less) who told him it was in the spot accessed through the trunk behind the backseat that you refer to.

He did have a small auxiliary battery on hand, he found info in a search that pointed that direction, 4 or 5 inches long, maybe 3" square, and a video showing how to remove it from behind the pop off panel between the door and the instrument cluster. I removed that, there was no battery in there.

When we pulled off the panel in the trunk behind the backseat, OMG, there was a large battery on the right side, maybe 16 inches wide, and the voltage was zip, nada.

I wrestled it out, and another miracle, he took it down to O'Reilly's about 10 minutes away, and they actually had the exact unit on the shelf! $275 large.

I put it in and the car started up sweet as you please. It only has 30k on the odo, it's a nice car.

Glad you guys got it sorted -- thats the "old" design IIRC. Good for another 10 years now.

cmac2012 · #6 05-07-2025, 11:39 PM

It's a new one on me. One thing that was odd is the car would not remotely try to start up until we replaced that extra battery, which is referred to as "the consumer battery" in some of my reading. Like I said, the main battery's voltage was like 12.6. Whereas the old battery in the trunk read zero. Maybe it powers all that is normally parasitic draw.