Went out to car today and it said Auxiliary battery malfunction. I had replaced main battery 2 weeks ago. part number on battery is A0009829308 and there is a Battery backup power relay Part number A0009822023. I think I only need to replace the battery. I hope the back up relay is ok. Does anyone have a suggestion on the best place to buy this battery ? Only needed a 10mm wrench to take lock down off to get part number. Hopefully I will not have to reset anything.

Check voltage of your aux battery and see if it is withing spec. Before you replace the battery or relay, I would find out why the error message is occurring. Certainly could be the battery, or the relay, or the wiring, or the connectors, or the computer ...... ........ Look at the evidence and let that lead you to the correct answer.

This is one of the items I replaced in the past month but forgot to add it to my list of items I posted here yesterday. I replaced it for the same reason you're concerned. I got the Aux Battery Malfunction 2 times when it was really cold out (10 degrees) so I ordered it but the message never came back again so I just held onto it for a few months until recently putting it in. I got mines from https://www.newegg.com/Product/Produ...oho000b2i00053

After reading a suggesting from a post on this forum see below:
https://mbworld.org/forums/e-class-w...acement-2.html

I will add that the tech said the voltage was a little low and that it wasn't OEM so he would advise next time to get an OEM Aux Battery. So far I've had no issues though. I don't think it's a change right now type of thing so you may have a couple weeks to a month but others might have had a different experience. Good Luck!

Motor werks in barrington Illinois had battery for $135 plus tax. They wanted $140 for labor. It took me all of 6 minutes to take out and put back in with 10mm wrench. Could not see paying $140 for 6 minutes work. Started car and everything is fine needed no reprogramming. I had asked dealer if it would need reprogram and they said sometimes. Unbelievable

When it comes to shops overcharging for Mercedes repairs, there is no number I would not believe.
I had battery messages in the past, where battery clamp developed some corrosion, or was not torqued properly.
Those things are not easy to test, but lately bought Solar brand battery tester and it is amazing gizmo.

I agree. If I had a newer car like you guys, I'd be more inclined to strictly go to the dealer or pay whatever but with a 7-8 year old 144k mile vehicle it no longer is a 70k car in reality. It is what it is, an extremely nice used vehicle that hauls *** lol.

you are forced to buy the auxiliary battery from dealer since I could not find anyone near me that sells them Main battery is different story. Pep boys is best for that

Glad to know the 2013 and later aux battery is an easy DIY replacement. Your battery is the larger auxiliary battery used when they added eco stop/start to these cars and move the aux battery from the dash to the trunk. The part number comes up as a 12ah battery which is larger than the aux battery on the non-eco stop/start cars. Before then (2012 and earlier) it was a smaller ~1.2ah battery.

Either way, they both can be recharged. When I got the aux warning, I pulled out the battery and charged with my ctek and it's been fine going on 4 months and colder days since then.

This battery is also connected to a battery backup power relay Part number A0009822023 I suppose that is how it is supposed to stay charged.

I recently purchased a 2014 E550. How long should I expect the engine shutoff to last at a red light? Sometimes it will last a couple of minutes other times it's only 10-20 seconds. I imagine it depends on what the car needs to maintain but I'm thinking I must be getting close to having to replace the auxiliary battery as well.

just go to ebay and look for eco shut off device for Mercedes benz. You never have the delay again.

All this time, I never noticed the Aux battery changed significantly on newer models. I guess I should of figured due to all the new features they are responsible for like stop/start.

On early W212 the small battery in dash was just for gear shifting, when on ECO models the rear battery restarts the engine.

It was also for the "HOLD' feature on the early W212 as well.

And probably a dozen other things that we might never experience. But it is MB afterall.
Lot of things in those cars that exceed average human perception, so Stuttgart never tells about them.