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How to Wire Multiple Batteries for Car Audio

Every car out there whether it runs on gas or diesel, is hybrid or electric, has a battery. The battery is the car’s power bank. It stores the power your vehicle needs to start the motor and provides the zap of electricity needed to put electrical components to work whenever the engine isn’t running.

A different component called “the alternator”, is responsible for charging your car battery when the engine is running.

In some cases, one battery just isn’t enough. This is especially true when you have a high-performance car audio system that requires a staggering amount of power, and that the stock electrical system in your vehicle just isn’t up to the task. In such case, the solution is to install a high output alternator, but that only has output when being driven by the engine.

So, if you want more power when the engine is off, whether to run a powerful car audio system or anything else, then the best option is to install an auxiliary battery.

Before You Get Started

It must be noted that unless you want to listen to music with your engine off for extended periods of time, adding a secondary car audio battery isn’t going to do you any good — and it may actually worsen situation.

That might seem counterintuitive, but it makes sense. I mean, think about it: the battery in your car is there to serve one purpose: to turn on a vehicle’s ignition system and start the engine.

After your engine is started and running, and the alternator is spinning, the battery actually acts as a load. Therefore, if you add a second battery, it’s basically just going to act as an extra load when the engine is running due to the fact that the alternator has to keep both batteries charged up.

So, always make sure the alternator in your car can handle the extra load.

Furthermore, keep in mind that an auxiliary battery won’t make up for a weak primary battery. In other words, if your car won’t start in the morning due a weak battery, adding a second battery won’t fix the problem.

Do You Need an Extra Battery for Car Audio?

Before we get into how to wire multiple batteries for a car audio system, you need to make sure an extra battery will solve whatever problem you’re trying to deal with.

If one battery is good, then two or multiple batteries must be better, right? Well, not always. Here are some of the situations where an auxiliary battery can help:

  • Provide power for a high-performance car audio system: Power hungry amplifiers, and other components, often require more power that far exceeds what the stock charging system is capable of providing. This is especially true if you use your car audio system when the engine turned off, like at competitions.


  • Listen to your car stereo with the engine turned offl: If you make a habit of parking your car and listen to the radio for hours on end, then a second battery might be for you.


  • Provide “engine off” power source for the electrical components: If you enjoy activities like camping, and tailgating, then an auxiliary battery will come in handy. It’ll provide the juice required to power up radios, televisions, electric cooler, and anything else you need to keep the party going.

How to Safely Install a Second Battery for Car Audio

When you’re in the market for more power for your audio system, what you’re really looking for is more reserve capacity.

Batteries all have a number of different ratings, but “reserve capacity” and “cranking amps” are two of the most important ones.

Cranking amps refers to how much amperage that a fully-charged battery can discharge at one time under a heavy load (i.e. when you are cranking the engine).

Battery reserve capacity, on the other hand, denotes the number of minutes a fully​ charged battery can sustain a designated constant load — usually 25 amps — before it is discharged down to 10.5 volts. It plays a major role in determining how long a battery can last with the engine off.

That means, if you have a high-performance audio system, or you make a habit of parking your car and using the audio system for an extended amount of time, consider a high-performance battery that offers a lot of reserve capacity.

There are two different ways to get more reserve capacity. The easiest way is to replace your stock battery with an aftermarket one that has a significantly larger reserve capacity (this is assuming you’ve got enough space for a bigger battery).

Replacing a small OEM battery with a larger capacity one is basically just a matter of pulling the old battery, putting the new one in, and hooking up the cables.

The second way to get more reserve capacity is to install a second battery. In this case, it’s highly recommended to ditch your existing battery and putting in two matched batteries for the best results. The two batteries should be the exact same brand, group, and age.

The two new batteries don’t have to be the same group as the original battery, however, they should be identical in size, brand, model, and production date. Preferably, they should also be at the same state of charge when you install them. This is ensure that one battery doesn’t end up getting overworked and that neither battery tries to pull juice out of the other when the engine is turned off, which can lead to a shortened lifespan.

If you’re installing two new matched batteries, one of them should go right where the original battery was, and the other needs to be wired in parallel.

Parallel wiring means that the terminals of each battery are connected to the same things — positive to positive, and negative to negative.

To prevent voltage drop, it’s highly recommended that you use heavy gauge battery cables, and the positive cable should have an in-line fuse. To further protect your equipment, consider installing a fuse at both the original battery and the second battery.

Furthermore, it must be noted that both batteries need to be connected to the chassis or some other ground spot.

dual battery wiring diagram car audio

Technically speaking, you could leave the second battery ungrounded, or ground both batteries without connecting their negative terminals. But experience has taught us that doing both (grounding both batteries and connecting their negative terminals together) can prevent a lot of problems and headache down the line.

Using a New Battery With Your Original Battery

If you’re on a tight budget and don’t want to splurge on a set of car batteries, then keep in mind that you can keep using your existing battery in conjunction with a deep cycle or marine battery.

It must be noted, however, that going this route is a little bit different from wiring both batteries in parallel.

In fact, if you’re adding a new battery to the an existing one, you’ll have to wire them in such a way that each battery is isolated from the electrical system and, more importantly, from each other.

The reason to do this is to use the existing battery when you’re driving, and the bigger deep cycle battery when you’re parked. The main benefit to this option is that you won’t ever accidentally be left with a drained battery that struggles to crank engine.

How Long Can My Car Battery Last When the Engine Is Off?

This is not an easy question to answer. A wide variety of things must be factored in in order to determine how long a car battery will last with the radio on including your car audio system, battery type, battery age and average use…etc.

To estimate how long you can run your radio before the battery runs outs, the formula is pretty straight-forward.

P.S: We will assume 100% efficiency between the battery and the audio system for purposes of this discussion. Although this seldom is the case in the real world.


We use the formula:

[(10 x RC) ÷ Load] = Operating Time.

In this formula, RC stands for reserve capacity, which is the number of minutes a fully charged battery can sustain a designated constant load until the battery drops below 10.5 volts. The load part of the equation refers to wattage pulled by your car audio system or other electronic devices.

Let’s say that your car audio system represents a 200-watt load and your battery has a reserve capacity of 90. This would result in numbers that look like this:

[(10 x 90) ÷ 200] = 4.5 hours.

In this case, the amount of time you’ll be able to run your stereo with the engine off is 4.5 hours. However, the practical limit is around two hours since it’s highly recommended to not discharge a battery more than 50%.

Alex Brown

Hey There, my name is Alex Brown, I'm an LA-based sound engineer with over 10 years experience installing, troubleshooting, and repairing commercial, automotive, and household sound equipment. I've installed highly competitive car audio systems, and everything from navigation systems to full car stereo systems, remote starters, alarms and beyond. I enjoy creating solutions and simplifying everyday needs. I also love helping people get great sounding gear, thereby, saving the world from bad sound one customer at a time.

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