Fix Mercedes-Benz C 300 Auxiliary Battery Malfunction

What is the Auxiliary Battery all about?

The vast majority of passenger vehicles on the road today have only a single, large battery. It’s generally located under the hood, although some makes and models have the primary battery in the trunk, near the spare tire. This battery’s job is simple – it provides the electricity needed to start the engine. All other power generated during operation to power the radio, signal lights, and other accessories comes from the alternator.

However, some vehicles (notably higher-end European imports) have more than one battery. There’s the primary battery under the hood, but there’s also an auxiliary battery. These are used to power other systems on the car that require 12 volts of power, and which the alternator is not capable of powering directly. In these instances, the alternator is responsible for charging both the primary and the auxiliary battery. The primary battery delivers voltage during cranking, and the auxiliary battery runs some or most of the car’s accessories.

Like all batteries, auxiliary batteries will eventually wear out or fail. When this happens, they’ll need to be replaced.


What is A0009829308 auxiliary battery ?

One more type of the battery for mercedes cars are A0009829308, it is very similiar looking for the previous model described above, but it has little bit different shape and different location for positive and negative terminals.

A0009829308 battery has few comaptible mercedes-benz oem part numbers A0019822708, A0009829608, A0049820008 and all of them are exchangable and compatible with each other.

As you can see there are many different OEM part numbers for mercedes-benz backup batteries A0009827008 A000 982 70 08 and so on.

Why does Auxilary Battery Malfunction come up? 

When the auxiliary battery fails to hold a charge to recharge, you will get an error message such as “auxiliary battery malfunction” or, in 2015 and newer models, “stop vehicle shift to P leave engine running” warning message.

These two error messages are common for Mercedes-Benz cars, and other than the error message, the vehicle usually runs and drives fine. 

If the car runs and drives fine, the problem is an auxiliary battery, but you keep seeing the auxiliary battery malfunction error on the dashboard. 

We have seen Mercedes-Benz try to read the codes with a generic OBD-II scanner, but no fault codes come up. Using a generic scanner, you will not find fault codes that indicate a bad auxiliary battery since generic scanners cannot read Mercedes-Benz-specific fault codes. 

You will know that the secondary battery is at fault because the car will still turn on, and some functions will work. The auxiliary battery does not affect the car’s mechanical functions. 

Where is auxiliary battery located ?

As you did understand from our blog there is usually three different locations for auxiliary battery and all depends on the car model.

The main three locations are: trunk (boot) near fuse box, at passenger side under footwell light or driver side under upper side panel.

What Is An Auxiliary Battery?

Auxiliary batteries work with your main starter battery, and as a backup battery in the event the main battery becomes disabled. It prevents your vehicle’s electrical components from dropping when starting/stopping your engine and keeps your radio, GPS, and other in-vehicle electrical components turned on.

Any failure with your voltage converter will typically depict an error fault message on your instrument cluster. It may appear as a small battery symbol or show an onboard computer error fault message displaying an “auxiliary battery malfunction.” You will notice it immediately every time you start your vehicle until a proper diagnosis (and potential replacement) by yourself or a licensed technician can be performed.

Our recommendation:

Over time and through normal use, your battery deteriorates. Eventually, it will fail completely. Most have a lifespan of about five to seven years, but this is only an estimate. Once your auxiliary battery fails, it puts a strain on the rest of your system, and some of your accessories may not operate properly or at all. Have the problem professionally diagnosed by one of our expert mechanics as electrical issues can stem from many different problems, not just battery failure.

What type of Mercedes Auxiliary Battery does my vehicle have?

Mercedes vehicles come with 3 different types of auxiliary batteries which is something you’ll want to keep in mind. Depending on your specific make and model, the location of the battery may vary also which is why it’s important to get that information before attempting to proceed with repair or replacement once you identify an auxiliary battery malfunction.

To check exactly which battery variant is used in your specific vehicle, you will be required to open the trunk and hood. Inside of your trunk you will find fuses if you don’t see a small auxiliary battery nearby, your vehicle does not use a voltage converter. Underneath the hood, you will be able to locate your main battery which can give you a quick insight into the system that your particular model uses. If you don’t see a small auxiliary battery underneath your hood, that means that your car does not have one.

How Does a Faulty Auxiliary Battery Affect My Vehicle’s Performance?

An auxiliary battery malfunction can cause your vehicle’s electrical systems and operations to display errors. These include your entertainment system, GPS, PDC sensors, radio, and start/stop functionality. You will start to notice vehicle safety features like Blind Spot Assist, Lane Keeping Assist, and similar lighting notifications fail to activate. An AUX battery light may also activate in your instrument cluster.

Note, these are not critical components, so it won’t cause your vehicle not to start. However, they can affect your concentration on the road, especially if you are reliant on any of these helpful safety features like Lane Assist. Imagine having to drive without your radio on during a long road trip? Your fellow passengers may not be able to forget it for months afterward.

Get Mercedes Repair Work Done at EurAuto Shop in Plano TX

We specialize in service and repair of high-end and exotic European vehicles. When you entrust your car to us we’ll give it the care and respect it deserves. Make an appointment today or call (214) 552-4194 to discuss your automotive needs.


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