R231 SL63 Auxiliary Battery Malfunction

[SL63 Mark]

An "Auxiliary Battery Malfunction" message came up on the dash on my SL, on Christmas Day, a nice Christmas present, as I was on my way to the in-laws for Christmas dinner.

Anyway I parked the car, hopped in the Landie and forgot all about it.

Last night I did a bit of reading on the internet, and it turns out this is a common problem, The small 12V 12 Ah auxiliary battery lasts about 7 years, so mine was due for replacement, as the car is seven years old.

The battery sits behind the drivers seat (UK car). I took some photos as I went, so I thought I would share them, and the steps for replacing the battery. Most of the internet write-ups say it is impossible to remove the battery without cutting the lugs on the battery cage. I found this is not necessary if you unclip the trim covering the rear section of the centre console.

The first step is to remove the two trim pieces on the shelf behind the driver. You can see where the clips are in the photo below.





Now you can see the battery cage revealed. First remove the 8 T30 Torx screws.

 

[SL63 Mark]

Now lift up and unclip the trim covering the rear section of the centre console. You can see how it attaches in the photo below. It pulls up and away. Below that is a thin strip of red trim , that also needs to be pulled out. You can see it in the second picture below. Again use easy gentle pressure. Pull towards the side of the car, not up.

 

[SL63 Mark]

Here is the battery cage, removed intact, with all lugs intact and unbroken. You can see the two on the right that go under the centre console trim.






And here are the fuses revealed, with the auxiliary battery cover on the left.

 

[SL63 Mark]

The next step is to remove the battery terminals, (negative first). The battery cover has two clips, hidden under the side trim. The cover hinges up, revealing the battery. Remove the acid drain tube, and the battery can be lifted up. The system is extremely sensitive to power spikes, so be very careful with the spanners.

As you can see from the photo below, there had been some serious acid leakage. The battery was not charging correctly, which is probably why the battery malfunction message had appeared.

 

[SL63 Mark]

Despite the leakage, the battery is holding 12 volts. I will ring MB in the morning to order a new battery. I believe they are around £100, probably cheaper alternatives could be found in the internet, but I will keep it original. I will update this thread when I have fitted the new battery.

 

[EmilysDad]

the last time I saw & used an AVO 8 was at school 40 odd years ago ;-)

 

[SL63 Mark]

the last time I saw & used an AVO 8 was at school 40 odd years ago ;-)

I like them. I have an 8 mk III, a nine and an 8 mk. 6. They are nice to use. I can't really justify a Fluke, so the Avo stays.

 

[sonic]

I like them. I have an 8 mk III, a nine and an 8 mk. 6. They are nice to use. I can't really justify a Fluke, so the Avo stays.

I have a Fluke which I kept when I retired. There was an Avo 8 the workshops which dissapeared shortly before I retired, but it wasn't me.

 

[EmilysDad]

My multimeter is coincidently an AVO, though it's 'only' about 25 years old but is digital. And work bought me mine.

 

[Monday]

The next step is to remove the battery terminals, (negative first). The battery cover has two clips, hidden under the side trim. The cover hinges up, revealing the battery. Remove the acid drain tube, and the battery can be lifted up. The system is extremely sensitive to power spikes, so be very careful with the spanners.

As you can see from the photo below, there had been some serious acid leakage. The battery was not charging correctly, which is probably why the battery malfunction message had appeared.

View attachment 87688

Useful thread, thank you. Couple of questions though:

Is this battery/location unique to the R231 SL63? Or applicable to the R231 SL400?
And what does the auxiliary battery do? Is this the one that runs the stop/start system? As I understand it, the R231 abandoned the twin consumer/starter battery system of the R230 for a single battery which is buried under the boot floor. But this 12Ah battery seems big to run the stop/start function?

Forgive my ignorance!
M

 

[SL63 Mark]

Useful thread, thank you. Couple of questions though:

Is this battery/location unique to the R231 SL63? Or applicable to the R231 SL400?
And what does the auxiliary battery do? Is this the one that runs the stop/start system? As I understand it, the R231 abandoned the twin consumer/starter battery system of the R230 for a single battery which is buried under the boot floor. But this 12Ah battery seems big to run the stop/start function?

Forgive my ignorance!

All R231 SLs have an auxiliary battery in the same location. Your car like mine, is over seven years old, so I would expect to see the Auxiliary Battery Malfunction message at some time in the near future, if it has not yet been replaced.

I believe the auxiliary battery performs a number of functions, including maintaining a steady voltage for sensitive electronic components during the stop start cycle, e.g. while the main battery is starting the car.

Also I believe it performs important functions in the case of failure of the main 90Ah battery in the boot, including the ability to shift the gearbox out of Park, unlock the car, and open the boot.

As soon as the car senses the auxiliary battery is no longer charging or maintaining a charge correctly, the auxiliary battery malfunction message will appear.

Hope that helps.

 

[Monday]

Thank you, that's very useful. As you say, my car is 7+ years old so I expect this may become relevant soon. I remember having the auxiliary battery changed on an E-class where it was much smaller and buried in the passenger end of the dashboard. It was much smaller and failed much earlier such that it was replaced under warranty. Perhaps that's why it is now so much bigger?!

 

[SL63 Mark]

You can see the numbers "41 15" on the battery in the photo above. I believe this signifies a manufacturing date of 41st week of 2015, so somewhere in October 2015. That means the battery had lasted more than 8 years, which is not bad going.

I picked up a new battery from MB this morning for £118. It has a manufacturing date of "46 23" (November 2023), so should be good for a few years yet. They had one on the shelf, so I suspect this is a high demand item.

I will likely install it tomorrow.

 

[SL63 Mark]

I can confirm I installed the new auxiliary battery today and the car works perfectly. The auxiliary battery malfunction message has disappeared.

Problem fixed,