How to Charge Mercedes Auxiliary Battery? [Full Guide]

Auxiliary batteries are responsible for running all the important accessories of your car. Such as headlights, tail lights, sound systems, digital devices, etc. Charging your car accessories regularly is important in order to keep them powered up.

Thus, here comes the question, how to charge mercedes auxiliary battery?

You can charge Mercedes auxiliary batteries using the car charger, alternator, or from another car. The safest method of charging the auxiliary battery is using the car charger. But using the alternator can wear your engine over time. You can also charge the auxiliary batteries from another car.

Before we jump into the detailed discussion of these three methods. You might think, can auxiliary batteries be charged? Let’s find out.

Can Mercedes Auxiliary Battery Be Charged?

Yes, it’s possible to charge the Mercedes auxiliary battery. You can charge your Mercedes auxiliary battery in three simple ways. 

The first method is using the car charger that comes with the vehicle. The second method is using the alternator. Which converts your vehicle’s mechanical energy to electricity. 

However, using the alternator for a long time can wear off your vehicle’s starting battery. So what’s the solution? Patience my friend, stay with us and we’ll find the solution together. The last method is using another car as a charger,

It’s similar to the reverse charge technology for mobile devices where you can use one phone to charge another phone. Remember the Mr. Bean episode where he charges a battery car from an ambulance? See the image below,

Can Mercedes Auxiliary Battery Be Charged

Image Source: Youtube

Now you remember? This is what charging your car battery from another car looks like.

How Do I Know If My Mercedes Auxiliary Battery Is Low?

Your Marcedes auxiliary battery is low if the smart gauge shows the error ‘Auxiliary Battery Malfunction.’ Typically auxiliary batteries become weak over time. If you have any late version of Marcedes you may come across this error message.

Although it’s a quite common issue to encounter, you shouldn’t ignore it. You can easily replace or charge the auxiliary battery with the three methods we’ve previously mentioned. Now, let’s talk about these three charging methods in detail, shall we?

3 Methods to Charge Mercedes Auxiliary Battery

You already know about the three methods by now. So, let’s check out some factors of these three charging methods in the table below.

Charging FactorsCharging with The car ChargerCharging using The AlternatorCharging from Another Car
Tools RequiredCar ChargerAlternator DeviceJumper Cable
Charging TimeFrom 2-4 hours30 Minutes on Average5-15 Minutes to Jumpstart
PrecautionsCheck Precautions.Check Precautions.Check Precautions.

Method 1: Charging with Car Charger

Let’s start with our first method, using a car charger to charge the battery. Most people prefer this method since it is the easiest. Before starting with the method, there are some tools you’ll need to gather first.

Tools Required


  • Make sure you wear proper safety gear. Such as protective clothes, rubber gloves, and safety glasses.
  • KEEP AWAY every kind of ignition spark source. It can result in fire or even battery explosion. So be careful with cigarettes, sparks, or any kind of flames.
  • Make sure there’s enough ventilation in the area where you’re charging the batteries.
  • Use the metallic tools carefully, they might cause an unintentional spark.
  • Avoid direct skin contact with the battery fluids. In case of an emergency situation, immediately consult with a doctor.

Alright, now that we have learned about the precautions, let’s move to the steps.

Step 1: Remove The Battery for Charging.

First of all, you have to prepare the battery for charging. Each vehicle model requires a different method for preparing the batteries. In most cases, the battery must be removed before the vehicle can be charged.

Step 2: Power Off All The Car Electronic Devices

The battery can be charged once all the electronic devices have been turned off. Such as lights, sound systems, air conditioners, etc. While charging, these electronics can be damaged or cause short-circuit if turned on.

Step 3: Remove the Cables

Now, you’ll find two colored cables attached to the battery, one is red(+) and the other one is black(-). We all know that the red one is an electrical positive cable and the black one is a grounding cable. 

First, remove the black cable and then remove the red one. Make sure the two cables are separated and not touching each other.

Step 4: Connect the Car Charger

Now, you have to connect the car charger to the battery. Please note that chargers come with specific instructions. So, make sure to go through the instructions before hooking up the charger. 

Alright, let’s check out the charging procedure. Follow the instructions below.

  • First, make sure the charger has been turned off.
  • Hook the positive (red) cable of the charger to the battery’s positive terminal.
  • Next, take the negative (black) cable and hook it to the negative terminal of the battery.
  • Now that you’ve connected the cables, connect the charger plug to an electrical outlet and turn it on. Then, begin by setting your desired charge rate. However, if you’re using the charger for the first time, set the lowest possible charge rate. 
  • Lastly, if the charger has a timer feature, set the timer accordingly. It will take around 2-4 hours to charge the battery. Note that car battery models and charger amp rates affect the charging time.

Step 5: Safely Removing the Charger

After you have done charging the battery you need to safely remove the charger. To do that, remember the rule of thumb, turn off the charger first. Then, do the opposite we did earlier. Before removing the negative cables, remove the positive ones.

After removing the cables from the charger, hook the car cables to the battery carefully. Then disconnect the charger from the electrical source. Lastly, tighten the screws with a wrench.

Now, let’s check out the pros and cons of using the car charger real quick.

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

Alright, let’s move on to the next method.

Method 2: Charging Using the Alternator

Let’s move on to the second method. As we previously mentioned, using alternator devices has one downside. To understand that downside, you’ve to properly understand how an alternator device works.

The alternator device usually converts your vehicle’s mechanical energy to electrical current. It uses the produced current to charge the primary battery. This way it makes sure the primary battery has enough power to start the vehicle when needed.

Modern cars have built-in alternators to charge the auxiliary battery. Old model vehicles have dynamo instead of alternators. Alternator devices require parallel wiring of the both primary and auxiliary batteries. 

That means the electronics use the primary battery when the engine is off. As a result, an excess electrical load on your vehicle will wear down the primary battery. That’s why we warned you before. 

However, you will be happy to know that we’ve got reliable solutions to prevent this issue. You can charge your car’s auxiliary battery without draining your primary battery with two devices. We have discussed them in detail below.

DC-DC Charger

A DC-DC charger is quite popular. It functions with the help of the primary battery to charge the auxiliary battery. It operates in multi-stage charging technology. 

There are two types of DC-DC chargers: Single Input and Dual Input. 

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Image Source: Panbo

Single-input DC-DC Charger

Single-input DC-Dc chargers, as the name suggests, only operate in one way. It charges your car’s auxiliary battery using the alternator and nothing else. If you don’t know how to install a DC-DC charger, consider watching this video.

Step-By-Step DC-DC Charger Install

Now, let’s go through all the pros and cons of the single input DC-DC.

Easy to use and install.Many amperage varieties to choose from.Portability issue.May need to install an Ignition circuit.
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Image Source: Jaycar

Dual Input DC-DC charger

Dual input DC-DC supports charging batteries from the solar panel along with the engine. So it provides two ways of charging instead of one.

Some people find it an easy choice but some find it complicated due to parallel wiring. Watch this step-by-step guide to installing a dual-input DC-DC charger. 

installing Powertech dual input DC/DC multi stage charger.

Two ways of DC-DC charging.Super easy to install.No ignition circuit is required.Some find the charging parameters rigid.Solar integration requires manual assembly.

Now that we have seen the DC-DC charging device, let’s move on to the next device.

Battery Isolator

Battery isolator is an excellent device that isolates primary and auxiliary batteries while charging. It flows the current in one direction isolating the other, how cool is that? 

That means you can charge the auxiliary battery without affecting your starter battery. Primarily, there are three types of isolators to choose from. These are

  1. Solenoid Battery Isolators.
  2. Diode Battery Isolator and,
  3. Smart battery Isolators.

Solenoid isolators work in a complex way, it has multiple electrical relays. One relay receives the electric current flow from the battery, the other relay monitors it. 

The additional relay delivers the current to the auxiliary battery accordingly. Make sure you choose the solenoid carefully because a bad solenoid can drain your battery.

On the other hand, diode battery isolators work in a simpler way compared to solenoid ones. As the name suggests, the diode isolator uses heavy-duty diodes. It delivers current flow from the battery to the alternator device.

Lastly, we have smart diodes, which are the best ones to choose from. The smart battery isolator smartly detects the current voltage of the primary battery.

If the primary battery is fully charged, it automatically delivers the current flow to charge the auxiliary battery. Also, it smartly isolates your auxiliary battery from the primary battery to prevent battery draining. 

Just like every electronic device these devices also have pros and cons. You might be thinking about when to use the isolators and when not to. Let’s talk about this.

The Battery Isolators: When to Use And When Not to Use?

You should only continue getting a battery isolator if the below conditions are met.

  • If you have budget issues you should get a battery isolator instead of a DC-DC charger. However, the DC-DC chargers have other advantages which are definitely worth the money.
  • If you use a lead acid battery as an auxiliary battery. Normally, auxiliary batteries don’t work with lithium-ion batteries. There are lithium-ion-specific isolators available but they are hella expensive.
  • Battery isolators don’t work properly with vehicles having smart isolators. If your Mercedes model has a smart alternator device, a battery isolator will malfunction.

Dc-Dc Charger Vs Battery Isolator, Which One Should You Use?

Now if you were to choose between a DC-DC charger and a battery isolator, which one will you choose? Let me help you with both their advantages and disadvantages.

DC-DC Charger
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
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?

No one can ideally say how long it will take to charge the auxiliary batteries with alternators. It’s because there are multiple factors to think about. For instance, the car speed, the RPM, the riding condition, etc.

It also depends on the car and the alternator device model. Typically, for most Mercedes cars, it takes around 30 minutes to charge the auxiliary batteries. But note that this data is an average calculation, not an accurate one. 

Method 3: Charging from Another Car

The last method we have is charging the auxiliary device with another car. Remember we told you about a Mr. Bean episode? Yes, that’s the method we are going to talk about. It is one the easiest and lifesaver methods to jump-start your car or charge.

Tools Required


  • Use protective clothes, safety glasses or goggles, and rubber gloves.
  • Check whether there are any leakages or visible cracks in the battery.
  • KEEP AWAY from all types of ignition sources like matches, cigarettes, flames, and sparks.

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

Step 1: Park Both Cars in Parallel Positions

First park your car and the donor car in parallel positions. That means, parking both cars in front of each other. 

Step 2: Check the Battery Cables

Make sure you recognize the negative and positive terminals of the battery. Usually, the positive terminal has red cables attached to it. And, the negative ones have black cables. So, we recommend you double-check the battery cables.

Step 3: Connect the Jumper Cable

Connect one end of the positive jumper to the positive terminal of your car’s battery. And another end to the positive terminal of the donor car’s battery. 

Then, connect the negative cable to the negative terminal of your car’s battery. If the battery terminals have any caps make sure to remove them first.

Step 4: Ground the Jumper Cable

Now connect the other end of the negative cable with a metallic part of your car for grounding. It can be the chassis or any metal part under the car bonnet.

Step 5: Start the Engine

Now start the donor car’s engine and leave it for around 2-3 minutes. Next, start your car’s engine and leave it for 5-15 minutes. Sometimes, you’ll hear the engine cranking sound but the car won’t start

If the car has started successfully leave it for a couple of minutes. The jumper cables will help to automatically charge your car’s auxiliary battery. 

Step 4: Remove the Jumper Cables

Before removing the jumper cables, turn off both engines. Now remove the jumper cables in the opposite manner. Take out the negative ones first then the positive ones to prevent sparks. 

You are done, now let the car’s alternator device do the charging for you.

Handy and a lifesaver in crucial times.Minimum tools required.Time-saving.Requires another car with a fully charged battery.Bad battery conditions can cause severe consequences.Setting up the cables can be confusing.Needs a clean surface for the negative jumper cable.

How Long to Charge Mercedes Auxiliary Battery This Way?

Usually, it takes a maximum of 2-5 minutes to jump-start a battery. However, if you want to fully charge the batteries it would take around 12 hours. 

Jump Starter Precautions and Safety Tips

Batteries are prone to catching fire and explosion. So, stay cautious while working with car batteries. Using the jump starter method is easy but there are safety precautions you need to keep in mind. These are given below.

  • Make sure you turn off the engine of both cars and put them in parking gear.
  • Make sure the cable’s negative and positive terminal never touches.
  • Be extra careful while hooking the cables to the battery terminal.
  • While hooking the cables always hook the red or positive cable first. And, while removing, always remove the black or negative cable first.
  • Make sure you properly ground the negative cable. Sometimes a bad negative cable can cause your car not to start. Know the symptoms of a bad negative cable and how to replace it.
  • Wear protective rubber gloves and safety goggles.

Mercedes Auxiliary Battery: Recharge or Replace?

Normally, a Mercedes auxiliary battery lasts up to 3-4 years smoothly. Consider replacing the auxiliary battery if it has reached the maximum lifespan. You can easily find the desired car battery for your car model here.

However, if the battery has enough voltage capacity left then it’s good to go. Ideally, a car auxiliary battery has 12V, if the voltage is below that it needs to be replaced. You can easily check the battery voltage in simple ways.

The light indicator will show the status while you charge the battery. Meanwhile, you can use a voltmeter to check the battery voltage. With the voltmeter, you can confirm whether its operating voltage is 12 volts or not. 

Replacing the Mercedes auxiliary battery can be pretty costly. A new Mercedes auxiliary battery costs you from $150-$00 maximum depending on the model. So, unless your battery is completely dead, replacing it will cut some slack.

Now let’s check out how to replace a Mercedes auxiliary battery.

Tools required

Now follow the below instructions thoroughly.

Step 1: Remove the Auxiliary Battery

  • Take the trim removal tool and remove the fuse box access panel.
  • Reach the lower panel and remove the Phillip screws.
  • Now find the panel that holds the auxiliary battery. Remove the panel screws and detach them using the ratchet and socket.
  • Remove both the positive and negative cables from the battery terminals.
  • Now remove any extra panels or supporting screws with the sockets.
  • Then remove the auxiliary battery from the battery panel.

Step 2: Replace the Auxiliary Battery

  • Place the new battery in the battery panel.
  • Attach the screws and supporting screws if have any.
  • Connect the positive and negative cables to the battery terminals accordingly.
  • Now place the battery bracket and snug the screws.
  • Place the lower panel of the battery and attach the Philip screws.
  • Turn on the ignition. Make sure “Battery protection: Convenience functions temporarily unavailable” message is gone from the gauge indicator. See the below photo, it looks something like this.
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Image Source: Stimul KZ

Congratulations you’re done, you’ve successfully replaced the auxiliary battery by yourself. Put that in your achievement list.


What Is An Auxiliary Battery In A Mercedes?

An auxiliary battery in Mercedes is a secondary battery that powers up the electronic devices of the car. The previous version of Mercedes used auxiliary batteries along with the starter battery. But it was low efficient so they moved to one battery concept to prevent battery draining and eco mode.

How to Test Mercedes Auxiliary Battery?

To test the Marcedes Auxiliary Battery always check the gauge indicator for information and errors. If any error appears on the screen, search for it online or consult with a mechanic. Some errors are easy to fix on your own so make sure to follow the user manual for troubleshooting.

Why Do Mercedes Have Two Batteries?

Marcedes have two batteries to support the extra electric loads of electronic devices. Its 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 started battery free for the engine.


We’ve reached the end of the article. Hopefully, we have clarified your query on how to charge marcedes auxiliary battery

One extra tip, while jump-starting your car from another car, don’t attempt to fully charge the battery. Instead, let your car’s alternator do the job for you.

Until next time!