thefisch

https://mbworld.org/forums/e-class-w...ml#post5855871

See the post linked above for pdf with photos.

Edward993

Someone correct me on the model year, but I know for 2014 forward, this aux battery is in the trunk, under the spare tire panel. On previous models, it was located in the driver's side dash. I replaced it on my 2014 and the warning light is gone and the brake "Hold" function now works again. FWIW, I hate the "Eco" system so always switch it off as soon as I start the car, but this aux battery is purportedly for this engine-start system so as not to tax the main battery as the engine is restarted all those numerous times it is trying to save you gas.


Edward

edbro

It makes you wonder if not using ECO will prevent this battery from needing changing that often. It doesn't make sense to save a few pennies using ECO if it ends up costing hundreds in service calls. Now, if I can only find a permanent ECO bypass.

LILBENZ230

So what is the actual cost out of warranty? Anyone know?

davidjohnbamber

My local NAPA had a AGM battery for around $185 that meets OEM specs, and a better warranty.

LILBENZ230

That's for the ECO battery? Or the main battery? Seems steep for the eco.

Rogue Leader

Battery is in the trunk in the spare tire well. This only affects ECO start/stop and the brake hold function.

cetialpha5

It makes perfect sense. Eco mode will give you better gas mileage and allows MB to hit their CAFE numbers which is mandated by the government. Costing you hundreds in service calls isn't a factor in CAFE numbers. It's a WIN-WIN for MB, better CAFE numbers and extra revenue for their dealers. A little bad PR from owners that complain, but that's their cost of doing business.

ItalianJoe1

We replace LOTS of batteries under warranty, it isn't all a win for MB.

blegthbloo

http://www.midcityengineering.com/pr...s-benz-eco212/

BetterTomorrow

There seems to be confusion on where this battery is located. I see quite a few references that point to under the driver side. Perhaps this applies to pre-2014 face lift? I know my car (2014) has a battery in the trunk, right next to spare tire.

Anyhow, my car just threw the same "Aux Bat Malfunction" message yesterday. After that, I noticed that not only auto hold is gone, distronic plus cruise is gone as well. I wonder if this means auto emergency stop, front cross traffic alert/assistance, etc are gone as well?

Local MB dealer has been a bit of pain to get an appointment for this, kept telling me to just come in before 2pm and wait for it (no loaner, not taking appointment for this apparently). Basically forcing me to lose at least a couple of working hours.

blueshound

Just got this replaced under warranty at my local MB dealer. I asked him what it does. He said it operates the ECO and it also is a backup to the main battery so that if your main battery dies you can still get the car in Neutral so that you can get the car out of the garage to get a jump/on a tow truck/or whatever. I don't know how much it costs and it appeared to be in the trunk of my 2014 E350 as they had that open at one point when it was in the service bay.

RobRoy

Had the same problem at 23186 miles. It was corrected by the dealer in Columbus Ohio by replacing the auxiliary battery in the truck at no cost. Also got a free car wash.

Tump43

Same here. At about 25k miles the auxiliary battery on my '14 E350 went. The dealer replaced it under warranty in less than 30 min. in and out the door. No problems since. About 5 months before it failed, I had, however, as a precaution, charged that battery to full capacity. Apparently, some early batteries were no good, period.

sakodik

Ill just add to it: 2014 E350 23k miles and had a malfunction message upon pick up from used dealership. Had MB replace it under warranty about 9 months prior to expiration.

As above, lots of functions ceased to function - hold/hill assist, park assist.

Fidel Garcia

Hi I’m own a E350 and i just miss the warranty for a mont on the same problem with the auxiliary battery
and the estimated price is 370 dollars in the dealer it is expensive but i need to doit i dont want this problem to afect any other aistem in the car
but if it helps to keep the eco thing off i will thanks my car has 39k miles

ItalianJoe1

Can replace the battery yourself, it's quite simple. Keeping Eco off doesn't help the battery life, they have to pass an electronic resistance check, if they fail, you get the warning. Not much you can do to prevent it, it's quite sensitive, the value is only about 2 ohms, differs slightly from application to application.

Sunnyslope48

Great info. Thanks for posting this. I've never thought about checking the resistance in a battery - seems like it's checking to see if the fire is hot or ice is cold. Something new for me.

BeachBunny

My 2015 E350's been through THREE of these batteries in 48k miles. I also had one of the service loaners throw up an Aux Battery Failure at 4k miles too. Definitely a design flaw.

RSIKR

Thanks for the information, you solved my mystery. My 2014 E350 4matic with eco stop/start has been sending the message Auxiliary Battery Malfunction. I tested the battery at 12.95V which is good for a AGM battery. The internal resistance explanation is the answer. My systems battery under the hood was replaced by the dealer 2 days before the original warranty expired. At the time the SA told me that the Aux Battery is replaced more often under warranty.

nc211

RSIKR

A 0009829308 That is the MB part number. You will see it when you remove the hold down. $134.99 list price at a dealer.

nc211

nc211

justdonating

Old thread but my aux battery errored today. Local dealer wanted $379 including installation or $187 including tax for the part. I ordered the OEM part online delivered for $125. Took 10 minutes to install. Folks of your vehicle is out of warranty, do this install yourself and save the money.