Auxiliary Battery Malfunction
 

This message just popped up today! I had to make two stops and it occurred both times so I don't think it's intermittent. I have an appt with the dealer Thursday morning to get it replaced. Anyone with experience with this failure? Should I be ok to use the car until then?
If the battery goes belly up what happens? Would I still be able to get it to the dealer? The car is my 22018 S450. Thanks in advance for any input.

i had the same failure in my g550; the dealer replaced it under warranty. no probs since. you can drive the car. you might get a few other driver assistant systems failures; i did. they were all corrected with the new battery and everything returned to working normally.

Don't worry at all. The Aux battery will not stop the main functions from running. You will make it to the dealer fine, just have a few warnings on the dash, and a few systems that may go offline. Just be sure to check blind spots and pay extra attention to lanes etc, as those systems may be offline.

I had the same issue in the past month. As others have stated you will get distronic inoperable etc.

I think yours will be a capacitor and not a battery. Dealer said they switched in 2015 and if it had a battery iwould have had to pay, if capacitor it would of been covered under my CPO.
Turns out I had the battery still so had to shell out 282.

Aux battery failure
 

When I had my 2015 S550 I experienced the auxiliary battery failure at the start of my drive from southern Florida to RI. Made the trip fine but distronic did not work for most of the trip. That was annoying but you can still drive the car. It was fixed under warranty. Read somewhere that if you do not operate car for over a week Mercedes recommends putting on trickle charge. I do that now with my 2018 560 and 2016 GLE 400 and haven’t had a battery problem since.

I don't think the Trickle charge will take care of both batteries. The AUX is stand alone when the car is off. You would need one for the Main Battery and one for the Aux.....

I drive the car daily so I don't think it's a charging issue.

On average, those batteries last 3 to 5 years.

I only have had the car for 18 months, so a little early for failure. The dealer replaced the battery and updated the software today. All back to normal...:)

Battery malfunction
 

I jinxed myself. Turned on ignition this morning and what pops up? Auxiliary battery malfunction! Took delivery of my car in September 2017. Made appointment for replacement under warranty Monday.

I bought mine in Oct of 2017, maybe a bad batch of Aux Batteries???

Nothing major...don't worry about it. The aux battery runs the car when you use auto start/stop. When the battery goes out, you just won't have the auto start/stop feature working.

The dealer should replace it free of charge if the car is still under warranty.

Other way around. The main battery is the starter battery. The aux battery powers other systems. (If the starter battery goes, it's not just auto start that doesn't work -- it's any start.)

So a failed aux battery can give you a lot of odd alerts for all kinds of systems, although none will usually prevent the car from operating.

Recent cars have a capacitor bank instead of an actual battery. This "voltage converter" is what the system calls the aux battery. They seemed to have had a batch of bad capacitors that leak and cause this problem.

Ok, Who sent the Virus..... My Aux Battery just went today. Eco Start Stop Inactive, Distronic Inactive, Active headlight inactive....

So battery lasted 5 Years....

Yep. Those are the odd alerts. But at least the car starts. :)

My 2015 S550 with 62k miles just gave me this warning. Where is the Aux battery located on a 2015 S550? It's under the hood in my E55. I've only owned this car for 30 days, so I haven't spent much time under the hood yet.

On the right at the trunk. A DIY task but more complicated to detach than one would assume.

Yep, the battery is far more likely to fail early if you use the start/stop function which I immediately turn off. It's annoying as heck that this cannot be programmed to be off when I start the car in the morning.

Thank you.
My local MB dealer parts counter has the best people working there. They are always very helpful with advice if asked. I'm choosing to replace this myself rather than deal with the hassle of dropping the car off, then getting a ride back to the dealer.
They advised me this battery is buried in behind the fuse box on the right side of the trunk. How bad could it be? I may take a few pictures to help those who are also dealing with a timed out Aux battery in a 2015 S550.

This is a very common failure. My 2015 E400 aux battery failed the first time when the car was only 6 months old. Dealer replaced under warranty. A couple of days ago, go the warning again -- 3.5 years later. Back to dealer, just before the factory warranty expires, to get another battery. Hopefully, should be good for another 3-4 years, by which time, I may be looking for a different car. BTW, I NEVER use auto start-stop. Part of my startup routine is to shut that "feature" off. Didn't help battery life at all. I think all of the other features, like blind spot monitors, collision prevention systems, etc., take more power. I also think that these batteries are built to the lowest possible specification/price point, so they will always have a shorter life span than the main batteries.

Also, in our 2015 GLK, the aux battery required replacement at 3.5 years. Wife ALWAYS uses auto start-stop. We get same service life of battery with or without the start-stop usage.

https://cimg3.ibsrv.net/gimg/www.mbw...ad777e01fd.jpg
Remove the flip up floor panel 2 T20 screws
https://cimg4.ibsrv.net/gimg/www.mbw...383c456034.jpg
remove the rear trim panel. Two push/pull clips, then pry up to lift the panel straight up
https://cimg7.ibsrv.net/gimg/www.mbw...74423d096e.jpg
Play safe and disconnect the neg battery term from the main battery.
https://cimg6.ibsrv.net/gimg/www.mbw...f12260cc91.jpg
View of the goal. The Aux battery is in a black plastic case that has a snap on lid. You don't remove the case, just pop off the lid
https://cimg7.ibsrv.net/gimg/www.mbw...a65db545c7.jpg
10mm nut x 2 that hold down the fuse box. You need to just move this a little to the side to gain full access
https://cimg9.ibsrv.net/gimg/www.mbw...d477247fbc.jpg
Flip up the Aux Battery cover, then unbolt the neg then pos terminal. The battery will lift out

I took a voltage reading of the aux battery and it appeared to be the same as the new battery. So I guess it's more than a voltage issue. The install was pretty straight forward. The one thing I forgot to do was reconnect the plug for the 12v power outlet. I almost took everything apart to reconnect that plug, but I don't think I'll ever need it. Don't forget to plug that thing back in. One surprise was I didn't have to relearn the windows or sunroof. It seems all memory was saved, even with both batteries disconnected.

One thing I learned a long time ago (from a Lexus tech when I had weird alerts coming from my LS-430 at that time) was that the new cars are very sensitive to low voltage or poor performing batteries. When you start to get all kinds of weird "fail" messages, check the battery. Iven if its within spec, but on the low end of it, you could have issues. When in doubt, replace the battery. Low and behold, once he replaced the battery all the issues went away. So now its the first thing I check if I ever get weird failures.

Yup, its all about the amps. In my industry ( Security ) we have a tester for Alarm and Fire Control Panels. The tester will measure the Voltage / Temp. / Amps. Curious what your reading was on the voltage. 12.x The amps are what gives you the standby time.

The rule about battery testing.
If the battery tests bad, it's bad.
if the battery tests good, it could still be bad.