Auxiliary Battery Malfunction Mercedes: 5 Causes and How to Fix It

Tim Miller

Seeing the “Auxiliary Battery Malfunction” warning on your Mercedes?

Don’t worry, though! I’ll break it all down for you. 

This article is your guide to understanding the meaning behind this warning, its causes, and, most importantly, how you can fix it. 

So, buckle up, and let’s get started!

What is the Auxiliary Battery?

Modern Mercedes-Benz vehicles are equipped with an auxiliary battery, also known as the secondary or backup battery. 

The location of the aux battery in a Mercedes can differ based on the vehicle’s model and year. However, there are common locations where you can typically find the auxiliary battery:

  • Engine Compartment
  • Trunk
  • Under the Rear Seat
  • In-Car Compartment

To locate the aux in your specific Mercedes-Benz model, it’s best to refer to your vehicle’s owner’s manual, consult a qualified technician, or contact a dealership.

This battery serves as a power source for essential electrical systems, such as the keyless entry system, interior lighting, and electronic modules, when the main battery is unable to deliver sufficient power. 

The auxiliary battery also helps to stabilize the voltage and prevent voltage fluctuations, ensuring the vehicle’s electrical components function optimally.

Similar to the primary battery, the backup battery recharges while the engine is operational. Likewise, it also has a limited lifespan, typically lasting between four to six years. 

However, once it loses its ability to retain a charge, the vehicle’s computer system activates a malfunction warning message.

What Does the Auxiliary Battery Malfunction Mean?

Auxiliary Battery Malfunction Mercedes

The “Auxiliary Battery Malfunction” warning typically refers to a problem or issue with the auxiliary battery.

When the auxiliary battery malfunctions, it can lead to various problems related to the vehicle’s electronic components and systems.

Here are some common symptoms of an aux battery malfunction on a Mercedes:

  • Warning Messages: The vehicle’s onboard diagnostics may detect the backup battery malfunction and display warning messages on the instrument cluster or central display.
  • Keyless Entry Issues: Difficulty in unlocking or locking the doors remotely using the keyless entry system.
  • Interior Lighting Problems: Flickering or failure of interior lights to illuminate as intended.
  • Electronic Malfunctions: Abnormal behavior of electronic systems, including the infotainment system, climate control, power windows, or other electrical components.

When you encounter this warning, it’s crucial to address it promptly to prevent further complications and ensure the proper functioning of your Mercedes-Benz’s electronic systems.

Can I Drive My Mercedes with Auxiliary Battery Malfunction?

Yes and no. While it is generally possible to drive a Mercedes vehicle with an auxiliary battery malfunction, it is not recommended to do so for an extended period or under certain circumstances. 

The auxiliary battery supports essential electrical systems and components, and a malfunctioning battery can lead to unpredictable behavior or failure of these systems.

Additionally, if the aux battery is severely depleted or fails completely, it could potentially impact the overall performance and drivability of the vehicle.

To avoid unexpected complications or further damage, it is advisable to have the auxiliary battery inspected and repaired or replaced by a qualified technician as soon as possible. This will help ensure the proper functioning of electrical systems and maintain the optimal performance of your Mercedes.

What Causes Auxiliary Battery Malfunction Mercedes?

Several factors can contribute to auxiliary battery malfunctions in Mercedes vehicles. Some common causes include:

Bad Auxiliary Battery
The primary cause of the secondary battery malfunction is a faulty auxiliary battery itself. As the battery ages, it may lose its ability to hold a charge or provide sufficient power.

Solution: Replace the backup battery with a new one that meets Mercedes-Benz specifications. Regularly replace it to prevent future issues.

Malfunctioning Auxiliary Battery Relay
The auxiliary battery relay is responsible for determining when the aux battery needs to be charged. It monitors the voltage and, when necessary, signals the charging process to ensure the battery remains in good condition.

If this relay malfunctions, it can disrupt the charging cycle, potentially leading to an auxiliary battery malfunction.

Solution: Replace the faulty auxiliary battery relay with a new one to restore proper charging and management.

Faulty Battery Control Module
The battery control module measures the voltage and monitors both the primary and secondary batteries. When this module fails, it can result in inaccurate battery management, leading to issues with the auxiliary battery.

Solution: Seek professional help to diagnose and replace the faulty battery control module, ensuring proper battery management.

Defective PolySwitch Fuse
The PolySwitch fuse protects against surges and overcurrents, maintaining the electrical flow within acceptable limits. Unlike traditional automotive fuses, the PolySwitch fuse automatically resets itself after a fault is resolved.

However, if there’s a problem with this fuse, it can lead to auxiliary battery malfunctions and trigger warning lights.

Solution: Replace the PolySwitch fuse with a new one that matches the specifications recommended by Mercedes-Benz.

Wiring Issues
Damaged or corroded wiring in the auxiliary battery circuit can prevent the battery from receiving power or communicating effectively with the battery control module. Wiring problems can disrupt the proper functioning of the backup battery and result in malfunctions.

Solution: Have a qualified technician repair or replace damaged or corroded wiring in the auxiliary battery circuit to ensure reliable connections.

How to Fix Auxiliary Battery Malfunction Mercedes

Step 1: Inspect Battery Connections

Check the secondary battery terminals and connections for cleanliness, tightness, and corrosion. Ensure the connections are secure and in good condition.

Step 2: Test Battery Voltage

Use a multimeter to measure the voltage of the backup battery. A healthy auxiliary battery should typically read around 12 volts when the vehicle is not running. If the voltage is significantly lower, the battery may need recharging or replacement.

Step 3: Scan for Trouble Codes

Use a diagnostic tool to scan for trouble codes. This step can help pinpoint specific issues related to the aux battery and the vehicle’s electrical system.

Step 4: Recharge or Replace the Battery

Depending on the condition of the auxiliary battery, you may need to recharge it using a compatible battery charger or replace it with a new one that meets Mercedes specifications. Make sure to choose the right battery for your vehicle model.

Here’s how you can do it:

Step 5: Reset Trouble Codes

After addressing the battery issue, reset the trouble codes using the diagnostic scanner or OBD-II scanner. This will clear any stored fault codes related to the auxiliary battery malfunction. 

Make sure to follow the instructions provided by the scanner or consult the vehicle’s owner’s manual for the exact procedure.

What’s Next?

There you have it! We’ve demystified the “Auxiliary Battery Malfunction” warning in your Mercedes-Benz, which signals issues or failures in your auxiliary battery. But fret not; with the insights from this article, you’re in control. 

So, next time you encounter this warning, don’t panic. Instead, take a deep breath, refer back to this guide, and confidently tackle the issue.

Moreover, to ensure you fully understand what’s going on with your Mercedes and to pinpoint any potential issues accurately, investing in a diagnostic tool can be a valuable addition to your toolkit.

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Information sourced from the owner's manual.