I just got the "Auxiliary Battery Malfunction" notice pop up on the instrument cluster. It was super easy to do it and I made a DYI video. Figured it might help some of you, if you decide to tackle this yourself. Admin, please delete if not allowed. Thanks and good luck!

most 212's use a 12AH, not a 1.2AH, and its mounted in the trunk, not in the dash. afaik, onlyt the first couple years had the in-dash small battery.

I believe mine is in the dash. 2012 pre facelift here. Thanks for the video! I normally get this notice when it's cold outside.

"10 through '13 are behind the left dash, looks like an alarm system battery, '14 through "16 are in the trunk, so three years of each. There are video links in the sticky posts showing you how to take off the left dash panel on the side of the dash to access it.

Yeah I figured as much. I'll be doing this soon thanks.

Thanks for posting.
My 2011 E550 has been displaying this warning for a couple of months.
Just this past weekend I noticed that the Hold feature was not working for a period of time.
So it seems I had better get this attended to before I am stranded somewhere.
At least now - with your video & others confirmation ... I know what my mechanic should be doing.
I would never attempt even a “super simple” thing on my own.
Thanks

Did my 2012 E350 dash battery. More of a PIA than in the trunk but doable for the average joe thanks to on-line videos. Got a Duracell replacement at Batteries+ for much less $ than MB. The battery alert pops up in cold weather when the battery starts to become marginal since cold temps reduce battery perfromance.

For my 2011 E550 - I was just quoted $ 355 + tax (Canadian = about $ 260 USD) to replace the Auxiliary Battery.
I have seen the behind the dash replacement video & there is no way I would try that myself.
Does this quote seem reasonable?

Only you can be the judge of that, It's a 50 dollar battery so 200 in labor, about an hour to change. USD.

That battery is more like $12-15 on Amazon and it's not really that much more complicated to change out than a trunk battery at least it doesn't weight much at all. So yes that amount is way high. Maybe they're thinking it's the more expensive aux battery that was in the trunk where the battery alone was in the $100 range, the real MB battery is about $50.

as shown on most videos. Once you get access to the side of the dash, you can get into the holes on the side of the dash and replace the battery. You can't see what you're doing but finding the battery and its connector is obvious to the touch.

I just replaced the original dash battery in my 2011, got the MB branded replacement from FCP Euro.
IMO easier if in addition to pulling the side panel, you also undo 3 fasteners and drop the lower dash cover.
Allows you to monkey with the battery with better access instead of trying to do it all through the small side dash opening.

Agreed! So much easier to drop the bottom of the dash.

Good info in this thread.

OP, thanks for making the video