Mercedes-Benz W211 auxiliary battery: location, symptoms and replacement
Mercedes-Benz W211 auxiliary battery: what is it and where is it located?
The auxiliary battery is a small extra battery that supplies power to the SBC brake control. It is in the engine bay near the bulkhead, under the air filter housing behind a plastic cover. When the power is switched off, an isolation relay separates it from the rest of the car.
How can I recognise a fault in the W211 auxiliary battery?
Typical symptoms are a red battery warning and a message about electrical devices being switched off. A resting voltage of about 10.4 V indicates an empty battery, and about 10 V points to a broken cell. Measure the main battery and the auxiliary battery separately, because replacing only the auxiliary battery does not always solve the fault.
What should be noted when replacing the W211 auxiliary battery or main battery?
First let the car go to sleep before disconnecting the batteries. A new battery should be charged full, because a partial charge can cause comfort function errors. After the replacement, the clocks and radio are set again and the windows are run up and down once. If needed, have the brake system checked at a workshop.