Do I need an extra battery for my high end car audio system?

Question: Do I need an extra battery for my high end car audio system?
I’ve upgraded my head unit and speakers, and added an amplifier and subwoofer, and it sounds great. Does all that extra gear mean I need to add an extra battery?
Answer:
Unless you want to listen to music with your engine off a lot, adding an extra 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.
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.
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 afterwards. 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.