The battery in your car is there to serve one purpose: to provide enough amperage to start the engine. After the engine is running, and the alternator is spinning, the battery acts as a load. If you add a second battery, it’s going to act as a second load when the engine is running because the alternator keeps both batteries charged.
When One Battery Just Isn’t Enough
One battery is good, so two must be better, right? There are a few situations where that’s the case. When the 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 a second battery, you end up with extra reserve power. 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 quickly. If you add a second battery, you can go longer without running the engine and charging it back up. If you make a habit of parking your car and using the audio system for hours on end, a second battery might be in order. In all other cases, it’s probably not going to solve the 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, or you’re going camping and want to power several devices, the battery has a limited capacity to work with. 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, here’s the formula: 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 power the 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 the battery has a reserve capacity of 70. This would result in the following calculation: 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, that means it will take 70 minutes for a 25 amp load to drain the battery 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
Adding a second battery can cause problems because it acts as an additional load whenever the engine runs. In other terms, an electrical load is anything that draws current. Your accessories—from the headlights to your car stereo—are loads, and so is the battery. While the battery provides current to the starter motor to get the engine going, it draws current from the alternator afterward. That’s why driving with a dead battery is hard on a charging system—alternators aren’t meant to be worked that hard. When you add a second battery to your car, you’re adding another bucket for your alternator to fill. If the second battery is discharged to any great degree, it may overtax the alternator. So if you’re dealing with issues like dimming headlights when you turn up the music, adding a second battery can make the problem worse.
