Mercedes auxiliary batteries occasionally operate in conjunction with the main systems battery or take over for it when that battery dies or has extremely low voltage. When the main battery has moderately low voltage, the vehicle computer connects the two batteries, allowing the main battery to return to normal, while charging the auxiliary battery at the same time. Many Mercedes vehicles feature a dual battery system to prevent failures in their combined hydraulic and electric braking systems.
Mercedes vehicles equipped with a dual battery system have several different modes, depending on the status of the main systems battery. The most common mode links the two batteries together, taking the load off the main systems battery and charging both batteries. The dual battery system also has two shutdown stages as well as an emergency mode that completely switches all functions from the main battery to the auxiliary battery.
During a stage one shutdown, some of the vehicle's electronic components stop working. A stage one shutdown only disables a few of the vehicle's electronic features, such as the rear windshield defroster and heated seats. A stage two shutdown disables internal power outlets and lowers the climate control fan speed. During an emergency mode shutdown, the battery control system switches all functions to the emergency battery, and only essential electronic systems continue operating at full capacity.