Hi. I have a 2011 MB E350 4matic w212. The vehicle has 119000 miles on it and ran beautifully until the red battery light came on. After it came on, different things started to shut down automatically such as power brakes, power steering, comand, etc until the dash went black and all that was left running was the engine and the headlights. Within a couple of minutes, the accelerator would no longer work and after I finally got the thing pulled off to a safe spot, I could not turn it off even when I pulled the key out. I finally had to remove the battery connections and the engine shut down. I have since replaced both the main and the auxiliary battery. The car starts but the red battery light comes on within seconds after I start it. If I let it go, the electrical items or any item that's got any electrical current going to it for whatever the need may be for that system. It I let it go, the battery drains to zero ultimately and the show starts over again from the beginning. Does anyone have any ideas as to what may cause such a thing? Alternator has voltage, battery has voltage. I've thought about the voltage regulator, the bcm, and the alternator. Please, if you have any thoughts on this at all, hit me up and let me know. Thank you

 

Has to be the alternator

 

On dual -battery system there is much more than that.
When charging modules are hard to test, scanning for codes and measuring car voltage when running is easy and good start.

 

I thought the trunk battery didn’t come until ‘12, and the shifter back up battery was the only other battery on board. First check is definitely the battery SOC while running. 14+

 

early W212 have aux battery in the dashboard.

 

Yes, the shifter battery

 

I'm not 100% sure of how this is on the cars we drive but in older cars the battery warning light connection was a very simple one and only lit up if the alternator stopped charging. For me this sounds like a charging issue, either the alternator itself or the regulator.