crock

My 2015 has been through three auxilliary batteries in 55k miles. First you get some battery malfunction warnings on the dash, then a few days later your distronic and a few other doo dads start to fail, and THEN you car goes into limp home mode, and THEN it gets towed to the dealer after it stops by the side of the road. So moral of the story is to get the battery replaced within a week or two of first seeing the warning on the dash. Ask me how I know......

amirijaz

Very helpful pics on how to change out the aux battery. Quick question: how did you remove the right side trunk panel? There is a 12v outlet on that side and it seems that must be disconnected and removed — but I can’t figure out how to do that. Any tips?

amirijaz

So I did this aux battery change out on my 2014 S550. Some tips:

1. Remove 3 nuts holding the fuse box (not just the 2 as mentioned above). The 3rd nut releases the battery box so it easily slides out. The battery is super heavy to remove otherwise

2. The 12v clip can be removed by reaching down once the right panel is loose.

3. Remove all the panels in order to more easily remove the right side panel. This includes removing the chrome trunk sill. Remove 2 clips and then pull it up....it’s tight, feels like you will break it....but it doesn’t break...it releases additional tension clips underneath.

Nice Ride

Additional look when you pull cover away. 2 Push clips on top under rear window need to be removed.

I had to remove this additional nut to move the module out of the way.

This black clip under the battery case can be pushed down to allow the battery case to slide out. ( You will need to remove nuts around the fuse panel to move it out of the way.

Old Battery Voltage reading after removed

Old Battery Amp reading

2nd test to confirm

2nd test to confirm

Notice Sell By Date TOP Right of sticker.
UPDATE: So I replaced my AUX battery today. I tested the Failed One. Interesting the Voltage was 12.18 the amps were between 8 and 10. So, the computers must me monitoring how many amps are available. The first reading with the tester was 8.5AH, 2nd reading was 10Ah, 3rd reading was also 10Ah. All tests were above 12V.

ECO turned Green immediately, Distronic worked immediately. ( Both showed error every time car was started.) Adaptive Headlight error will not clear...All else OK. Start stop worked immediately, no long drive needed to charge.

Anyone have an idea about the adaptive headlight error?
Thanks

whoover

12.18 V is essentially dead. Anything below 12.4 on a charged battery means it needs replacement. The low amp-hours confirm it, of course, but the electronics are affected by the low voltage. It's just that because the battery can't put out the current, the voltage drops when the electronics try to draw some.

David Wms

I have a 2014 E350 with same error message. Dealer replaced it once under warranty but would not honor warranty on replacement battery. After getting quote from dealer, I purchased a battery from Autozone with same specs. It worked fine for while, then intermittently, then error message returned and stayed on. Autozone checked battery and it's still good. Dealer now says I have to have OEM battery and wants $475 (really!!??) to replace with OEM. That price sounds outrageous! I'm deferring that expense for now as they're charging $675 for B service and additional $650 for transmission service. I see auxiliary batteries with MB logo advertised for $129 (https://www.fcpeuro.com/products/mer...BoC_RMQAvD_BwE) and something advertised as "genuine" MB but without logo on Amazon for $49 ( ) Replacing the battery is very simple, so I plan to buy one myself. Has anyone else had experience with "genuine" MB batteries from non-dealers? Thanks.

whoover

Neither of those batteries looks like the OEM battery, which is this:
https://www.eeuroparts.com/Parts/861...BC-0009829308/

The OEM battery is AGM. A cheaper flooded cell battery, even if it physically fits, will operate at a lower voltage and cause problems. Always go by the MB part number, A 000 982 93 08.

David Wms

Great. Thanks for that information and link!

amirijaz

For comparison, my local dealer charges $125 for the battery. Or $260 installed.

flint350

Drat! My 2018 S560 just had the same failure and less than 15 months old. Made an appt for repair - anyone know approx how long the dealers take with this?

amirijaz

About an hour. Should be covered under new car warranty. If CPO warranty, then not covered.

flint350

I just had my aux battery replaced and the service rep expressed surprise that it failed so early (14 mos and 6k miles). What really surprised me was when he said that the aux battery fails are not covered under extended warranty protection, only under the normal 48mos/50k. I have the 7 yr extended warranty and found this to be annoying, especially given the number of failures being reported. I don't know if he is right or not, though.

whoover

Yes, there are numerous posts here about the aux. battery not being covered by the extended warranty.

Nice Ride

Just a question, what makes you think it will operate at a lower voltage? AGM has the ability to drain very low and recover close to 100% where a WET battery will be compromised each time the voltage is low. Other than that Volts are volts and amps are amps...

whoover

AGM batteries operate at a slightly higher voltage when charged than flooded cell. A standard battery is considered fully charged at 12.7 V. An AGM will be more like 12.9. It's just a characteristic of the AGM cell. It's one way that sensitive battery monitors can be thrown off by the wrong battery type and is one reason the aux. battery has a "Replace only with same technology" warning label.

Quadrobenz

The Aux battery in my 2015 S65 triggered the failure warning 2 weeks ago. I had it replaced, the original lasted just over 5 years. That is consistent with the life of the AGM Consumers batteries in the SL's.
In my opinion the SL has a much better battery management system since all the consumer loads begin working as soon as the AGM battery reaches 50% charge while the car is running. That usually takes no more then 10 minutes even with a weak battery that will not hold a charge after shut off. The S does not work this way.

Mike_Metal

I picked up my 2018 S450 4Matic from MB of Owings Mills, MD, last Saturday. As soon as I got on the highway, got the message 'Auxiliary Battery Malfunction'.
No Distronic Cruise control/stop start functionality for a 300+ mile drive. Not sure what other functions are covered by auxiliary battery? Possible Hold brake function?
Called MB roadside assistance , who confirmed that auxiliary battery always needs to be replaced when this message appears.
Local MB dealer in Hartford replaced it for free as it is under warranty.
Bad experience after purchasing the CPO car, that this happened immediately.

whoover

It's more complicated than that. When everything's working, the aux "battery" powers everything but initial starts. (A supercapacitor's quick charge and discharge cycle makes it ideal to take the load off the main starter battery, which would have a reduced lifetime from frequent starts. A lead-acid battery has a fixed number of charge cycles before it is no longer functional but a supercap doesn't.) But if the aux battery fails, the system switches over to the main battery but limits current draw by shutting down consumers. It does this in a phased manner, depending on total charge available. First to go are high-current, non-essential things like stop-start, seat warmers and electric windshield and steering-wheel heaters. If voltage is low enough, pretty much everything not related to safety is shut down. If the monitor determines there's not enough charge left to start the car you get the warning to not turn off the engine until you're at a shop.

Supercaps are an important new technology and will be in all cars in the near future. MB was unusually innovative in exploiting their capabilities fairly early, and we are seeing the price of early technology adoption. As annoying as it is, I'm glad that Mercedes engineering isn't the conservative organization it once was, when its customers watched BMW and others bringing all the new technology to market a few years before MB.

mmnguye3

Would you mind sharing the battery model number?

Nice Ride

Old Note, but did you know that FCP will take back your old battery and replace it for free if you purchased it from them. No questions asked, just have your original receipt. Don't take my word for it, just call them.

Nice Ride

Found this.... Purchase once from FCP, Free Shipping and Free Replacement for Life....

https://www.fcpeuro.com/products/mer...mer-0009829308

2012 merc amg

Anyone know if a 2017 s550 has the battery or the Capacitor? My 2015 had the battery and it was a bad setup. It would Die every 30k miles.

parker15555

Just ran Xentry on my 2016. It shows that the Aux Battery has high internal impedance. >1000 mohm. With that impedance it is on the verge of failing. Still is working fine no other problems. From what I can see this is a standard motor cycle battery which are readily available for about $50. Still have some warranty left. Will try to persuade the dealer to replace the Battery.

fun33

I have 3 cars. I don't drive any of them often. I have used a separate on each car for the past 3 years to charge the main battery. They have worked flawlessly. One car is only driven twice a year and the CTEK always keeps it ready to go.

I decided to purchase a 4th CTEK 40-206 and install in on my 2018 MB S560 Auxillary battery. Took the CTEK to my MB dealer, and they refused to install it on the S560. I explained that Mercedes sells their own MB branded model of the same CTEK 40-206, the answer was still no. I asked to speak to the service manager and he told me "we install trickle chargers all the time, but ONLY ON THE MAIN BATTERY". I tried explaining MBWorld forums are filled with owners who have 2 chargers hooked up to their Main and Auxillary batteries but the Service Manager insisted the CTEK would "mess with the electronics" and void the new car MB warranty.

I asked the SM how the CTEK might "mess with the electronics". The SM said the CTEK was made for full-size batteries only and might overcharge the Auxillary. I tried showing the SM the motorcycle symbol (indicating a reduced charge for a smaller battery) but he blew me off. The SM then told me the CTEK would continue to overcharge the auxiliary because it "would not recognize when the battery was fully charged." I tried explaining there is circuitry in the CTEK to stop the charging once a full charge is achieved and then it regularly checks the battery, as long as it is connected and adds charge whenever needed. Again the response, "it would void the warranty".

It was apparent, the SM was not going to budge. In total frustration, I called another local MB dealer and spoke to a service advisor. I received essentially the exact same answer. Have I misunderstood the postings on MBWorld? If using a CTEK on the auxiliary battery will indeed destroy my electronics and void my warranty, I should call the SM and thank him profusely for his warning. But if a second CTEK can successfully be installed on my 2018 S560, I'll simply take it to an independent shop. I would like to do the same install on my 2017 SL550.

I realize this installation would be extremely simple - just attach a quick connect to the positive and negative battery terminal and plug it into the CTEK. I would do it myself (as I did all three of the main batteries on my other cars) but the auxiliary location on my S560 requires the removal of more parts than I am comfortable doing.

Would appreciate any of you confirming the successful install of two trickle charges on your own main and auxiliary batteries.

Nice Ride

External charging if you connect a trickle charger to Main Batt, will not charge the Aux Battery, only the Main Battery, also, you cannot use the Cig Outlet in the trunk/boot, it shuts down a bit after you turn off the car. Average lifespan 3 Years... You can do a direct connect to the Battery Terminals in the Trunk or a Direct connect to the Jumper location under the hood. Depends on year and model where you battery is located.