Unless you want to listen to music with your engine off a lot, adding a dedicated car audio battery isn't going to do you any good — and it may actually hurt. That might seem counterintuitive, but the reasoning is pretty simple. Basically, the battery in your car is there to serve one purpose: provide enough cranking amperage to start the engine. After your engine is running, and the alternator is spinning, the battery actually acts as a load. If you add a second battery, it's basically just going to act as a second load when the engine is running due to the fact that the alternator has to keep both batteries charged up.
When One Battery Just Isn't Enough
One battery is good, so two batteries must be better, right? Well, there are a few situations where that's actually the case. When your engine isn't running, any accessories you turn on pull current directly from the battery. That's why you'll come back to a dead battery if you accidentally leave the headlights on overnight. If you add a bigger battery or even a second battery, you end up with a lot of extra reserve power.Westend61 / Getty
The main reason to add a second battery to a car or truck is if you need to use your accessories when the engine isn't running. If you take your vehicle camping, that's a good example. You may be out for a weekend, or longer, without running the engine, and that can drain the battery down pretty fast. If you add a second battery, you'll be able to go longer without running the engine and charging back up.
If you make a habit of parking your car and using the audio system for hours on end, then a second battery might be in order. In all other cases, it's probably not going to solve whatever problem you're trying to deal with.
Listening to Your Car Stereo With the Engine Turned Off
Whether you have a high-performance car audio system that you want to show off, you just want to listen to music with the engine off, or you're going to be camping and want to power various devices, your battery has a limited capacity to work with. In fact, the battery your car came with may only be able to run your stereo for an hour or so with the engine off.
If you want to estimate how long you can run your stereo with the engine off, or figure out how much reserve capacity to look for in a second car audio battery, the formula is pretty simple.10 x RC / Load = Operating Time
In this formula, RC stands for reserve capacity, which is a number, in amp-hours, that indicates how much juice your battery has available on a full charge. The load part of the equation refers to the sustained load power, measured in watts, pulled by your car audio system or other electronic devices.
Let's say that your car audio system represents a 300-watt load and your battery has a reserve capacity of 70. This would result in numbers that look like this:
10 x 70 / 300 = 2.33 hours.
If your car audio system has an aftermarket amplifier and a correspondingly higher load, the amount of time you'll be able to run your stereo with the engine off will go down. If you add a second battery, the time will go up.
In many cases, a battery will show a reserve capacity in terms of minutes rather than amp hours. If your battery shows that it has a reserve capacity of 70 minutes, what that means is that it will take 70 minutes for a 25 amp load to drain the battery down below 10.5 volts. In reality, the real number will differ depending on the ambient temperature and the condition of the battery.
Car Audio Batteries: What a Load
The reason that adding a second battery can actually cause problems is that it will act as an additional load whenever the engine is running. In plain terms, an electrical load is anything that draws current. All of your accessories — from the headlights to your car stereo — are loads, and so is your battery. While the battery provides current to the starter motor in order to get the engine going, it draws current from the alternator afterward. That's why driving around with a dead battery is so hard on your charging system — alternators just aren't meant to be worked that hard.
When you add a second battery to your car, you're basically adding another bucket for your alternator to fill. If the second battery is discharged to any great degree, you may even end up overtaxing the alternator. So if you're trying to deal with issues like dimming headlights when you turn your music up, adding a second battery can actually make the problem worse.