This Is Why Your Mercedes Has 2 Batteries (Replacement Cost & What They Do)

If you own a Mercedes Benz, you may have seen the “Auxiliary Battery Malfunction” warning light on your vehicle’s dashboard. It may be confusing if you don’t know the function of the Auxiliary battery.

The Mercedes vehicles are designed to have two batteries, namely the main and the Auxiliary battery. These batteries cater to the different electrical needs of the car. In addition, the auxiliary batteries can be seen as a support or a secondary power source in the car. 

Furthermore, the main battery helps start and power the vehicle’s electrical system. In contrast, the auxiliary battery serves as the backup battery.

Why Do Mercedes Cars Have 2 Batteries?

Mercedes cars are advanced vehicles with heavy power demand to cater to their electrical components and needs. To deal with the demand for power without straining the main battery, Mercedes came up with the two-battery concept.

This design involves having a main and a secondary (auxiliary battery). The main battery is found in the trunk, while the auxiliary battery is located close to the windshield and is usually smaller in dimensions.

mercedes main and auxiliary batteries

Before the advent of two batteries, cars were designed to have a single battery that was used to start the car and supply power to electrical components.

Soon automakers began to see the efficiency of having a backup battery and the main battery. Some vehicles began to allocate roles to the auxiliary battery, including supplying power to minor lighting components in your car. The main battery was left to start the car, powering the ECM and other major components in the Mercedes.

Another major characteristic of the dual battery concept was that the auxiliary battery was usually limited to 12V and powered the minor components in the car. Nowadays, all modern vehicles make use of two batteries. The functions of both batteries depend on the battery concept your Mercedes uses.

Related content: Car Is Slow to Start, but Battery Is Fine – (Here are 7 Causes)

What Is the Second Battery in a Mercedes?

The second battery, also known as the auxiliary battery, plays different roles in your Mercedes,  depending on the vehicle model.

In some Mercedes models, the auxiliary battery is just a backup, while the battery powers minor electrical components in others. 

Also, some cars use the auxiliary battery to start the vehicle. The battery had a lower capacity and could not power any other vehicle parts. The main battery was responsible for powering the main electric components.

mercedes auxiliary battery location

However, as time went on, with newer models of Mercedes, producers discovered that the auxiliary battery could do more than start the vehicle. They made the battery more powerful, making it assume the position of the backup battery. The auxiliary battery kicks in as a backup when the main battery is down. It also powers electric components like the car’s automatic transmission.

Modern auxiliary batteries can successfully power more electrical components. When you turn off your engine, the power changes from the main battery to the auxiliary battery, allowing you to enjoy features like the radio and other electrical systems in your vehicle. When the engine is switched on again, the main battery takes over, powering the vehicle.  

Can I Drive My Mercedes Without an Auxiliary Battery?

It is possible to drive your Mercedes without an auxiliary battery, but it is unsafe. The battery is a backup and can save you from a lot of stress if the main battery drains. Also, a major part of the vehicle’s electrical system will fail because they rely on the battery. 

The possibility of your Mercedes without the auxiliary battery depends on the model of the car. If the vehicle relies solely on the auxiliary battery to start the vehicle, it will be near impossible to start the engine, or it will take several cranks. If, in your model, it’s the main battery that starts the engine, you will have no problem with starting.

In addition, it will be risky to drive without a backup battery, especially when you are not sure of the condition of the main battery. If the main battery eventually drains, you will be stuck in a difficult situation.

Also, the auxiliary battery ensures that many electrical components do not lose power, especially when the engine is switched on or off. You will find that many electrical components will continuously trip off, or some will not come on.

When driving without an auxiliary battery, it is important not to drive too long. Also, try to get the battery fixed as soon as possible to avoid any possible problems.

Related content: Why Does My Battery Light Come On When Idling? 

Can an Auxiliary Battery Drain the Main Battery?

The auxiliary battery may drain the main battery; this could result from a wiring issue or a malfunction in the system. A medium is put in place to ensure this does not happen, but you must have your vehicle checked if it does. 

There have been several complaints about the auxiliary battery draining the main battery, which is not meant to be so. Manufacturers use isolators or separators to ensure that the auxiliary battery does not affect the main battery. 

The battery isolator stops the auxiliary battery from draining the main battery. It does that by isolating each DC bus and allowing electric current to flow through a particular direction. The isolator allows the batteries to receive and transmit current without being connected.

On the other hand, the battery separator functions quite differently and is more complicated. It works by monitoring the activities of both batteries; when it is time for charging, that is, when the engine is switched on, the separator automatically switches from the auxiliary battery and monitors the charge needed for both batteries. Whichever needs more will get charged, while the other will have to wait. In this way, both do not get connected and drain the other. 

When either the isolator or separator is damaged or malfunctions, the batteries will easily get connected, and one will drain. 

Can I Drive My Car with a Failed Auxiliary Battery?

We do not recommend driving your vehicle with a failing auxiliary battery. The failing battery can affect the main battery and cause it to drain faster and affect your vehicle’s electrical system, making it malfunction. 

Driving your vehicle with a failing auxiliary battery does not pose any immediate threat. Still, as time goes on, you will begin to notice a change in the driving experience of your vehicle, mainly because most of the electric systems relying on the failing system will also begin to stop working. If you do not manage the situation well, the damaged battery could damage some electrical components in the vehicle.  

Also, the damaged battery could cause the main battery to get drained easily, leaving you with no battery to power your vehicle when this happens. 

We advise that you do not go more than 100 miles with a damaged or failing auxiliary battery to be on the safe side. 

Related content: Can a Weak Battery Affect Engine Performance?

What Happens If the Auxiliary Battery Dies?

When your auxiliary battery goes bad, your vehicle will exhibit some symptoms showing that you need to change your battery. Some of these symptoms include the following.

1. Vehicle will not start

In many vehicles, the auxiliary battery is responsible for starting the engine. When the auxiliary battery gets damaged, it will be unable to carry out this function, which means the vehicle will not start. Sometimes, it will take a few cranks before the engine finally picks up. 

2. Battery light 

Another thing that will happen when your auxiliary battery dies is that your battery warning light will be triggered, coupled with warning error messages.  

3. Failed electrical system 

With a dead auxiliary battery, your electrical system will not function as it used to. The electric components relying on the battery will immediately fail. 

Another symptom that you may notice is that there will be obvious leaks on or around the battery. 

Can You Charge Mercedes Auxiliary Batteries?

You can not personally charge the auxiliary battery when it is drained, as the battery is charged with the help of the system out in place. The auxiliary battery gets charged like the main battery when the engine is running. Your car’s alternator is responsible for charging both batteries.

When the auxiliary battery hits below a certain voltage, the system will not be able to charge it anymore, which is when you have a dead battery. When this happens, you will need a new battery because neither you nor the system will be able to charge the battery. This takes us to the next point, which discusses the cost of replacing the batteries on your Mercedes.

Related content: What Could Cause Battery Discharge When the Engine is Off?

How Much Does It Cost to Replace a Mercedes Battery? (Auxiliary & Main)

The price range of a Mercedes battery depends on several factors. The average cost of getting a new battery will be between $300 and $700 for the main battery and between $200 and $300 for the auxiliary battery. The price stated excludes the cost of labor.

Fixing a battery is easy and requires no professional knowledge, just the right instructions. However, if you want professional help with the installation, you should budget an extra $50 to $100. However, some places will install the battery free of charge if buying it from a physical store.

Other factors that may affect the batteries’ price may include the battery’s brand, model, and vehicle’s year. 

Final Thoughts 

Mercedes uses dual batteries because they have realized that one battery cannot cope with their vehicle’s needs. They make use of two batteries which are the main and the auxiliary battery. 

The auxiliary battery has gone from being the backup battery to powering major parts of the vehicle. As automobiles become more advanced, some manufacturers might consider adding a third battery.