Good morning, I wonder if anyone could help, please. I had the usual messages for convenience features, as usual wait for the battery to charge and it goes away but then I had a red battery message pop up 'Visit Workshop' so I thought ok, aux battery needs replacing. I replaced it this morning let it idle for a while to charge the battery but it still shows me the message and turns off convenience features. What would be next to check after changing the aux battery? Thank you in advance.

 

How many miles are on your cdi? My thoughts are if you changed the aux battery and it is still giving you that message, it is either a bad BCM, or a bad voltage regulator. You will need to use a volt meter and check both batteries to see if they are charging. Please post the voltages before starting and while your cdi is running. By checking the voltage through the instrument cluster, you should see a steady 14.1 volts while driving, but you cannot tell if the aux battery is charging.
Good luck!
Mike T.

 

The car has done 103k miles. It has had both batteries replaced but still shows this message. How do I have the car show me it's voltage on the instrument cluster? Thank you.

 

I have checked the voltage of the battery while running, it's only 11.8v

 

Alternator failed.

 

Take a look at this post on how to check battery voltage from your instrument panel:
https://mbworld.org/forums/e-class-w...ery-light.html
With what you are saying in your previous post that the battery is only showing 11.8 volts, that as ItalianJoe1 said, you have a bad alternator. You can try changing the voltage regulator first but make sure you purchase a Bosch regulator or you will have problems.
Mike T.