Merserybenze

Replaced it. Easy to do. Cost $150 for part from MB. Takes about 30 mins to do. After you remove the three screws holding the fuse box in place don’t be afraid to yank on it so it brings the battery out so you have space to remove and replace. Thx all for comments.

seamus2154

That is my exact same story. Didnt use the car for about 2 months. Planned a roadtrip and that morning aux battery message came up. Figured it would charge up going down the road. Well of course 25 miles down the road the distronic and cruise would not work. 300 miles later it did. Then it didnt. It worked intermittently till we got home, 900 miles. Changed out the aux battery, no issues. So there is something in that circuit that effects it. Good luck

blevinson

I just had mine replaced on my 2014 with 112K miles. I was told the only thing the Aux battery powers is the eco start/stop and it is a backup to release the parking brake if the main battery dies.

seamus2154

I was told the same thing. But was the first time the aux battery went bad on this car and I immediatly has cruise control issues and distronic issues. Never had one issue with the car. Got home from the trip, changed the aux battery and back to normal ever since. All modern cars go crazy if any battery is not up to par.

Tom in Austin

You can also put in one of these motorcycle batteries that have similar specs: .

As people said it can be a little fussy to change, particularly figuring out that the metal tray slides out with the battery in it. Also, the motorcycle batteries have a different terminal attachment but not too difficult to mate to the W222's wiring.

I spent a couple extra bucks and got an AGM version.

2012 merc amg

I wonder if a lithium like this battery would be ok. I'd think something like this and you'd never have to replace the aux battery again. On my 2017 E550 the Aux batt went twice. So for on my 2018 S560 no issues yet but waiting for the message to appear one of these days as it's a real problem with these cars.

amgs63dxb

...unless it caught fire for reasons unknown or due to being overcharged, since there is no lithium charge controller as the car's electronic expect AGM...

vinflouen

Thanks Cosmic Trucker, did this in the parking lot of my office this am.
Took about 15 mins.
Your pics were very helpful.

Orange Tesla

Hi,

2019, S560.

Upon start up I receive “auxiliary battery malfunction”

After a bit of research, some people are replacing their auxiliary (small) battery while others are replacing a voltage converter.

Does anyone know specifically which one the 19’ model year s560 has? Any tips/tricks for replacement is greatly appreciate!

Thank you!

2012 merc amg

Probably the same as my 2018. Battery on pass side of the trunk remove lower trunk trim and then side panel. Getting the battery in and out was tougher than I thought. Ended up taking it to an independent repair shop and they put in the new one I’d got at batteries and bulbs. 85 dollars for the battery and 100 dollars to put it in. Tried to talk them into installing it at batteries and bulbs they would not do it. Said it was more involved than replacing a regular under the hood battery. They were right.

W205C43PFL

The best way is to actually remove some of those floor covers and see for yourself, it is either a capacitor or an actual battery, there you will find the part number as well so you can order accordingly, I suggest this because I heard it is market dependent on what auxiliary battery is installed and if it is installed (in your case, it is installed)

gr1nch

I've a 2017 W222 S350D UK model (RHD SWB) and for the past few months errors on start regarding Lane Assist. For the past couple of days on a trip away there have been the "Auxiliary Battery Malfunction" message and no more Distronic - which I can't do without, particularly as it helps so much with regular driving and if the vehicle in front suddenly brakes.

Thanks to all who have posted solutions and details, I'm going to give it a go to replace the battery myself.

Will this likely fix the other "auxiliary" functions that are reporting as unavailable/faulty?
​

seamus2154

Thats exactly what happened to me. See my post#104. Distronic would not work on my roadtrip. Aux battery light on. I could care less about auto stop start, but apparently there are other circuits effected by a low aux battery. Changed it out and not one issue since. Good luck