When a faulty auxiliary battery in your vehicle its back up power supply is no longer functioning correctly. This backup system for the electrical needs of a vehicle was introduced to many models over a decade ago. When the auxiliary battery no longer is capable of holding a charge, a light on the dash will illuminate. For mercedes benz vehicles, this is the "visit workshop" red light. The battery can become non-functional when it loses a cell and shorts out or the water and acid level in it drops below the required level causing the metal there to become exposed to air and removing its ability to transfer electrons.
The main battery on almost all vehicles is located in the engine compartment. The auxiliary battery is at the other end of the vehicle in most instances or in the trunk. This auxiliary power supply is a supplement to the electrical system to ensure there is enough power for the vehicle to operate as it was designed too. On many vehicles, the auxiliary battery is of the same size and power capabilities as the main battery in the engine compartment. They are both supplied with renewable power from the alternator on the engine when it is being operated.
The replacement of the auxiliary battery is similar to the replacement of the main battery. The first step is to disconnect the main battery by removing the battery terminals. That same procedure should then be carried out on the auxiliary battery. If there is acid on the exterior of the battery casing, it can be cleaned up by pouring a simple coke or other soft drink on it, then rinsed with water.