How to Fix an ‘Auxiliary Battery Malfunction’ on a Mercedes

How to Fix an ‘Auxiliary Battery Malfunction’ on a Mercedes

If you’re seeing an ‘Auxiliary Battery Malfunction’ message in your Mercedes-Benz, it’s a signal that there’s an issue with the secondary battery on the car, which supports the transmission.

This guide will walk you through a few simple steps to identify and resolve the problem!

Quick Answer: The most common solution for an ‘Auxiliary Battery Malfunction’ message on a Mercedes is replacing the auxiliary battery.

What does Auxiliary Battery Malfunction mean?

If your Mercedes is from 2005 or newer, it will have two batteries.

The main (or ‘starter’) battery starts your car and powers most electrical systems. The auxiliary battery is a backup.

The main difference between the starter battery and auxiliary battery on a Mercedes is that the starter battery is the main battery, while the auxiliary battery ensures that certain electrical components, particularly your car’s electronically-controlled transmission, keep functioning even if the starter battery fails or runs low.

Main / ‘Starter’ BatteryAuxiliary BatteryAlternator
Starts and runs the carProvides backup power, primarily for the transmissionCharges both batteries
Overview of the Mercedes-Benz battery system

When you see the ‘Auxiliary Battery Malfunction’ alert, it indicates that this backup battery isn’t working as it should.

It can also trigger warning messages like ‘Stop Vehicle Leave Engine Running’ or ‘Stop Vehicle Shift to ‘P’ Leave Engine Running.’

The ‘Auxiliary Battery Malfunction’ message can happen on various Mercedes models, including the E350, C300, CLA 250, ML350, and GLA 250.

Plus, this malfunction can have several causes.

Even though the official Mercedes Operator’s Manual states that it’s a charging issue, we found that the most common cause is simply that the battery is old and can no longer hold a charge effectively.

Auxiliary Battery Malfunction Mercedes Operator's Manual

Other causes might include a faulty Battery Management System (BMS) or problems with the wiring, but these are very uncommon.

5 Steps to fix an ‘Auxiliary Battery Malfunction’ message

If you see an ‘Auxiliary Battery Malfunction’ message on your Mercedes, you can often fix it yourself by replacing the auxiliary battery. A genuine Mercedes auxiliary battery costs around $160, while an OEM auxiliary battery costs around $70.

Here are the five steps to fix an ‘Auxiliary Battery Malfunction’ on a Mercedes:

  1. Get a Torx set and take safety precautions.
  2. Locate the auxiliary battery.
  3. Remove the old battery.
  4. Test and replace the auxiliary battery.
  5. Check the new battery.

Step 1. Get a Torx set and take safety precautions

Before we replace the auxiliary battery, you should gather the necessary tools and take some safety precautions.

Here are the tools you will need for the job:

  • A Torx bit set
  • A multimeter

Here are the safety precautions you should think about:

  • Disconnect the main battery. Before you start, make sure to disconnect the main battery. This is crucial to prevent any electrical shocks or short circuits.
  • Avoid a terminal short. Perhaps the most important thing to remember during a Mercedes battery replacement is to prevent contact between the battery’s positive and negative terminals. Also, the positive contact of the battery mustn’t make contact with any metal parts of the car.
  • Turn off your Mercedes. It might sound obvious, but it’s easy to overlook. You don’t want the keys in the ignition when you’re working on the battery.
  • Be careful of metal objects. Here’s something many forget: remove any metal jewelry or watches before starting. These can conduct electricity and cause short circuits or even burns. Trust me, it’s better to be safe than sorry.
Mercedes battery replacement terminal short illustration

Step 2. Locate the auxiliary battery

The location of the auxiliary battery in your Mercedes-Benz can vary based on the model of the car.

Generally, a Mercedes auxiliary battery can be found in one of these places:

  • In the engine bay. For some models, like the E-Class W211, the auxiliary battery is located in the engine bay, often on the passenger side.
  • Under the passenger seat. In many SUV models like the ML, R, and GL, you’ll find the battery under the front passenger seat.
  • Behind the driver’s seat. In models like the SLK and SL, the auxiliary battery is situated behind the driver’s seat back panel.
  • Near the trunk’s fuse box. Some models have the auxiliary battery located in the trunk, near the fuse box.
Mercedes E350 Auxiliary Battery Location

To find the exact location in your specific model, it’s best to search Google. For example, ‘Mercedes E350 2017 auxiliary battery location.’

Step 3. Remove the old battery

Now that you’ve located the auxiliary battery, let’s remove it safely. Follow these steps:

  1. Ensure that the main battery is disconnected. Double-check that the main battery is disconnected to avoid any electrical hazards.
  2. Access the auxiliary battery. Depending on its location, you might need to remove some panels or covers. In the case of under-seat locations, you might have to slide or tilt the seat for better access.
  3. Disconnect the auxiliary battery. Start by disconnecting the negative terminal first, followed by the positive terminal. Be careful not to let the tools touch both terminals at the same time.
  4. Remove any securing brackets. Some models have a bracket or clamp holding the battery in place. Use the appropriate tools to remove these.
  5. Carefully take out the battery. Once everything is disconnected and unscrewed, carefully lift the battery out. Remember, even small batteries can be heavy.

Step 4. Test and replace the auxiliary battery

Once you have removed the old auxiliary battery, it’s time to test it and, if necessary, replace it.

Testing the old battery:

  1. Use a multimeter. Connect a multimeter to the battery terminals. The positive lead goes to the positive terminal, and the negative lead to the negative terminal.
  2. Check the voltage reading. A healthy battery should show around 12 volts. If the reading is significantly lower, it’s a clear sign that the battery is failing.
  3. Decide on replacement. If the voltage is low or if you have other reasons to believe the battery is faulty (like age or visible damage), it’s time to get a new one.
Measuring a car battery - Illustration

Replacing the battery:

  1. Get the correct replacement. Purchase a new auxiliary battery that’s compatible with your Mercedes model. AutoZone has auxiliary batteries in stock for every Mercedes model.
  2. Position the new battery. Place the new battery into the designated spot where the old one was.
  3. Secure the battery. If there was a bracket or clamp holding the old battery, use it to secure the new one in place.
  4. Connect the terminals. First, connect the positive terminal, and then the negative. Make sure the connections are tight and secure.

Step 5: Check the new battery

With the new battery installed, it’s time to reconnect everything and check if the issue is resolved.

  1. Reconnect the main battery. Reconnect the main car battery that you had initially disconnected. Make sure the connections are secure.
  2. Check the electrical systems. Turn on your car and check the electrical systems to ensure everything is working correctly. There is no need for a reset after replacing the auxiliary battery.
  3. Look for the warning message. Start the car and look at the dashboard. The ‘Auxiliary Battery Malfunction’ message should no longer be there.
  4. Run a final diagnostic. If you have access to an OBDII Scanner, run a final diagnostic to confirm that no fault codes related to the battery are present.

After these steps, your Mercedes should run without the warning message.

It’s a great feeling to solve car issues yourself, isn’t it?

Can you continue driving with an Auxiliary Battery Malfunction?

Auxiliary Battery Malfunction Mercedes

It’s a common question: “Can I keep driving my Mercedes if the ‘Auxiliary Battery Malfunction’ message is displayed?”

The short answer is yes, but with caution.

The most important thing to remember is that you must manually set the transmission to ‘Park’ before you switch off the engine. Also, before leaving the vehicle, apply the electric parking brake.

As mentioned before, the auxiliary battery is a backup battery.

If it’s malfunctioning, you shouldn’t have any problems, except if the voltage of the main battery is cut.

In case the main battery fails, having a malfunctioning auxiliary battery can lead to situations where you can’t shift your car into Park, or worse, damage to your transmission.

Mercedes transmission selector

Plus, continuously driving with this malfunction puts extra strain on the main battery and other electrical systems.

So, while you can technically continue driving with an ‘Auxiliary Battery Malfunction’ alert, it’s not advisable to do so for an extended period.

Can I charge a Mercedes auxiliary battery?

Another question that often comes up is whether you can simply charge the auxiliary battery in your Mercedes-Benz instead of replacing it.

The answer depends on a few factors.

A Mercedes auxiliary battery lasts for around four years.

If your auxiliary battery is relatively new and hasn’t shown signs of significant wear (like swelling or leaking), charging might be an option.

Most auxiliary batteries in Mercedes cars are AGM (Absorbent Glass Mat) batteries. These can be recharged, provided they are not too far gone.

Here’s how to charge a Mercedes auxiliary battery:

  1. Use an appropriate charger. Ensure you have a battery charger that is suitable for AGM batteries. Using the wrong type of charger can damage the battery.
  2. Connect the charger. Attach the charger to the auxiliary battery, connecting the positive lead to the positive terminal and the negative lead to the negative terminal.
  3. Monitor the charging process. Keep an eye on the charging process. Most modern chargers will indicate when the battery is fully charged.
  4. Check the battery voltage. After charging, use a voltmeter to check the battery voltage. It should read around 12 volts if it’s charged properly.

Remember that your car’s alternator should also charge the auxiliary battery. If you find that the ‘Auxiliary Battery Malfunction’ message reappears, it might be because of a defective alternator.


I hope you found this guide helpful and easy to follow!

If you’ve followed the steps and fixed the issue, great job!

But if you ever feel unsure or if the problem seems bigger than expected, it’s totally okay to ask for professional help.

Take care!

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