Auxiliary Battery Malfunction: How To Fix? (Step by Step)

Auxiliary Battery Malfunction

While not all vehicles have an auxiliary battery, those that do rely on it quite heavily.

It is a source of backup power and can be used to keep certain vehicle functions operating, although rarely the more important ones.

Like with your standard battery, it is not uncommon to have an auxiliary battery malfunction. But, what does this mean? How can you fix it? Let’s take a look.

Auxiliary Battery Malfunction

An auxiliary battery function likely means one of three things:

  • The auxiliary battery has been completely drained of power.
  • The auxiliary battery is old and needs replacing.
  • There is a wiring issue in your vehicle.

The problem is that it is tough to work out what the actual problem is. When you get an auxiliary battery malfunction, literally all you will be told is that the battery has an issue. Nothing more. The only way to really find more out about the issue is to use an OBD2 scanner.

This device taps into the vehicle’s onboard computer and reads various error codes. These error codes can then be used to work out what the issue is.

You can pick up an OBD2 scanner for not a whole lot of money, although we suggest that you head to your local garage as they will be able to fix the issue.

That being said, there are a few methods that you can try first. You will need to read through your vehicle’s manual to work out where the auxiliary battery is. It is likely going to be located close to your standard vehicle battery, but it will be much smaller.

We won’t talk about wiring issues here. They are tricky to deal with, and it isn’t really something that you should be trying to fix if you don’t have a clue what you’re doing.

The issue could be anything from a frayed cable to a rusty terminal or a complete breakdown of the electrical components in your vehicle. That’s why an OBD2 scanner is used.

Charging Your Battery

Just like your standard vehicle battery, your auxiliary battery should charge when your vehicle is in motion.

This means that it is highly unlikely that your vehicle is going to drain. In fact, the only time that your auxiliary battery should drain is if your vehicle is stopped and you decide to run your vehicle’s electronics.

The vehicle’s main battery will drain first, and then the auxiliary battery will drain shortly after.

While running your vehicle should kick a bit of life back into your vehicle’s auxiliary battery, it may not help if that battery has been fully drained.

To deal with a fully drained battery, you will need to remove it from your vehicle and use a standard car battery to recharge it. The process should be fairly self-explanatory.

You just connect the charger’s battery clips directly to the battery.

Charging should take a few hours. If you have a smart vehicle battery charger (which you should have if you have purchased a modern battery charger), then it should charge overnight without any issues. 

We do want to point out here that your auxiliary battery should rarely be put in a position where the battery has completely drained. If your vehicle’s auxiliary battery is fully drained, then it is likely that you also have an issue with the main battery in your car too.

This is because the auxiliary battery should barely be using any power when that main battery has power. So, you may also want to double-check to make sure that the main battery hasn’t completely drained too. 

Replacing Your Battery

An auxiliary battery, just like any other car battery, has a limited lifespan. If you have not changed the battery in 4-5 years, then it is likely due for a replacement.

You can pick up a new auxiliary battery from any decent car parts supplier. They will cost you a bit of money, but nowhere near as much as a standard battery!

Some people may find that they are able to squeeze a few more years out of their battery but, honestly, if your auxiliary battery has managed to get to the point where it is flashing up battery malfunctions, then it is probably long past due to being replaced. 

Read also >> Can You Leave a Battery Charger Connected? (Is It Safe?)

Read also >> How Long to Leave a Car Running to Charge Battery (Do This)

What If The Auxiliary Battery Malfunction Doesn’t Disappear?

Your vehicle’s computer works by keeping a constant eye out for any issues that may be occurring with the various systems in the car.

If you have a battery malfunction, then your vehicle will be constantly checking to see whether the issue has been dealt with.

If the issue has been dealt with then, hopefully, that warning is going to eventually disappear. If it disappears, then you know that the problem is fixed.

If the auxiliary battery malfunction warning still doesn’t appear after either replacing or charging your vehicle’s battery, then there is a good chance that your vehicle has a wiring issue. As we said, this is not something that you should be dealing with yourself.

Instead, you will head to your local garage. If possible, you should try and head to a garage that specializes in your particular vehicle brand. 

Can You Drive With An Auxiliary Battery Malfunction?

You can drive with an auxiliary battery malfunction, although your vehicle isn’t telling you that there is a malfunction in the battery for fun. It is telling you that your battery probably needs to be dealt with in the very near future.

While you probably won’t need to deal with the problem within a couple of days, do not wait more than a week or two.

For the most part, if your auxiliary battery fails, nothing will immediately happen to your vehicle. However, if your vehicle’s main battery lacks sufficient charge, then you may find it difficult to start your vehicle, perhaps impossible.

You may also find that certain electrical features in your vehicle fail to work, although they are rarely going to be the most important ones.

What Happens If The Auxiliary Battery Dies?

If the auxiliary battery dies, nothing will happens. The job of the auxiliary battery is to provide backup power. Assuming your standard vehicle is fully charged, then it is unlikely that you will notice any major issues with your vehicle. 

There are some vehicles that have functions that rely on the auxiliary battery and these may stop working. However, for the most part, you probably won’t even notice them working.

For most people, the only time you will ever know that the auxiliary battery has stopped working is because you notice an alert on your vehicle’s dashboard, or perhaps the radio stops working, or your vehicle’s air conditioning system is acting a little sluggish.

The latter is much more likely to occur if your vehicle’s main battery doesn’t have sufficient power.

Can An Auxiliary Battery Drain The Car Battery?

No, an auxiliary battery shouldn’t be able to drain the car battery. However, that isn’t to say that it won’t.

An auxiliary battery will only drain the car battery if the wiring in your vehicle is poor. If an isolator or charge controller has been installed, then the auxiliary battery shouldn’t drain the car battery at all.

If you notice that your car battery is draining rapidly and you suspect it may be down to the auxiliary battery, then we suggest that you have the vehicle’s wiring looked at. This is not something that should be happening at all. 

One of the most common reasons why that auxiliary battery is draining the car battery is because the wires are in the wrong order.

So, rather than the vehicle sending generated power into the auxiliary battery, the vehicle is taking power from the auxiliary battery and feeding all of it straight into the main vehicle battery.

There is nothing charging the auxiliary battery at all. Although, this is something that you will notice at a pretty early stage. It doesn’t take long to completely drain an auxiliary battery if it isn’t being charged, so you should spot the issue within a few hours. 

Will a Bad Auxiliary Battery Cause a Check Engine Light?

Yes a bad auxiliary battery can cause a check engine light.

Only the more modern vehicles will give you a proper indication that there is an auxiliary battery malfunction. This will normally happen if your vehicle has some sort of digital screen inside it.

If your vehicle does not have a digital screen and you have a simple dashboard, then there aren’t that many lights that can indicate issues, so the vehicle may flash up a check engine light instead.

There is a major problem here, though. This is the fact that the check engine light could indicate one of hundreds of potential problems in your vehicle. It doesn’t necessarily mean that there is a problem with your auxiliary battery.

So, unfortunately, the only option that you really have here is to either get yourself an OBD2 scanner or to head to your local garage.

Honestly, if your vehicle is displaying a check engine light, it is highly unlikely that it is going to be a problem with your auxiliary battery.

In the grand scheme of things, an auxiliary battery malfunction is going to be far, far less common that any other issue in your vehicle. 

Remember, most vehicles don’t have an auxiliary battery. Some people assume their vehicle has an auxiliary battery, but it normally isn’t the case. 

Auxiliary Battery Malfunction: Mercedes E Class W212 Replace >> Check out the video below:


It is rare that a vehicle will have an auxiliary battery malfunction, although that is mostly because many vehicles don’t even have an auxiliary battery.

If you notice an auxiliary battery malfunction warning in your vehicle, then it is likely to be one of three issues. The most common two will be an old or drained battery. Both of those problems can be rectified easily.

In rare cases, there may be a wiring issue in your vehicle. This is something that can only be dealt with by a trained mechanic.


Steve P.

Steve is an automotive technician, technical writer, and Managing Editor. He has held a lifelong passion for cars, with a particular interest in cars like the Buick Riviera. Steve is based in Boise, Idaho.

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