Backup battery in Mercedes-Benz: function, error message, replacement
Many Mercedes-Benz models have a small second battery in addition to the starter battery: the backup or auxiliary battery. If the instrument cluster reports an auxiliary battery malfunction, it is usually at the end of its life. Here you can read what it is for and what to consider when replacing it.
What is the backup battery for?
The backup battery is a small 12 V battery that stabilises the electrical system. Depending on the model, it performs different tasks: it keeps important consumers alive during a voltage drop of the main battery – for example when the engine restarts via the start-stop system – or safeguards functions such as engaging the park position with an electronic selector lever. Which task it performs in your vehicle is stated in the owner's manual.
Signs of a weak backup battery
- A message in the instrument cluster, such as “auxiliary battery malfunction” or a prompt to visit a workshop
- The start-stop function remains off – on models in which the backup battery safeguards the restart
- Entries in the fault memory relating to the auxiliary battery
The backup battery ages like any battery. Because it is small and frequently loaded, it often fails independently of the main battery. A test of the starter battery therefore says nothing about the condition of the backup battery.
Which backup battery fits?
Compare the part number on the old battery or in the parts catalogue. According to the item description, our 12 V, 12 Ah AGM backup battery is a replacement for the Mercedes-Benz numbers A0009829308 and A2115410001 as well as for the BMW numbers 61217586977 and 61219394648. The dimensions are 151 × 87 × 145 mm (L × W × H). Also check the terminal arrangement and installation location. You will find the backup batteries in the Start-stop batteries category.
Replacing the backup battery
- Find the installation location: It depends on the model. The owner's manual or your workshop will state the location.
- Switch off the ignition: Switch off the ignition and keep the key out of range of the vehicle.
- Disconnect: First loosen the negative terminal, then the positive terminal. Open the holder and remove the battery.
- Install: Insert and secure the new battery, connecting the positive terminal first, then the negative terminal.
- Check: If the message remains after a few drives, have the fault memory read out and cleared.
Work on the vehicle electrical system should only be carried out if you are familiar with it. Otherwise, a workshop will take over the replacement.
And the main battery?
If the starter battery has also aged, it is worth checking its condition. Many vehicles with a start-stop system need an AGM battery, and after replacement the new battery often has to be registered in the battery management system.
Frequently asked questions
What does “auxiliary battery malfunction” mean on a Mercedes?
The vehicle has detected a problem with the backup or auxiliary battery. It has often aged and no longer supplies enough voltage. Have the fault memory read out and replace the battery if necessary.
Can I keep driving with a faulty backup battery?
The vehicle will generally keep driving, but the functions that the backup battery safeguards may not be reliably available. Have the fault rectified promptly.
Where is the backup battery located on a Mercedes?
That depends on the model. The owner's manual or your workshop will state the installation location.
Which auxiliary battery fits my Mercedes?
Compare the part number of the old battery. According to the product description, our 12 V, 12 Ah AGM auxiliary battery replaces A0009829308 and A2115410001, among others.
Does the auxiliary battery need to be registered?
If a message remains after replacement, the fault memory should be read out and cleared. Whether further steps are necessary depends on the model.