There is a mix of incorrect and correct information on the internet about the battery locations & replacement procedure for the R231 SL. I wrote up a little guide to help owners find the information they need quickly and in one place.

The following information is taken from Daimler EPC Group 54, Subgroup 30. I do not guarantee its accuracy, nor do I guarantee that part numbers have not changed. If there is any doubt, contact a trained professional.

All of the following cars in all regions from 2013 to present use the same battery configuration. There are two batteries in all R231 SL models, in the same locations in the cars. Explicitly, this applies to:

SL 350 231.457
SL 400 231.465 / 231.466
SL 500 / SL 550 231.473
SL 63 AMG 231.474
SL 65 AMG 231.479

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The "Main Battery" is A0019828208, a 12V 92Ah 850CCA AGM battery. In my region, this is a relabelled Varta SilverDynamic AGM 95Ah with OEM code (595 901 085 / G14 / 019AGM). This is responsible for the majority of the electrical power requirements of the car. Prior to July 2015, the old part code was A0055411001.

This battery is located in the rear "boot" or "trunk" of the car, under all liners and inserts. To replace it, you will need a 10mm and a 13mm socket. To replace: Use the 13mm socket to release the hold down strap, then use the 10mm socket to disconnect the terminal clamps (negative terminal first, do not let terminal clamps touch anything else once loose).

The battery date code is stamped into the top of the metal of the negative terminal. The upper two numbers will be the week of the year, and the lower two are the year itself. This battery should be good (assuming normal usage of the car) for at least 4-5 years.

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The "Auxilliary Battery" is A2115410001, a 12V 12Ah 170CCA battery. In my region, this is a FIAMM brand battery, and to the best of my knowledge there is no standardised size or format for these batteries. The only way to guarantee the fit would be to buy the official part.

This battery is located in a dummy compartment behind the right-hand-side seat. To replace it, you will need a Torx T30 screwdriver and a 10mm socket. To replace: Remove both leather trimmed panels from the top and side of the dummy compartment by pulling firmly but not excessively. This will reveal a black plastic cage, shown in a photo below (originally by "touchyu2"). Remove 8 Torx T-30 screws from the cage. Pull free the coloured leather trim on the side of the central rear sliding compartment, which will otherwise impede the removal of the cage. You may also need to remove the trim that covers around the sliding compartment. Pull the cage clear, and the Signal Aquisition Module (SAM) as well as fuse banks will be visible, along with the top of the battery holder. This is visible in the photo below. Undo both terminal screws with the 10mm socket (negative terminal first, do not let terminal clamps touch anything else once loose), then unclip the top of the battery holder. Ensure that during replacement that the second wire attached to the positive terminal under the terminal screw is correctly replaced.

The battery date code is printed on the side of the battery, in the exact same format (week & year codes) as the main battery. Sadly, these batteries do not seem to live for very long, even with regular use. If you see the message "Auxiliary Battery Malfunction" in your SL R231 instrument cluster, it is indicative that this battery may be dying. Simply leaving the car to sit for a week may let the battery go flat. It may have a life of ~3 years in my estimate.

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These batteries are separate, isolated and have separate functions. As such, battery trickle chargers will only charge the Main Battery unless explicitly connected to the Auxiliary Battery terminals. Should one wish to keep a SL in long term storage (perhaps over a winter), two trickle chargers would be advised. For anyone interested in the precise duties of each battery, excellent information on this forum is available from "MBtech1098" in February 2013.

I hope this helps!

Plastic cage of dummy compartment hiding the Auxiliary Battery (photo by "touchyu2")



Under the plastic cage:

 

This post is awesome. Thanks for posting it. Do you know of any instructional videos on how to remove those panels to get to the battery?

 

Great post. I was disappointed to learn that my CTEK maintainer that I use does not help maintain the auxiliary battery.
Anyone know what is the typical cost of dealer replacement of the auxiliary battery?

 

They quoted me at $439 for Aux battery install at the dealership in Orlando, FL.

 

In the same boat with one battery charger on my sleeping SL. But wouldn't it be cheaper to just get a second battery tender?

 

I have another CTEK but connecting it is not so easy. I use the posts under the hood for the main battery. I was hoping the secondary battery would also benefit from this location but this is not the case according to this thread. I guess I could access the secondary battery and run a cord into the passenger area but this is not an easy job since it involves removal of several panels which I am not willing to tackle.

 

Dang - was hoping you'd blaze the trail

 

I might this Spring.

 

As a reference, I bought an auxiliary battery from a dealership directly for the equivalent of ~ $/€ 65-80.

 

Did you install it yourself? If yes, comments/advice?

 

Curious about its make-up. It looks like a motorcycle battery, but with no starting duties, it would be free to be a deep-cycle unit. I wonder if there's an equivalent available online somewhere. I don't think I'd be paying anyone to install it for me unless there's some kind of re-set required after the swap.

 

The second battery is specifically to support the start-stop function. It is a standard size, it cannot be a deep cycle.

 

Sooooo, if that battery goes dead will it disable that annoying "feature"? If so, I might just disconnect it manually. That's how I was able to disable it on my Jaguars.

 

Yes, not sure if you'll have an active dash error but you can give it a try.

 

I try to remember to deactivate this function every time I start the car.

I sure wouldn't be disappointed if it was permanently disabled.

 

I'm about ready to tackle the job of accessing the auxiliary battery in order to attach the wires for a battery maintainer. Can anyone shed light on the details of removing the trim pieces? Does the compartment cover then lift right off? Any help would be much appreciated.

 

For $129 you can permanently disable On/Off with an OBD module for sale on ebay. I have bought one for each of my MBs and they work just as advertised with no problems. ebay seller is ”vladimircdi”, listed as “Mercedes ECO start-stop OFF”. Installs in two minutes then forget on/off forever.

 

It also controls a "HOLD" function too because I lost it when auxiliary battery needed to be replaced. I do not recall now what else was "lost", but I think it was more than just Eco and Hold functions, at least on '15 SL.

Tried DIY and did removed the first panel...after which I promptly took the car to the dealer for them to replace it. DIY for the main battery in the trunk is not an issue, however auxiliary due to its location IMO is not a DIY for most people.

 

Something tells me @rmorin49 will take the challenge....

 

Taking the car to the dealer on Monday to have them check the non-functional AirScarfs. I'm going to ask them that if they have to access the fuse box next to the auxiliary battery if they will install the battery tender leads. If they don't or won't I'm planning to try to do it myself.

 

Did you damage the panel when you removed it? If so, do you have any suggestions on how to remove the panels without damage? I do have a set of the plastic tools that are used for panel removal but no other special tools.

I am also going to try to remember to ask my dealer about what functions the auxiliary battery supports.

 

I did not damage the panel when removing it, but could not put it back flush when I tried. So I stopped at that point and took the car in for them to do replacement of the aux battery since the car was still under warranty at that time.

 

Service Manager at my dealer discouraged me from accessing the auxiliary battery to install battery maintainer leads. He advised that this battery only supports the start-stop function and has no draw when car is parked. Since my car is still under CPO warranty I'm going to follow his advice. He said they will check the fuses to see if they are the cause of my non functioning AirScarf.

 

Just wondering if you ever to the air scarf problem resolved. It may be more than a coincidence that I came across your message when looking at how to change the auxiliary battery. So we both have bad aux batteries and we both have no air scarf function. I'm currently charging the aux battery while I look for a replacement. MB dealer was unable to troubleshoot what was wrong with the air scarf last year. Oddly it was just before I got the warning for malfunctioning aux battery. Hope the new battery fixed it for you.

 

My battery was fine. Dealer replaced a "control unit" or some other part. AirScarf works. Advisor said the aux battery is only there for the start stop function. I always turn mine off right after I start the car.