Auxiliary Battery Malfunction: What Does It Mean & How To Fix

Suppose you are getting an auxiliary battery malfunction; no need to panic! This problem is quite common with cars with a dual battery system like the Mercedes vehicles.

Mercedes models come with two batteries, a main and secondary battery. The secondary battery, also known as auxiliary battery, serves as a backup to the main battery. If the Auxiliary battery fails, your ECM will send the error message “Auxiliary Battery Malfunction” to notify you of the failure.

The malfunctions could cause many problems to your vehicle and make it difficult to drive. These problems include difficulty starting the vehicle and failure in electric components. 

When your auxiliary battery starts failing, it begins to warn you before it becomes completely dead. You will notice that your battery warning light is on, and the car will display the warning message described above.

What Does An Auxiliary Battery Malfunction Mean?

When the auxiliary battery has problems keeping its charge, you will receive a message on your dashboard that states, “auxiliary battery malfunction.” Your vehicle will still run, as this does not affect the car’s drivability. 

Auxiliary Battery Malfunction warning

This error appears when the auxiliary battery gets damaged or simply stops holding its charge after multiple years of use. When the auxiliary battery is not able to hold its charge properly, your vehicle’s electronic accessories might stop working and your instrument cluster will start throwing all sorts of warnings.

In order to fix this error message, you must replace your Mercedes auxiliary battery with a new one.

The auxiliary battery performs the role of powering most accessories in the vehicle that require 12V of energy. It powers electronic components like the Powertrain Control Module (ECM), headlights, air conditioner, and more. The auxiliary is seen as the second battery and serves as the main battery’s backup. In some cars, the auxiliary battery only stores energy when the main battery starts running low.

When the auxiliary battery goes bad, the electrical components that rely on the battery will stop functioning, and you might be prompted will all sorts of warning messages. You will have problems with some systems of the vehicle, like the A/C and headlights, which could either malfunction or not work at all.

Also, your vehicle’s safety functions that are powered by the auxiliary battery will be affected. The GPS, Entertainment system, PDC sensors, and many other sensors will be affected. In extreme cases, the vehicle may not even start or may randomly stop. 

Can You Drive With An Auxiliary Battery Malfunction?

You should be able to drive your vehicle for a while after your auxiliary battery fails. However, replacing the battery as soon as you notice it has failed is important. 

We advise you don’t drive your vehicle for more than 100 miles after you realize the battery is faulty. Drivers must pay attention to the warning signs that indicate a bad auxiliary battery.

In some vehicles, your engine will not start when you have a failing auxiliary battery. Without the engine starting, there is no way you can drive the vehicle. It might happen in a case where the main battery and the auxiliary battery are both either drained or bad. 

However, the auxiliary battery malfunction shouldn’t affect the mechanical function or drivability of the vehicle. It means that you should still be able to drive the vehicle. As you drive the vehicle, you will notice some symptoms that your vehicle will begin to exhibit, such as failing electrical components, battery lights, radio, and more. 

Once your vehicle shows obvious symptoms, you must immediately replace the battery. Failure to make a replacement may cause other parts of the vehicle to fail, leading to more cost of repair.  

How Much Does A Mercedes Auxiliary Battery Cost?

A new Mercedes auxiliary battery will cost between $70 to $200, excluding the cost of labor. Many factors determine the price of changing your Mercedes auxiliary.

Having a budget will help you with your purchase and save time. You should consider if you will purchase a new or used battery. Used auxiliary batteries are available for sale and will cost a little above $40. However, we advise you to use a new battery to enjoy the full benefits, including durability and warranty, for peace of mind. However, there are occasions when one might be able to source a used battery in very good condition.

Secondly, consider the type and brand of auxiliary battery you will be using. Auxiliary batteries come in different types determined by the components used in making them. Various types of batteries do not have the same price. You should use the same type of battery as your original part.

Finally, it would help if you considered whether you would be getting professional help. Installing an auxiliary battery is quite straightforward, but if you are getting help, it might cost an extra $100 or more. However, there are shops that will install your new battery for free when purchased.

What Does Mercedes Auxiliary Battery Control?

The auxiliary controls a major part of the electric system and serves as the backup battery when the main battery is faulty or drained. It runs components like the headlight, air conditioner, and more. 

Accessing the auxiliary battery on your Mercedes is pretty easy. The battery is usually located under the hood, near your windshield.

Mercedes-auxiliary-battery location

The role of the auxiliary battery depends on what model of Mercedes you are using. In some models, the battery only serves the backup function: store energy and wait until the main battery is down before it starts functioning. 

In some other Mercedes models, the auxiliary battery serves more than the function of a backup battery but also powers some electric components in the vehicle. Components like the headlights, air conditioner, audio system, and more rely on the auxiliary battery. 

The auxiliary battery is also responsible for powering the Powertrain Control Module (PCM). It might not be the case in some Mercedes models, as the main battery may power the PCM. Once your auxiliary battery is bad, you may have problems with your PCM. 

Will Auxiliary Battery Drain Main Battery?

The auxiliary can drain the main battery if the auxiliary battery is failing or malfunctioning.  

Having multiple batteries comes with its advantages and disadvantages. Multiple batteries can draw current from each other, causing the other to drain easily, which is why vehicles use isolators or separators.  

The isolator uses diodes and ensures that both batteries and the alternator are equally charged. On the other hand, the separator guarantees that both batteries receive current from a single source using a solenoid. 

When there is a battery malfunction, the separator or the isolator can be affected, allowing the auxiliary battery to drain the main battery. 

What Happens If Auxiliary Battery Dies?

When your auxiliary battery is drained, it has some symptoms that are pretty easy to spot. These symptoms make driving the vehicle quite difficult or, in some cases, impossible. 

1. Battery Lights 

The first symptom of a failing auxiliary battery is a failing battery. It will be triggered when there is a problem with the main or backup batteries. So when you see the battery light come up, check both batteries immediately.   

2. No crank 

The auxiliary battery does not serve the primary function of starting the car, but when the main battery is down, the auxiliary steps in to start the vehicle. When your vehicle does not start, it means that both the main and auxiliary batteries are bad. 

3. Failing electric components 

When the auxiliary battery fails, so will many parts of the vehicle—the battery powers major parts, including sensors, air conditioners, and more. Electric parts like this will not receive adequate current to power them, and if the battery is not fixed or replaced, it could damage these parts. 

These symptoms make it quite hard for your vehicle to function well and also make driving quite difficult. Yes, you can drive with a bad auxiliary battery if it does start, but the driving experience will be bad and frustrating.

When the auxiliary battery gets drained, do not attempt to charge the battery. You can not recharge the battery once the voltage hits below 4 volts and exceeds internal resistance; you would have to change the battery. 

Is Mercedes Auxiliary Battery Covered By Warranty?

A Mercedes warranty covers the Mercedes auxiliary battery. Their policy covers the original battery of the vehicle if found to be damaged or defective in respect of material and labor.  

The warranty covers up to 50,000 miles or 48 months of usage and will also cover a replacement done under warranty. It means when the company replaces the new one, the remainder of the original battery warranty term will cover the new battery. 

Final Thoughts!

Failing auxiliary batteries is not usually taken seriously by many drivers. Still, it could cause serious damage to your vehicle’s electrical system if it is neglected for too long. 

We advise that you pay close attention to the condition of your car’s battery, including the warning light and error messages that pop up. The best solution to a dead or drained auxiliary battery is to replace it rather than try to fix it.