dean1484 MegaDork
7/9/21 11:38 a.m.


There is a small battery in modern Mercedes that is located in the dash just above the driver's left knee (in a RHD car).  You access it through a panel on the side of the dash with the door open and you will most likely need to partially remove the panel under the dash as well. These batteries go bad and when they do all kinds of weird stuff starts to happen electronically with the car. Many of the automatic features will get wonky.  The biggest one was the car reporting signal lamp and running lamp failure and in the cabin the signal lights on the dash would flash really fast but the exterior ones work fine.  Other things that were reporting errors were the high and low beams and cruise control.  All kinds of weird errors on the dash info screen were popping up on startup.  The key one is this "Auxilery Battery Malfunction" .

Most people (including experienced mechanics) don't associate it with all the other errors.  Most assume that you have a module going bad and it is causing all the errors including this one.  I have seen many internet posts of people chasing these errors with no resolution.  However the real problem is the auxiliary battery needing replacement.

Replacement is kind of like playing Tetris when trying to get the battery out and back in and then plugging in the power and ground leads and having small hands is a big plus!!!

AND if you purchase a new one with Mercedes branding it can cost you anywhere from $50-$125 just for the battery online and who knows what a dealer would want.  There is an alternative.

Purchase one of these batteries.  It fits and meets all the specs and is less than $15.00



After the battery replacement let the car run / idle for 10 minutes or so then take it for an easy drive for 10-15 min. Shut it down and then restart and see if it cures all the weird error codes and weird acting signal lights and hi-beam stuff.

My car is a 2012 CLS 550 but I know this is something that is used in a wide variety of Mercedes over a very wide range of years.

I would recommend anyone that has a merc that is 8 years or older go ahead and replace the Auxiliary batter as a preventive maintenance thing.  Cheep easy thing that will only take 30 min or less to do and it will save you the panic of a car that starts to act really weird.


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