Can the Mercedes Auxiliary Battery be Charged?

If you own a Mercedes-Benz, you’re lucky enough to drive around in one of the most luxurious cars in the world. But, when it comes to its auxiliary battery, you might find yourself confused in some situations.

There are certain questions you must answer and certain procedures you must learn before investing in this luxurious car. For example, it’s important to know how to charge the Mercedes auxiliary battery. 

Yes, you can charge a Mercedes auxiliary battery. This battery connects to the main battery via the auxiliary battery relay, and the main battery connects to the alternator. 

Keep reading to find out how to charge, fix, or replace the Mercedes auxiliary battery.

Mercedes Auxiliary Battery

The Mercedes auxiliary battery is used only for a short period in case of emergencies when your car battery dies unexpectedly. However, you can also use this battery to charge electronic accessories in your car. Typically, an auxiliary battery lasts three years and has 12 volts and 200 amps.

Can the Mercedes Auxiliary Battery be Charged?

The auxiliary battery connects to the main battery via the auxiliary battery relay, which is actuated by a master control unit, controlling the car’s onboard electronic network. This master control unit is known as the Signal Actuation or Acquisition Control Unit. 

The Signal Actuation or Acquisition Control Unit utilizes an electrical signal line to monitor the auxiliary battery’s temperature and voltage. It actuates the auxiliary battery when its electrical voltage goes below 11- 14 volts. 

As a result, the auxiliary battery can now be charged by supply from the main battery. However, this is only possible if the car is already running and the alternator is charging the main battery. This prevents discharging the main battery.

How to Charge the Mercedes Auxiliary Battery

As we mentioned, the auxiliary battery is charged by the main battery, which is charged by the alternator. However, there are a few important tips to remember while charging your Mercedes auxiliary battery for safety and efficiency. 

Firstly, you must not forget to read the manufacturer’s instructions about charging your auxiliary battery or the specific charger. If you’re using an external charger to charge your auxiliary battery, disconnect the battery from your car before doing so. 

In addition, you must never leave your charging auxiliary battery unattended for a longer period or overnight. It doesn’t take too long to charge the battery completely, so check on it every 10 to 15 minutes and unplug it as soon as it’s fully charged. 

That’s because overcharging the auxiliary battery can decrease its lifespan of three to four years. You may also face issues like battery failure or reduced cranking power. 

Why the Mercedes-Benz Uses an Auxiliary Battery

Most Mercedes-Benz models have start/stop buttons, which means they run on an ECO (Engine Cut Off) system. This type of car powers off each time it’s stopped at a red light or intersection and is powered on once the driver lets go of the brake pedal. 

During this process, the auxiliary battery has the purpose of powering the vehicle’s onboard systems. That includes audio systems, climate control blower motors, and more. In addition, if the main battery is discharged, the auxiliary battery provides an Emergency Parking function.  

Each time the engine is cranked, the starter motor must draw approximately 200 amps, resulting in a minor voltage drop in the main battery. This 3 to 4 volts drop results in an 8 volts drop in the power supply, leaving insufficient voltage to power the vehicle onboard system. 

This is where the auxiliary battery comes into play, connecting to the power supply circuit. Now that the main battery is not responsible for powering other vehicle functions, it can power the engine efficiently. 

When the engine is turned off, the mechanical oil pump found in the gearbox stops working. In this case, the auxiliary battery powers the transmission auxiliary oil pump. 

How Long It Takes to Charge the Mercedes Auxiliary Battery

Typically, it takes about two to four hours to completely charge an auxiliary battery, but that period may vary based on the battery’s size and how much power it needs. If you’re not using the main battery to charge the auxiliary battery, an external wall charger may work too. 

You must monitor the auxiliary battery’s performance for at least one hour after a full charge. To prevent battery failure, you must ensure that the charger’s wattage matches the battery’s rating.

How to Check the Mercedes Auxiliary Battery

In some cases, you may need to replace your Mercedes auxiliary battery, but you must check it before doing so. Firstly, remove the battery cover to check the voltage; it should be at least 12 volts. If the voltage is under 12 volts, it’s time to replace the auxiliary battery.

You can look at the light indicator when you want to check whether the battery is charging. An increase in voltage on the voltmeter is another indicator that the auxiliary battery is charging.

To make the most of the auxiliary battery’s three to four-year lifespan, you must check the battery now and then. Unfortunately, it isn’t used regularly, so it’s easy to overlook its performance. However, this battery is essential in an emergency, so it’s best to check it regularly if you don’t want to end up stranded. 

Mercedes Auxiliary Battery Malfunction

If you ever see the “Auxiliary Battery Malfunction” message on your Mercedes-Benz screen, don’t stress; there are a few solutions to this issue. Typically, a faulty voltage converter is a reason behind this malfunction.

When this error shows up, the car’s entertainment system, start/stop action, radar, PDC sensors, and other factors will be severely impacted. However, it doesn’t impact your car’s drivability, so it’s safe to keep driving, but not for too long.

Another message may pop up when the auxiliary battery begins malfunctioning, saying, “Stop vehicle shift to P leave engine running.” In this case, it’s best to start the car to see if it’s your auxiliary battery causing this error. 

If the car starts, your secondary battery is probably causing the issue instead of the auxiliary battery. You can also use an OBD scan tool to diagnose the issue, as it will give you all the diagnostic details about your vehicle. 

When your car is showing the “Auxiliary Battery Malfunction” error, make sure not to drive longer than 100km to prevent damaging your battery further. Don’t put off fixing this error for too long; otherwise, it can lead to more serious battery issues down the line. 

Since you’re not in immediate danger, you don’t need to stop the car instantly. Instead, you may drive home or to the nearest authorized workshop. If the malfunction isn’t serious, it may be unnecessary to replace the battery instead of just correcting the onboard computer configuration. 

How to Fix Mercedes Auxiliary Battery Malfunction

Here is a step-by-step guide on fixing the Mercedes auxiliary battery malfunction.

  1. Diagnose the problem with the help of the tips mentioned above.
  2. Locate the auxiliary battery. In newer models, you can find it inside the passenger seat’s front footwell near the top. 
  3. Detach all connecting parts and the carpet to access the auxiliary battery.
  4. Use a 10mm socket to unfasten the screws securing the battery to the storage component. 
  5. Remove the primary car battery’s connectors to prevent sparks and shortages.
  6. Remove the auxiliary battery
  7. Peel back the small plastic tabs to remove the voltage converter or capacitor.
  8. Replace the old capacitor with a new one. 
  9. Follow steps 3 to 9 in reverse order to reinstall the auxiliary battery.

Also, this procedure is fairly easy to perform by yourself. Still, it’s best to let a professional finish the job to prevent accidents or further damage to your auxiliary battery. Then, head to the nearest authorized workshop to fix the “Auxiliary Battery Failure” issue.

Replacing the Mercedes Auxiliary Battery

To save money on traditional repair costs over and over again, it’s best to replace your auxiliary battery to prevent accidents in the future. The cost of replacing a Mercedes auxiliary depends on the make, model, and year, but the range is typically between $40 and $300. 

Choosing the Right Mercedes Auxiliary Battery

When it’s time to buy a new auxiliary battery for your Mercedes-Benz, you must consider a few factors. Firstly, the battery needs to be 12 volts and 200 amps, which is the minimum requirement for Mercedes auxiliary batteries. 

Other than that, check the make, year, and model of both the car and the battery to ensure that the two are compatible with each other. However, it’s not necessary to replace the battery every time you see a malfunctioning error. In most cases, you only need to correct the onboard computer configuration. 


The Mercedes auxiliary battery can be charged by supply from the main battery. You don’t need to trickle charge the auxiliary battery or connect it to an external battery charger, as this battery is only used temporarily. 

It typically lasts three to four years, and you won’t find this battery in the latest Mercedes Benz models. In case you need to replace the auxiliary battery, make sure to consult an authorized workshop. 

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