Can you charge a mercedes auxiliary battery?

Yes, it is possible to charge a Mercedes auxiliary battery using three different methods. The first option is to use the car charger that comes with the vehicle. The second method involves using the alternator, which converts the car’s mechanical energy into electrical energy. However, using the alternator for an extended period can wear down the car’s starting battery.

The third and final method is to use another car as a charger. This process is similar to reverse charging technology used for mobile devices. It involves using one car to charge another car’s battery. If you’re familiar with the Mr. Bean episode where he charges a battery car from an ambulance, that’s what this process looks like.

How to identify a low mercedes auxiliary battery and what to do?

One of the signs that your Mercedes auxiliary battery is low is when the smart gauge displays an error message such as ‘Auxiliary Battery Malfunction.’ Over time, auxiliary batteries may become weaker, and this issue is common in later Mercedes models.

It’s important not to ignore this error message and take action by replacing or charging the auxiliary battery. As we discussed earlier, there are three ways to charge the auxiliary battery. Let’s take a closer look at these methods to ensure your Mercedes stays powered up.

Charging your mercedes auxiliary battery: three methods to try

Charging Factors Charging with The car Charger Charging using The Alternator Charging from Another Car
Tools Required Car Charger Alternator Device Jumper Cable
Difficulty Easy Moderate/Expert Moderate
Charging Time From 2-4 hours 30 Minutes on Average 5-15 Minutes to Jumpstart
Precautions Check Precautions. Check Precautions. Check Precautions.

Method 1: How to Charge Mercedes Auxiliary Battery with a Car Charger

The easiest way to charge your Mercedes auxiliary battery is by using a car charger. However, before you start with this method, there are some tools and precautions you need to keep in mind. Here’s a step-by-step guide to charging your auxiliary battery with a car charger:

Tools Required:

  • Car Charger
  • Adjustable 8 Inch Wrench


  • Wear proper safety gear, including protective clothes, rubber gloves, and safety glasses.
  • Keep away from any ignition spark source to prevent fire or battery explosion.
  • Ensure there is enough ventilation in the charging area.
  • Use metallic tools carefully to avoid unintentional sparks.
  • Avoid direct skin contact with battery fluids and seek medical assistance in case of an emergency.


  1. Prepare the battery for charging by removing it from the vehicle.
  2. Power off all electronic devices in the car before charging.
  3. Remove the black grounding cable first and then the red positive cable from the battery.
  4. Follow the car charger’s specific instructions to connect the positive and negative cables to the battery’s terminals.
  5. Plug the charger into an electrical outlet and set the desired charge rate, starting with the lowest if it’s the first time you’re using the charger.
  6. Set the timer feature, if available, according to the battery model and charger amp rates. It will take approximately 2-4 hours to charge the battery.
  7. Safely remove the charger by turning it off, removing the positive cable first, then the negative cable, and reconnecting the car cables to the battery.
  8. Disconnect the charger from the electrical source and tighten the screws with a wrench.

It’s important to note that there are pros and cons to using a car charger, which you can weigh based on your needs and preferences.

ProsEasy and convenient.No extra tools are required.No risk of explosion or overcharge ConsTime consuming.Needs electricity.Mistakes can have consequences.

Method 2: charging using the alternator

Moving on to the second method, let’s discuss how to charge the auxiliary battery using the alternator. As we mentioned earlier, this method has one downside, which can be better understood by knowing how the alternator device works.

An alternator device converts the mechanical energy of your vehicle into electrical current, which it uses to charge the primary battery. This ensures that the primary battery has enough power to start the vehicle when needed. While modern cars have built-in alternators, older models use dynamos instead.

To charge the auxiliary battery, both primary and auxiliary batteries need to be wired in parallel. However, this means that when the engine is off, any excess electrical load on the vehicle can wear down the primary battery, as the electronics use it.

Fortunately, there are two devices that can help you charge your car’s auxiliary battery without draining the primary battery. The first is a DC-DC charger, which is quite popular and operates using multi-stage charging technology. There are two types of DC-DC chargers: Single Input and Dual Input.

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A Single-input DC-DC Charger

charges only the auxiliary battery using the alternator, and nothing else. If you are unfamiliar with how to install one, there are resources available to help. Now, let’s take a closer look at the pros and cons of the single-input DC-DC charger.

Dual input DC-DC charger

The dual-input DC-DC charger allows for charging of batteries through both the engine and solar panel, providing two charging options instead of just one. While some may find the parallel wiring process complicated, others may find it a simple choice. If you’re unsure of how to install a dual-input DC-DC charger, consider watching this step-by-step installation guide.

Battery isolator

Battery isolators are a useful device that separates the primary and auxiliary batteries during charging by allowing current flow in one direction and isolating the other. There are three types of isolators: solenoid, diode, and smart battery isolators.

Solenoid isolators have multiple electrical relays that receive and monitor the electric current flow from the battery, and then deliver it to the auxiliary battery. However, choosing the wrong solenoid can result in battery drain.

Diode battery isolators use heavy-duty diodes to deliver current flow from the battery to the alternator device, while smart battery isolators can detect the voltage of the primary battery and smartly deliver current flow to charge the auxiliary battery.

When deciding whether to use a battery isolator or not, consider your budget and the type of battery you have. Battery isolators are a good option for those on a budget, and if you use a lead-acid battery as an auxiliary battery. However, they do not work well with vehicles that have smart alternator devices. If your Mercedes model has a smart alternator, it is recommended not to use a battery isolator.

DC-DC charger vs battery isolator, which one should you use?

If you had to choose between a DC-DC charger and a battery isolator, which one would you choose? Allow me to assist you by outlining the advantages and disadvantages of each.

DC-DC Charger
Advantages Disadvantages
Compatible with modern battery isolators.Provides multi-stage charging.You can charge lithium-ion batteries. A little pricey compared to battery isolators.Beginners may find it difficult to install.May require an ignition circuit.

The advantages and disadvantages of the battery isolator are given in the below table.

Battery isolator
Advantages Disadvantages
Budget-friendly.You can charge AGM, Gel, and FLA batteries. Requires parallel connection.Incompatible with modern/smart isolators.May cause battery degradation.Voltage drop issues.

How long to charge Mercedes auxiliary battery this way?

It’s difficult to determine exactly how long it will take to charge auxiliary batteries with alternators due to various factors such as car speed, RPM, and riding conditions. The duration also depends on the car and the alternator model. On average, it takes around 30 minutes to charge auxiliary batteries for most Mercedes cars. However, this is just an estimate and not a precise calculation.

Method 3: charging from another car

The following method is charging the auxiliary device using another car, which we previously mentioned in a Mr. Bean episode. This method is not only easy but also a lifesaver when it comes to jump-starting your car or charging it.

Tools needed:

  • Jumper cables
  • Another car with a fully charged battery

Safety precautions:

  • Wear protective clothing, safety glasses or goggles, and rubber gloves.
  • Check for any battery leakages or visible cracks.
  • KEEP AWAY from all types of ignition sources like matches, cigarettes, flames, and sparks.

Now, let’s move on to the charging procedures. Follow the steps below.


Park your car and the donor car in parallel positions, i.e., facing each other.


Ensure that you can differentiate between the negative and positive terminals of the battery. Typically, red cables are attached to the positive terminal, while black cables are attached to the negative terminal.


Connect one end of the positive jumper to the positive terminal of your car’s battery, and the other end to the positive terminal of the donor car’s battery. Next, connect the negative cable to the negative terminal of your car’s battery.


Ground the other end of the negative cable to a metallic part of your car, such as the chassis or any metal part under the car bonnet.


Start the donor car’s engine and let it run for 2-3 minutes. Then, start your car’s engine and let it run for 5-15 minutes. Sometimes, you may hear the engine cranking sound, but the car may not start.

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If your car starts successfully, leave it for a couple of minutes. The jumper cables will automatically charge your car’s auxiliary battery.


Before removing the jumper cables, turn off both engines. Remove the negative cables first, followed by the positive cables, to prevent sparks.

Now, let your car’s alternator device do the charging for you.

Pros Cons
Handy and a lifesaver in crucial times.Minimum tools required.Time-saving. An additional car with a completely charged battery is necessary. Poor battery conditions can lead to serious consequences. Setting up the cables can be perplexing. A clean surface is required for the negative jumper cable.

How long does it take to charge a Mercedes auxiliary battery with a jump starter?

When jump-starting a battery, it typically takes between 2 to 5 minutes to get the engine running again. However, if you want to fully charge the auxiliary battery, it could take up to 12 hours.

Jump starter precautions and safety tips:

Working with car batteries can be hazardous, as batteries can catch fire or explode. Here are some safety precautions to keep in mind when using a jump starter:

  • Turn off the engine of both cars and ensure they are in park.
  • Ensure that the positive and negative terminals of the cables never touch.
  • Be extra careful while attaching the cables to the battery terminal.
  • Always connect the red or positive cable first and remove the black or negative cable first.
  • Properly ground the negative cable, as a bad negative cable can prevent the car from starting.
  • Wear protective rubber gloves and safety goggles.

Recharging or Replacing a Mercedes Auxiliary Battery

Generally, a Mercedes auxiliary battery can function smoothly for up to 3-4 years. However, it is recommended to replace the auxiliary battery if it has reached its maximum lifespan. If the battery has enough voltage capacity left, it can still be used. The operating voltage of a car auxiliary battery should be 12 volts. If the voltage is below that, it needs to be replaced. You can easily check the battery voltage using a voltmeter or the battery light indicator.

Replacing a Mercedes auxiliary battery can be expensive, ranging from $150-$500 depending on the model. Therefore, unless the battery is completely dead, replacing it may not be necessary.

If you do need to replace the battery, you will need some tools such as protective gloves, a trim removal tool, a T15-bit socket, an 8 mm socket, a 10 mm socket, and a ratchet. The following steps should be followed:

Step 1: Remove the auxiliary battery

  1. Use the trim removal tool to remove the fuse box access panel.
  2. Remove the Phillip screws from the lower panel.
  3. Find the panel that holds the auxiliary battery and remove the panel screws using the ratchet and socket.
  4. Remove both the positive and negative cables from the battery terminals.
  5. Remove any extra panels or supporting screws using the sockets.
  6. Remove the auxiliary battery from the battery panel.

Step 2: Replace the auxiliary battery

  1. Place the new battery in the battery panel.
  2. Attach the screws and supporting screws if needed.
  3. Connect the positive and negative cables to the battery terminals accordingly.
  4. Place the battery bracket and tighten the screws.
  5. Place the lower panel of the battery and attach the Philip screws.
  6. Turn on the ignition and make sure the “Battery protection: Convenience functions temporarily unavailable” message is gone from the gauge indicator.

It is essential to follow safety precautions when working with car batteries. Wear protective rubber gloves and safety goggles, turn off the engine of both cars, and put them in parking gear. Ensure that the cable’s negative and positive terminal never touch and be extra careful while hooking the cables to the battery terminal. Finally, always hook the red or positive cable first while hooking the cables and remove the black or negative cable first while removing them.

Frequently Asked Questions about Mercedes Auxiliary Battery

What is an auxiliary battery in a Mercedes?
The auxiliary battery in a Mercedes is a secondary battery that powers up the electronic devices of the car. In the past, Mercedes used both an auxiliary battery and a starter battery, but they have since moved to a single battery concept to prevent battery draining and enable eco mode.

How do you test a Mercedes auxiliary battery?
To test a Mercedes auxiliary battery, always check the gauge indicator for information and errors. If an error appears on the screen, search for it online or consult with a mechanic. Some errors can be fixed on your own, so be sure to follow the user manual for troubleshooting.

Why do Mercedes have two batteries?
Mercedes have two batteries to support the extra electric loads of electronic devices. The high-tech sound system, automation, and high-powered lights put a heavy load on the battery. The auxiliary battery manages these heavy loads and keeps the starter battery free for the engine.