When Do I Need an Extra Battery for Car Audio System? [EXPLAINED]

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When Should I Add an Extra Battery to My Car?

You need an extra battery for the car audio system when you have to run your stereo with the car engine turned off or when you are using a very powerful aftermarket setup of amps, subs, and speakers.

No audiophile would advise you to add an extra battery for your stereo system just for the sake of it. A second battery with no reasonable use only acts as a load for the car’s alternator. However, there are some instances where you might need more wattage than what your stock battery can produce to run your car audio.

If you are searching for “when do I need an extra battery for car audio”, continue reading this post. Here, we will discuss when you need one.

You Plan to Use Your Stereo System with the Engine Turned Off

If you use your vehicle for more than just an everyday commute, you might listen to your stereo even with the engine turned off. This particularly holds if you do a lot of camping or take your vehicle to tailgate parties.

Running an audio system with the engine turned off can quickly drain your car battery. Therefore, getting an extra battery for your car audio is a more sensible decision when you have to use its stereo system with the engine off.

You Are Using a Powerful Stereo System

Manufacturers pick a battery for a vehicle in line with its stock features. Therefore, when you are replacing your stock head unit and speaker set with aftermarket e options, you have to factor in if you need an extra battery or not.

With an amplifier cranking various thousands of watts and sets of sprawling subs and speakers, you might not be able to run your upgraded sound system on your car battery for long.

If the beat drops and high reverbs on your aftermarket system are causing dimming and flickering of headlamps, it is a sign that your existing battery is not meant for that degree of power handling.


You can also calculate the “runtime” of your existing battery to determine if you need an extra battery for your car audio.

Use this formula: Battery runtime in hours = 10 X (Battery capacity in amp hours) / (Power load in watts)

You can find the amp-hours rating of a battery on its label. Some manufacturers give RC(reserve capacity) rating instead of amp-hours. However, you can easily convert RC value into amp-hours. The power load will be the total wattage your stereo system (amp, subs, and speakers included) needs.


Suppose you are using an average car battery (70 amp-hours) to power your audio system with the collective wattage of 500 watts, you can use it to blast your favorite music for 1.4 hours (84 minutes).

With such a powerful audio system, especially when you run it with the engine off, you will need an extra battery for car audio. It will ensure that the rest of the electrical functions of your vehicle don’t suffer due to your overpowering audio system.

Hey there mobile audio lovers! My name is Vincent Talbot, founder and chief editor at 99carstereo.com. Ask any mobile audio fanatic, installer, or company rep what makes a good car speaker, sub or amp, or, better yet, why he or she prefers a certain brand over another, and be prepared to endure a litany of opinions, viewpoints, and passion-fueled perspectives. To be honest, mobile audio shopping can be a daunting task without a guide, so I’ve assembled what I feel are the best products to consider to make things easier for you. Welcome aboard!

Why you should trust me?

Mobile audio is my passion, so before I put anything in front of you I exhaustively research a broad range of products, review all the available information on them and ultimately make a curated list of recommendations. My goal is to recommend products that will serve you well so that when I extend this site to include more products, you will come back.