Every driver knows what the battery does. But if you notice an extra one lying around in your vehicle, this article can help you understand what it is and what it does, among other crucial info about it.
What Is an Auxiliary Battery?
The auxiliary battery is typically a secondary battery that keeps certain electrical systems running even when the engine is off. Vans and pickups built with a camper package will have an auxiliary battery to power the lights in the camper.
Likewise, bobtail box trucks equipped with 12 volt lights and accessories in the box part of the truck that are routinely used for extended periods of time will have an auxiliary battery to power those units. The auxiliary battery on these vehicles is isolated from the vehicle’s main electrical system to prevent the primary battery from being drained. However, when the engine is started, a solenoid will close, and the auxiliary battery will be charged as the vehicle is driven.
Some utility vans will have auxiliary batteries to power active anti-theft systems.
All hybrid and electric vehicles (EVs) have a high-voltage battery to provide drive power to the vehicle, but they also have a 12 volt battery to power everything else. The 12 volt battery can be referred to as an auxiliary battery, but it’s just as important as the high voltage battery that powers the motors that drive the vehicle down the highway.
The 12 volt battery is charged through a DC-to-DC converter built into the vehicle’s high voltage battery system.
Where Is the Auxiliary Battery Located?
As is the case with many ordinary single battery vehicles that have wide variations in battery location, auxiliary batteries can be mounted just about anywhere. Single battery vehicles might have the battery in the trunk, under the back seat, hidden in the wheel well in front of the left front tire, and other places.
For example, Toyota Prius 12 volt batteries are in the left rear trunk area, Ford Focus hybrid batteries are under the hood, as are Tesla EV batteries. Toyota hybrids typically use a glass mat 12 volt battery that requires special charging procedures.
The location and type of the auxiliary battery depends on the vehicle’s specifications.
How Does an Auxiliary Battery Work?
Most light vehicle diesels will have two main batteries wired in parallel. Neither of these is considered an auxiliary battery. Some diesels even have 2 alternators that work together to keep the batteries up.
In a conventional system (non-hybrid, non-EV) with a primary and auxiliary battery, the auxiliary battery supplies power to certain electrical components that would kill the primary battery and cause a no-start situation if used for an extended period.
The auxiliary battery on non-EV or hybrid vehicles usually operates in an isolated circuit upon engine startup.
But on hybrids and EVs, the 12 volt battery is charged when the main battery’s contactors are engaged through the DC-to-DC converter. If the 12 volt battery on a hybrid or EV is dead, the contactors that energize the main high voltage battery won’t close.
If the engine computer detects a low voltage output from this battery, one of the contact switches will remain closed while the engine is running. This lets the alternator recharge the auxiliary battery.
Once the auxiliary battery is recharged, a contact switch will open to reduce engine load. This reduces exhaust emissions and optimizes fuel economy.
Installing a Dual Battery Setup for Your Car
A dual battery setup isn’t suitable for all vehicles, but if you’re planning to install one on your car, you should know that the process can be tricky for newbie DIYers.
You can see how a dual battery setup is wired by looking at the diagram shown below, but you need a suitable relay that will consistently carry 60 to 100 amps. This should also be normally open so that the auxiliary battery will be suitably isolated when the key is off and the accessories are drawing power.
Take note that setting up a dual battery system requires extensive automotive wiring knowledge.
So if you think you don’t have the necessary skills for this project, it’s always a good idea to let a professional mechanic handle the job. This will ensure that all steps and protocols are followed correctly.
Some popular models have installation kits tailored to their specifications, making the task a lot easier and less time-consuming.
How Long Does an Auxiliary Battery Last?
An auxiliary battery can last anywhere between five to eight years, which is almost the same lifespan as the main battery.
However, keep in mind that the lifespan can be shorter, depending on how you take care of your battery and vehicle.
What Happens When an Auxiliary Battery Malfunctions?
It’s unlikely that you’ll notice any drivability problems with your vehicle once the auxiliary battery fails.
The worst thing that could happen is that you won’t be able to use the electrical components that get voltage from the auxiliary battery.
Some of these parts include the lights, audio system, and start/stop feature (if applicable), among others.
How Much Will a New Auxiliary Battery Cost?
An auxiliary battery replacement can cost anywhere between $275 and $320, depending on your vehicle’s year, make, and model.
Battery Maintenance
The auxiliary battery needs to be taken care of the same way as the main battery. Here are some things you can do to extend its lifespan.
Clean the Battery Terminals
A poor battery connection is one of the most common reasons why electrical faults happen.
To prevent this from causing any trouble, make sure there are no signs of corrosion in the battery terminals.
Cleaning the battery terminals is a simple task that you can do with a few household items.
Simply combine a tablespoon of baking soda with one liter of distilled water, and brush the mixture onto the battery and housing to neutralize the acid.
Secure the Battery Hold-Downs
Excessive vibration because of loose brackets can damage the battery’s plates. So make sure the hold-down brackets are secure and tight but not to the point where they’ll crack.
Symptoms of Battery Failure
Overcharging is the most common cause of battery failure, as it can cause the active plate material to disintegrate and fall out.
It can be hard to determine whether or not a battery is overcharged at first glance. But there are a few telltale signs that point to a weak or failing battery.
Presence of Water in One or More Cells
Water in one or more cells is a sign that the electrolytes are separating into hydrogen and oxygen gases. It also indicates that the plates are sulfated during the charging process.
Excessive Corrosion
A sulfated battery produces hotspots on the plates. As the battery is charging, acid fumes surround the cables, connections, and battery tray.
Long Crank Time
Cold weather startup can be difficult with a weak or failing battery because it won’t have enough current to start the engine.
Keep in mind, however, that slower-than-normal engine cranking can also be the result of a defective starter or solenoid.
Wrapping Up
An auxiliary battery is a smaller battery than the main battery. In most cases, it supplies power to certain electrical components to take some load off the main battery and alternator.
The auxiliary battery also operates in a different circuit from the main battery, so it’s unlikely that you’ll experience drivability problems with your vehicle.
Lastly, it’s also important to check the auxiliary battery for any signs of damage or corrosion. Any issue with this battery can result in electrical troubles and might add more strain to the main battery and alternator.
Where to Get New Auxiliary Battery for Your Vehicle
If you’ve decided to try a dual battery setup for your car, the first thing you should look into is a high-quality auxiliary battery. Luckily, CarParts.com has a variety of batteries for you to choose from!
We can guarantee the quality of all our products because we source them from the most trusted manufacturers in the industry. These replacement parts are engineered to specifications in world-class facilities.
We also have strategically located distribution centers equipped with the latest technologies for efficient order processing and faster shipping. You can even expect your order to arrive in as little as two business days.
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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.
