Auxiliary Battery Malfunction

Auxiliary Battery Malfunction – Causes and Symptoms!

Auxiliary Battery malfunction Mercedes!

Auxiliary Battery Malfunction

An auxiliary battery malfunction is a common issue in Mercedes. The auxiliary battery, also known as a buffer battery or capacitor, is responsible for providing power to critical components of the car, such as the gear selector or ignition unit, in case the main battery fails. 

When the Mercedes auxiliary battery malfunction, it can cause an error message to appear on the dashboard and affect the car’s performance. While it is possible to continue driving with a malfunctioning auxiliary battery, getting it fixed as soon as possible is recommended to avoid any potential issues.

Don’t worry, after reading this article you will be able to understand the function of the auxiliary battery, how to diagnose the issue, and how to fix it.

What does the auxiliary battery do?

The auxiliary battery found in hybrid electric vehicles and electric cars supports all 12-volt electrical systems present in the vehicle, except for the air conditioning and heating systems. In simple terms, it powers computer controls, headlights, music systems, accessory systems, and other electrical accessories. 

Depending on the individual make and model of the car, the auxiliary battery or capacitor may occasionally be utilized to power electrical accessories like the radio or infotainment system.

Symptoms of Mercedes Auxiliary battery malfunction

1.   Error message on dashboard

If you see an error message related to the auxiliary battery on your car’s dashboard, it may indicate a malfunction. The error message may read “Auxiliary battery malfunction” or “Buffer battery of the electronic ignition switch AC is faulty.”

2.   Loss of voltage

If you have a diagnostic device for your car, you can measure the voltage of the auxiliary battery. If the voltage is zero, it may indicate a malfunction.

3.   Electrical issues

 A malfunctioning auxiliary battery can cause electrical issues in the car. This can include dimming headlights, flickering interior lights, and a weak or dead battery. If the car’s main battery fails, the auxiliary battery is designed to provide power to critical components such as the gear selector or ignition unit.

However, if the auxiliary battery is malfunctioning, it may not be able to provide enough power to keep these components running correctly.

  • Error codes

If you have a diagnostic device, you can check for error codes related to the auxiliary battery. The error code may indicate that the battery’s voltage is zero or that there is a general electrical fault.

5.   Overheating

A malfunctioning auxiliary battery may become hot to the touch. If the battery produces excessive heat, it may be a sign that it is working harder than it should be to keep the car’s critical components running. This can be dangerous and should be addressed immediately.

Causes of Auxiliary battery malfunction

1.   Overuse

The auxiliary battery is responsible for providing power to critical components of the car, such as the gear selector or ignition unit, in case the main battery fails. However, overuse of the auxiliary battery can lead to its malfunction. The auxiliary battery is not designed as a primary power source and can only provide temporary power in emergencies.

2.   Capacitor Failure

The auxiliary battery is a capacitor, not a battery, and can malfunction due to capacitor failure. Capacitors are electronic components that store electrical energy and release it when needed. When a capacitor fails, it can cause the auxiliary battery to malfunction.

3.   Broken Wires

Another possible cause of auxiliary battery malfunction is broken wires. If the wires that connect the auxiliary battery to the car’s electrical system are broken, the battery will not function properly.

4.   Fuse Issues

Issues with the fuse that connects the auxiliary battery to the car’s electrical system can also cause the battery to malfunction. If the fuse is blown or damaged, the battery cannot provide power to the car’s critical components.

How to test an Auxiliary Battery?

To test an auxiliary battery, you can follow a few steps.

  • First, connect the leads to the battery, with the negative lead connected to the negative terminal and the red lead connected to the positive terminal. The tester will turn itself on if the battery is partially dead.
  • Next, press the “menu” button on the tester and select “battery test.” The tester will ask if it is a 12 or 24-volt battery. Select the appropriate option.
  • Then, select “top post” or “side post,” depending on the type of terminals on the battery.
  • After that, select the appropriate standard for entering the battery’s cold cranking amps (CCA). For example, if the battery is lead-acid, select “CCA” and enter the appropriate number.
  • Once you have entered all the necessary information, the tester will begin testing the battery. If the battery needs to be replaced, the tester will indicate this.
  • Alternatively, you can test the voltage of the battery using a multimeter. Connect the black lead to the negative terminal and the positive lead to the positive terminal. The voltage should be above 12 volts for a 12-volt battery.
  • For a more specialized test, you can use a battery analyzer explicitly designed to test batteries. This tool will provide a more accurate analysis of the battery’s condition..

How to Fix the Auxiliary Battery Malfunction?

To fix the auxiliary battery malfunction, you must replace the capacitor, which is located under the passenger footwell area. The capacitor is not a battery but rather a device that holds a voltage or charge for critical components of the car, such as the gear selector or ignition unit, in case the main battery fails. 

Suppose you have a device or computer that can check the errors in the car. In that case, you will have an error message in the ignition indicating that the buffer battery of the electronic ignition switch AC is faulty and that a general electrical fault exists. 

To check what’s wrong, you can measure the voltages using live data, and if the voltage of the battery is zero, then the capacitor needs to be replaced. You can replace the old capacitor by taking it out and putting a new one in, then putting the connector in and tightening the nuts. 

You don’t need a diagnostic device to reset or do anything, but there will be some error in the system that won’t appear on the dashboard. The new capacitor should come with a warranty. It’s essential to fix the auxiliary battery malfunction as soon as possible to avoid problems with the main battery.

Final Thoughts

Modern cars frequently experience auxiliary battery problems, which can impair the functionality of vital parts like the gear selector or ignition system. It is crucial to understand the auxiliary battery’s purpose, the signs of a malfunction, and the reasons why it occurs. 

To prevent potential problems with the main battery, it is advised that you get it fixed as soon as you notice any symptoms. 

You can ensure that your automobile’s electrical system operates smoothly and effectively by changing the capacitor and correcting any damaged wires or blown fuses.


A: If the car starts, you can drive with a bad auxiliary battery, but the driving experience will be wrong and frustrating. However, fixing any fault as soon as possible is always recommended.

A: A Mercedes-Benz auxiliary battery replacement typically costs between $210 and $450 at a dealership or auto repair, with parts costing between $45 and $258 and labor costs between $50 and $110, depending on where it is done. However, the price to do it yourself ranges from $30 to $95, and the new auxiliary battery you install doesn’t require programming.

A: If the system is improperly wired or the auxiliary battery is defective or malfunctioning, the auxiliary battery could drain the main battery. Auxiliary batteries can drain power from the primary battery by being wired into a car’s electrical system, making it difficult to start the engine and perform other crucial tasks.

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