Adding a Second Car Battery for High Performance Audio

Tokyo Auto Salon 2015
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High performance car audio systems tend to require a whole lot of juice, and the original electrical system in some cars just isn't up to the task. The solution in some cases is to install a high output alternator, but that only actually works when the engine is running. If you want more power when the engine is off, then the best option is to install a second battery.

Powering Performance Car Audio Systems

If you want to add some extra juice to run your performance audio equipment, you have two basic options. The first option is to ditch your original equipment manufacturer (OEM) battery for the biggest, highest capacity battery that will fit in the available space. This is the easiest solution, and it’s typically good enough for most situations.

The other option is to replace your single battery with matched brand new batteries or add a deep cycle backup. This is more complicated, but it can potentially give you even more reserve amperage, and it has the added advantage of allowing you to install the second battery close to your amplifier.

Of course, it's also important to remember that there are situations where a stiffening cap or a high output alternator will be a better idea than an extra battery. Adding a second battery is a good idea if you want to be able to run your car audio system longer when the engine is off, but it won't do you any good when the engine is actually running.

High Performance Batteries for High Performance Audio

When you find yourself in the market for more power for your performance audio equipment, what you’re really looking for is more reserve capacity. Batteries all have a number of different ratings, but two of the important ones are cranking amps and reserve capacity.

Cranking amps refers to how much amperage the battery can provide at one time under a heavy load, i.e. when you are cranking the engine, and reserve capacity, usually given in ampere hours, refers to what the battery can deliver over an extended amount of time. That means you’re looking for a high performance battery that offers a lot of reserve capacity.

Depending on what car you’re driving, you may or may not have some extra space to work with where your battery is concerned. As long as a replacement battery physically fits into the allotted space, and you can safely strap it down, it’s perfectly fine to replace an OEM battery with an aftermarket one that has a significantly larger reserve capacity.

If you have space for a bigger battery, then that's the simplest option. 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 battery cables. It doesn't get any easier than that.

Second Batteries for High Performance Audio

The other way to add extra reserve battery capacity is to actually add a second battery. In this case, you’re typically going to get the best results by ditching your existing battery and putting in two matched batteries. The batteries should be the exact same brand, group, and age.

The new batteries don’t have to be the same group as the original battery, but they should be the same group and same production date as each other. This is essentially just to ensure that one battery doesn’t end up getting overworked, and that neither battery tries to pull juice out of the other when the car is off, which can lead to a shortened life expectancy.

second battery high performance audio wiring diagram
There are multiple ways to wire a second battery into a car, but this is a good place to start.

If you’re installing new matched batteries, one should go right where the original battery was, and the other needs to be wired in parallel. You can install the second battery in the passenger compartment or the trunk, although you need to take precautions if you install it in the passenger compartment, and it's a good idea to use a battery box or some other kind of protection even if it goes in the trunk.

When you wire the batteries together, it's essential to wire them in parallel. This means you connect the negative terminal on one battery to the negative on the other, and also connect the positive terminals together.

It's also important to use heavy gauge battery cable, and the positive cable should have an in-line fuse. For extra protection, consider installing a fuse at both the original battery and the second battery.

Both batteries also need to be connected to the chassis or some other good ground location. While you could technically leave the new battery ungrounded, or ground them both and omit connecting the negative terminals, grounding both batteries and also connecting the negatives together can solve a lot of problems before they ever happen.

For the best results, the amplifier should be wired directly to the new battery and be located in close proximity to it. For instance, you could install both the second battery and the amplifier in the trunk. It's also extremely important to use an inline amplifier fuse between the new battery and the amp.

Using a New Battery With Your Original Battery

You can also keep your existing battery and add a deep cycle or marine battery. This option is a little different, because you have to wire it so that you can isolate each battery from the electrical system and, more importantly, from each other.

The idea is to use the original battery when you're driving, and the bigger deep cycle battery when you're parked. This has the added advantage that you won't ever accidentally leave yourself with too little power to start your car back up.

Whether you swap out for a bigger battery or install a second one, finding a spot with the right horizontal dimensions isn't enough. If the new battery is tall enough to ground out on the hood, you'll have to look for other options.

The Problem With Extra Battery Capacity

Whether you install a high capacity battery, or a second battery wired in parallel, it’s important to remember that you’ll only really see a benefit when the engine is off. That’s when the extra capacity really comes in handy. Whenever the engine is running, the extra battery is just an extra load as far as the alternator is concerned, which can overstress an old (or underpowered) unit.

Depending on the precise issue that you’re trying to address, you may actually be better off with a car audio capacitor than an extra battery. Although stiffening caps typically aren’t the best solution for anyone who actually enters car audio competitions, they can often solve minor problems like headlights that dim during especially loud or bass-heavy music.