Adding an Auxiliary Battery

When and how to add extra automotive battery capacity

Every car and truck, whether it runs on gas, diesel, or alternative fuel, has a battery. The battery is what allows the engine to start up, and it provides power to all of the electronics in the vehicle whenever the engine isn't running. A different component, the alternator, is responsible for providing juice when the engine is running.

In some cases, one battery just isn't enough. Most electric cars, for instance, have a high voltage battery that powers the motor and an auxiliary 12-volt battery to run other electronics like the radio. Other vehicles, like campervans and motorhomes, typically also come with auxiliary batteries to run everything from interior lights to refrigerators.

If you think that you could use some extra battery capacity in your car, whether to run a powerful car audio system or anything else, it is possible to install an auxiliary battery in just about any car or truck. However, it's important to understand that there are some problems that you can't solve by installing an auxiliary battery.

Who Needs an Auxiliary Battery?

Some of the situations where an auxiliary battery can help include:

  • Providing power for a high-performance car audio system: Extremely powerful amplifiers, and other components, sometimes require more power than the stock charging system is capable of providing. This is especially true if you use your car audio system when the engine isn't running, like at competitions.
  • Electricity for activities like tailgating: If you enjoy activities like tailgating, then an auxiliary battery can be helpful in providing the juice required to power up radios, televisions, cooking equipment, and anything else you need to keep the party going.
  • Primitive camping without giving up creature comforts: If you like to take your vehicle with you camping, it's pretty easy to hook up an inverter to power any electronics you just can't be without. Adding an auxiliary battery will ensure that the engine will still start up when it's time to go home.

Don't Install an Auxiliary Battery to Make up for a Weak Primary Battery

One situation where installing an auxiliary battery won't help is if the battery you already have isn't holding a charge. That means if you're experiencing a problem where your car won't start in the morning, adding a second battery won't fix the problem.

While a battery that won’t hold a charge is a clear indicator that it’s time for a replacement, it also means that there is some kind of issue that needs to be dealt with before worrying about installing an auxiliary battery.

In specific circumstances, like cases where you run a lot of electronics when your car is off, and then find that the engine won't start, then installing a high capacity battery or a second battery might be the end of it. If not, then it's a better idea to check for a parasitic drain, and fix it, before doing anything else.​​

What to Do When a Battery Keeps Going Dead

Before you replace your battery, let alone install an auxiliary battery, it’s important to make sure that there isn’t a parasitic drain in the system.

This can be accomplished with a test light, but a good ammeter will provide you with more precise results. is fairly straightforward, but it’s important to remember that certain components will tend to draw a small amount of current, which is normal.

You can also run into situations where it seems like a drain is present, but it’s just a relay that’s unable to energize and close.

If a drain is present, then you’ll want to fix it before you do anything else. That may be the end of your problem right there, although your battery may already be toast from all of those times that it went dead and you needed a jump start.

If the problem has been going on long enough, you may even find that your alternator’s operational lifespan has diminished due to the extra load that your constantly dead battery has placed on it.

How to Safely Add an Auxiliary Battery

add auxiliary car battery diagram

There are a few different ways to install an auxiliary battery, but the most important thing is that it needs to be installed in parallel with the existing battery. In simple terms, that means both negative battery terminals must be connected to ground, and the positive terminals may be connected together, with an in-line fuse, or to a battery isolator to prevent draining the battery.

It's also important to find a safe location for the auxiliary battery. Some vehicles have space in the engine compartment. If your vehicle doesn't, you may want to consider installing a battery box in the trunk or some other safe location.

Adding an Auxiliary Battery for High-Performance Audio

If you have a high-performance audio system that you enter in competitions, or you just like to use it when your car isn’t running, then you may want to add a second battery. This is perfectly safe, although it’s important to follow wiring and installation best practices.

The second battery should be wired in parallel with the original battery, and most car audio competition experts will suggest that you buy “matched” batteries instead of wiring a high-performance battery into a configuration that includes an existing battery that’s already old and tired.

The battery cables should be the thickest gauge you can reasonably use, and you need to be really careful if you place the second battery inside the passenger compartment of your vehicle.

Since batteries can and do explode, the battery should either be placed in the engine compartment, the trunk, or inside a solidly built battery or speaker box if it has to be inside the passenger compartment. Of course, you’ll typically want to locate it as close as possible to your amplifier.

In some cases, you’ll be better off with a single, high capacity battery than two lower capacity batteries wired in series.

You may also be better off with a stiffening cap located close to your amplifier. If you have a problem with your headlights dimming when your music is turned up, then a capacitor will usually do the trick.

However, more reserve capacity in your battery (or batteries) is what you’re typically looking for if you’re entering your system in competitions.

Adding a Second Battery for Camping or Tailgating

The other main reason to add a second battery is if you spend a lot of time tailgating or dry camping. In those cases, you’ll typically want to install one or more deep cycle batteries to power an inverter.

Unlike regular car batteries, deep cycle batteries are designed to run down into a state of “deep discharge” without being damaged. That means you can use your electronic devices all you want without any fear of damaging your battery.

If you do add a second battery for either camping or tailgating, the battery should still be wired in parallel with your original battery. However, you may want to install one or more switches that will allow you to isolate the batteries depending on whether you’re driving or parked.

When you’re parked, you’ll want to have it set up so that you only draw power from the deep cycle battery, and when your engine is running, you’ll want to have to option to isolate the deep cycle battery from the charging system.

Recreational vehicles are all wired like this with “house” and “chassis” batteries, but you can set up the same type of system yourself if you know what you’re doing.

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