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AutoFocus: One Mercedes, TWO batteries?!

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AutoFocus: One Mercedes, TWO batteries?!

The technician was working on a Mercedes. A red battery symbol with the message “Visit Workshop” was displayed in the message center of the instrument cluster. The battery and the alternator have been replaced with no results.

When the technician called Identifix, I asked if testing had been performed on the Battery Control Module (BCM) with a factory-compatible scan tool, and if the front battery had been tested with an electronic battery tester. The reply: “There’s a battery under the hood? Where is it??”

Starting in model year 2003, Mercedes came out with a new E-class model with the Mercedes chassis designation 211. One of the new features of the 211 is the Dual Battery System. A main systems battery is in the trunk of the car and an auxiliary battery is located under the hood, underneath the cabin filter at the right rear corner of the engine compartment.

The Dual Battery system uses a BCM, located next to the systems battery, that is in charge of the usage and the diagnostics of the auxiliary battery. It also monitors the voltage of the systems battery along with the amount of charging system output. The BCM uses the information to perform tasks in relation to both batteries. During normal operation, the BCM will perform a battery voltage test of the auxiliary battery. Depending on this voltage, the BCM can operate what is called the auxiliary battery relay. When this relay is energized by the BCM, the auxiliary battery becomes connected to the vehicle’s main battery. This, in turn, allows the charging of the auxiliary battery until the BCM turns off the battery relay. After that, the auxiliary battery will be disconnected from the vehicle. The BCM will then monitor the battery voltage and charge rate of the systems battery.

During a low voltage condition of the systems battery, the BCM is capable of prioritizing consumer shutoff operations in stages, in order to help the systems battery regain its voltage more quickly. During “stage 1” of shutoff, a message displayed on the instrument cluster states that the system is in consumer shutoff operation, and items such as the rear window defroster and heated seats will stop functioning. If the BCM determines that this is not enough to restore correct voltage of the systems battery, the BCM will then perform “stage 2” of shutoff. At this point, the driver may notice that any components plugged into their accessory outlets will stop operating, along with reduced blower motor speed operation.

Next is the “emergency mode.” For example, if the alternator fails to operate or if the systems battery voltage becomes too low for the operation of the vehicle’s electrical system, the battery control module will not only activate “stage 2” but also activate the auxiliary battery relay to connect the auxiliary battery to the vehicle’s electrical system.

Why add all these extra functions on a car that works just fine with one battery? One of the main reasons is that the Sensotronic Brake Control (SBC) brake system on the 211 series vehicles is part electronic and part hydraulic. This brake system has no substantial connection between the brake master cylinder and the rest of the brake system. The SBC system uses an electrical pump that will pressurize the brake system for operation. If there is an electrical failure of the vehicle’s charging system or an accident which damages the systems battery or alternator, the use of an auxiliary battery is needed to help maintain proper brake operation, and bring the vehicle to a safe assisted stop.

How do you know what to look for? Testing of the Dual Battery System requires the correct tools. The BCM operates on the vehicle’s data bus giving out valuable data that can be read with your scan tool, such as alternator charging current. It also stores fault codes along with maintaining a memory of consumer shutoffs. Not only is this system used in the 211 chassis but also in the 219 chassis, also known as the CLS class. The wiring diagram on the next page shows the BCM and its components.

In this Hotline subscriber’s case, a voltage test of the auxiliary battery found it to only have a rest voltage of 10 volts. Testing it with an electronic battery tester showed it to be faulty, so the battery was replaced. After the next key cycle, there were no more warning messages in the instrument cluster.

So the next time a 211 E-class comes in the door with a red light on the instrument cluster, be sure to check the other battery under the hood.

Service data provided by Identifix Inc. For more information, visit the company’s Web site at

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