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What Is An Auxiliary Battery and Does My Car Have One?

Power - by Joe Weber - updated on 12/28/2022

Replacing a battery

Did you know that your car might actually have two batteries? First seen in some higher end European cars in the early 2000's, secondary auxiliary batteries are becoming more and more common across all car brands. Learn more about what auxiliary batteries do and what you can do if yours fails.

What Is an Auxiliary Battery?

Starting to appear in the early 2000s, some European automakers began designing and engineering their vehicle electrical systems to have a second, smaller battery in their vehicles. These batteries are designed to run as a backup to the starting battery and provide power to some essential equipment like engine start/stop and other systems that require power while the engine is off to put less strain on the main battery and alternator. The exact systems that will be powered by the auxiliary battery, or secondary battery, will depend largely on the make, model and trim level of your specific vehicle.

As automotive technology continues to evolve, electrical design needs to evolve as well. The more electrical equipment installed in new vehicles, the more strain on the starting battery. Flash forward a few years and auxiliary batteries are still fairly rare but you will now see secondary auxiliary batteries in many cars ranging from Mercedes Benz, BMW, Chrysler, Jeep and more.

How Do I Know if My Car Has an Auxiliary Battery?

Not every car or truck has one so how do you know if your vehicle does? The easiest way is to enter your vehicle information on the fitment finder on to see if we have an auxiliary battery solution for your car.

If you can't find or no longer have your car's owner's manual to look up the location of the batteries, you could also get your hands a little dirty and look in the car. Auxiliary batteries are not always as easy to find as your main starting battery. It could certainly be under the hood but it could also be under a seat, behind the glovebox or somewhere in the trunk.

How Long Does an Auxiliary Battery Last?

It is difficult to provide a narrowed down estimate of how long an auxiliary battery will last. The lifespan of an auxiliary battery will depend on the systems it powers and if it's maintained properly during routine maintenance. Our Power Sonic auxiliary batteries come with a 6-month warranty but, generally speaking, auxiliary batteries should last around 5 years.

Auxiliary batteries, like your main battery, need to be checked regularly for corrosion, damage and leakage. Depending on the location of the battery, this could be difficult for you to do yourself and you may require the assistance of a mechanic.

What Are the Signs of a Bad or Failing Aux Battery?

Auxiliary batteries aren't as obvious as your starting battery when there's a problem. Your car will still start and drive like normal. What you may encounter is some accessories may not function properly or not at all. Many vehicles equipped with auxiliary batteries also have a warning light on the dashboard that will illuminate when the battery is starting to fail. When this light illuminates, that is your sign to take your car into your local Batteries Plus to have it tested to see if it may be time for a replacement.

One main system that may start to malfunction when the auxiliary battery is failing is the vehicle's start/stop.

What Is Start/Stop?

The idea behind automatic start/stop was brought up as a way to reduce CO2 emissions to help the environment. The idea is simple: if the engine is shut down for short periods while waiting at stop lights, emissions and fuel consumption are reduced. With this feature, carbon emissions can be reduced by 3-8% worldwide. The benefits to the environment and the increase in fuel efficiency have made this feature almost standard in many new cars. While the engine is off, your electronics need to continue to work, that's where the auxiliary battery comes in.

Where Can I Go to Have My Auxiliary Battery Replaced?

The experts at Batteries Plus will be ready to help you with your auxiliary battery needs. Are you starting to notice some odd behavior with your stop/start system or have a battery light illuminated on the dashboard? Let us test your auxiliary battery, and your main starting battery while you're here, to see how things are faring. If it's time for a new battery, we'll be ready and willing to install your new battery while you wait; most installations can be completed in under an hour.

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