When to Add a Second Battery for Car Audio? (With Extra battery Tips)




As an affiliate, ImproveCarAudio get small commissions for purchases made through links on this website from Amazon and other third parties.

Enjoyment from listening to music can be damaged quickly when you turn your engine on and find out that your battery is flat.

Unfortunately, having a standard battery dead is a common issue with powerful car audio. One of the solutions to fix that problem is adding an extra battery to your car. But when should you add an extra battery for car audio? Let’s find out.

A second battery can be added if you want to run high-power amplifiers, subwoofers, or other power-hungry components, especially with the engine turned off. Generally, the best time to add a second battery to your car audio system is when you’re experiencing power issues with your current battery, such as audio cutting out or an inability to power all car amplifiers.

Tip: Having installed a second battery in your car will boost your audio system and improve performance. But, to do it right, you have to make sure that you are using good power wires.

You have to remember that power and ground wires should have the same size (gauge) and high quality, ideally OFC.

I use the KnuKonceptz 4 gauge OFC Wires that I have found really flexible and easy to install. They are available in either red, blue, or black, depending on your needs, and in different lengths from 5 to 100 feet.

In the article below, I will show you more about car audio batteries and how you should install yours.

When Do I Need to Add Extra Battery for the Car Sound System?

Whether you need a second battery for car audio or not is determined by your audio system and how much power your amplifiers need. 

The easy way to find out if you need a second battery for your car is to check the headlights. If headlights are dimming while you play the music, it means that your amplifiers are drawing more power than your alternator and battery can produce.

Remember that your serial battery was designed to start your engine and to work with standard electrical equipment in your car, so adding powerful amplifiers to the vehicle increases a total load of your electrical system.

How Much Power Car Batteries Hold?

There is an easy way to find out how long you can listen to music in the car before the battery is drained. 

The calculation below shows how long you can listen to the music, depending on the amperage of the battery and amplifier’s power.

Running time (hours) = 10 x battery capacity (in Amp hours) / load power (in Watts)

If you have a 90 AH battery and 1,800 Watts RMS amplifier, the calculation looks like below:

10 x 90 / 1,800 = 0.5 h  

In other words, a 90Ah battery will be able to power your 1,800W amplifier for half an hour before it goes flat.

Based on the above, if you want to listen to the music with the engine turned off for a longer, or you have a more powerful amplifier, you need to install an extra battery for your car audio system.

You can also quickly determine how much amperage you need for your car audio system by using the below formula right of Ohm’s Law:

Current (I) = Wattage (P) / Voltage ( E )

Let’s take the above amplifier of 1,800W that is running at the standard 14.4V.

1,800 / 14.4 = 125 amps

But, an amplifier does not run at 100% efficiency, so you have to divide the 1,800W by an average of 70%.

In effect, your amplifier to supply 1,800W needs to receive from the battery 2,571 W.

Let’s now replace the 1,800W with the actual 2,571 in the calculation:

2,571 / 14.4 = 178.54 amps

This is the actual amperage the battery need for your 1,800W amplifier, but this is not the end.

The majority of the standard car electrical systems consume around 60 amps to operate, which should be added to the total requirement.

178.54 + 60 = 238.54 amps

If you want to install a 1,800W amplifier in your car. Your electrical system needs to supply a constant total of 239 amp.

Do I Need an Extra Battery for My Subs?

You can never have too much power, which is also true for the car audio system. Subs are the most powerful elements of any car sound system, and even if you do not have a strong amplifier for the speakers, I am sure you have one to power your subwoofer.

You need an extra battery for your sub, especially if it has a power of several thousand watts. In addition, if you have installed a professional competition system, you need to ensure the proper amount of energy is supplied to the system. 

Can I Use a Regular Car Battery for Audio System?

A battery for your car audio system works the same way that any regular automotive battery does, but instead of powering all vehicle electronics, it just powers the radio and amplifiers.

The vast majority of the regular car batteries are Lead-Acid batteries. Although these batteries are most popular in cars, they require regular maintenance.

You will have to check the water level in each cell regularly, and if it is too low, you need to add distilled water. Otherwise, this battery can be permanently damaged.

Another disadvantage of lead-acid batteries is that they tend to swell, so if you have an excellent finishing of your car audio area, the last thing you need is to have water or moisture there.

You can use a regular battery for the car audio, which is common practice. However, installing higher output AGM or Lithium-ion batteries will bring more benefits to the system than just a regular lead-acid battery.

The only drawback when installing an extra battery for car audio is that the second battery can cause problems for the serial alternator.

While the additional battery supports your car audio, it actually acts like extra load every time the engine runs, drawing more current than the standard alternator can produce.

Therefore, it is strongly recommended (not only by me) to upgrade your standard alternator whenever you decide to add a second battery to the car audio. Otherwise, you may find a flat, not one, but both batteries.

Are AGM Batteries Good for Car Audio?

AGM batteries are the best choice as a second battery to the car audio systems. Because of AGM’s low internal resistance, fast recharge, sealed, and maintenance-free, they work best for the car audio.

AGM batteries are an excellent solution for high-end and advanced car sound systems. They have significant power demands and have a longer life span than most acid cell auto batteries. 

But not only for car audio, but you can also install an AGM battery instead of any factory battery. An AGM battery will give the vehicle more cold cranking power and greater resistance to heat and vibration.

The only downside of the AGM batteries is their discharge rate, which cannot be larger than 50%, while you can discharge lead-acid batteries to 80%. In other words, it means that the lead-acid battery can run longer than AGM on every single charge.

Therefore, it is handy to have an AGM battery charged at home overnight and make sure that it is fully charged every time you leave the house.

Remember also to change an AGM battery before the first connection to the car audio. New AGM batteries come uncharged for safety purposes.

XS Power D3400 XS Series 12V 3,300 Amp AGM High Output Battery with M6 Terminal Bolt

What size second battery for car audio should I buy?

The size of the second battery for car audio depends on how much power your amplifiers need.

As a general rule, you need a 100Ah battery for every 1,000 watts RMS of your car audio.  

The table below shows a few examples of popular batteries with the RMS wattage that can support on top of standard electronics in the car. 

CCAAh MaxAmplifier Wattage (RMS)
Maximum Wattage that Car Batteries Can Support

When looking at the battery label, you will not always see both the CCA or Ah values. To calculate the rough estimation of the Ah of your battery, you can take CCA and divide it by 7.25.

Where to Place an Extra Battery in My Vehicle?

Most factory car batteries are installed under a hood, but there is not enough room for the second one in the engine compartment.

Additional car audio batteries should be placed close to the amplifier, and the most common places are either inside the cabin or in the trunk.

Batteries installed inside the car should be leak-proof and vibrations resistant, and this is another reason why you should not use standard lead-acid batteries for car sound systems.

If you install a battery close to the powerful subwoofers that make many strong kicks, the vibration resistance is crucial. If you mount the battery in the other than factory position or even on its size, its leaking proof is mandatory.

Which Additional Accessories Do I Need for Additional Battery?

When installing an additional battery for your car’s sound system, you need to ensure it works at its optimal performance. To do that, you need to use some high-quality accessories:

Terminal clamps

Terminals made of either copper or gold will prevent connections from any corrosion and avoid voltage dropping.

If you are adding a second battery, do not use old terminals from the acid lead battery, but install clamps with multi-connection points instead.

Power and ground wires

Power and ground wires used for a second battery should have the same gauge as the other power wires used in the car audio.

Remember never to use ground wire that is smaller (thinner) than the power wire. For example, if you use a gauge 4 of the power wire, use the same gauge 4 for the ground wire. 

I have been using for a while KnuKonceptz OFC wires, and they work really well.

KnuKonceptz Kolossus Flex Kable OFC 4 Gauge Power Wire Copper Cable Red (20 ft)

Battery Isolators

Battery isolators are used to disconnect both batteries from each other when the engine is off. This will keep your start-up battery able to start an engine even if your second battery is drained.


It does not matter which secondary battery for car audio you use. Fuse on the power wires is mandatory.

There are many models and amperage versions, but remember to use one fuse on the power wire between batteries and another between the secondary battery and amplifier.

Distribution Blocks

Distribution blocks are needed to connect several power and ground wires to the same battery terminal and are mainly required on the secondary battery.

You will have at the positive terminal input power wire from the main battery and output power wires to the amplifiers, so at least two solid and thick wires in the same place.

Wire connectors

Power wire connectors are as important as high-quality terminals, although they are not required for every battery or amplifier type.

If you do not use a proper size for either battery or amplifier, you risk sound quality because they can become the weakest points with the most resistance.

How to Add a Second Battery for Car Audio?

How to wire an extra car audio battery depends on the type of battery used and if it is the same battery type as your primary or a different one.

The simplest way to connect a second battery is when it is the same type as the primary one and is placed, for example, in the trunk.

However, if you add a second battery that is different than a start-up battery, you have to charge them separately to the same voltage level before connecting to the same circuit.

The new battery still has to be connected to the alternator, but you cannot wire both batteries together if they have different charge levels and, therefore, will supply different voltages. This will kill a weaker battery.

To wire the second battery, you should use the thickest gauge wires you can. If you have to use gauge zero for both power and ground, do it.

Also, try to place the extra battery close to the amplifier and make wiring connections as short as possible.

If your second battery is a different model, size, capacity, or technology than your primary one, the installation is more complicated, and you will need additional equipment to make it work.

You cannot connect batteries with different chemistry in parallel. So, for example, AGM and Lithium batteries can work in the same car but in separate circuits.

Connecting a second battery in the car, you start by running the power wire from the alternator to the place where you will install the battery. If this is a trunk, then you have to run a power wire through the entire car.

On the power wire, close to the second battery, you have to place the fuse, and it is a good practice to add a second fuse of the same size close to the alternator.

Next is the direct wiring from the fuse in the tunk to the positive battery terminal. Make sure that for all connections, you use wire of the same size (gauge).

Another wire from the positive battery terminal you connect with the power terminal in the amplifier. On this wire, you also have to add the fuse, so you will end up having two fuses in the trunk, one between the amplifier and the battery and the second between the battery and the alternator.

Now the ground wire from the amplifier and another one from the second battery. Here, you have two options: run a short wire to the chassis bare metal or directly to the battery.

Using a chassis to ground the wires allows you to use shorter wires, but the disadvantage is that in some cases, especially in older cars, the chassis can have high electrical resistance and limit the quality of the current flow. 

Because of that, I prefer to have a direct wire connection for both power and ground wire directly with the battery and not through the car’s chassis.

To summarize, wiring from the second car battery is as follow:

  • Two wires with fuses from the positive terminal. One goes to the amplifier, and the second connects with the alternator.
  • One wire from the negative terminal. It goes either to the chassis or to the main battery’s negative terminal.

How to Charge the Second Battery in Car?

There are a few ways to charge your second battery, and all of them are straightforward.

The easiest is using the alternator from the engine to charge your second battery when traveling on the road and using a voltage sensing relay (VSR) or an isolator switch in the SCA dual battery kit.

Another way to charge a second battery for car audio when the engine is off is to use a solar battery regulator.

The best is a DC to DC Charger with solar input. This is the more efficient way to regulate the input to your battery from both sources without damaging the battery over time.

All the above devices allow both batteries to charge while the vehicle is running and isolate the batteries after switching the engine off.

This way, you will keep your starting battery at full charge, while the second battery will power your amplifiers.

Can I Run Two Batteries on One Alternator?

You can run two batteries on the same alternator because alternators protect against excessive current drain and are not affected by multiple batteries.

When you connect the batteries, the voltage in the system equalizes, meaning that the lower voltage battery sucks power from the other.

If your batteries are the same type, the alternator sees your main and an extra battery for car audio as a single large battery and charges them the same.

The batteries self-regulate the amount of current they take as they charge. Therefore, you can use the same alternator for several batteries, but one additional battery is installed in most car audio systems. 


Installing a second battery is straightforward, and it does not take much time. 

When you need to add a second battery to your car audio system, it is worth looking at the AGM batteries.

They are the best choice because of their low internal resistance, fast recharge. AGM batteries are also fully sealed and maintenance-free. In addition, they can be installed in any position, unlike standard wet cell batteries that can be placed only vertically.


When Is It Necessary to Add a Second Battery for Car Audio?

If your existing battery is running low or draining quickly when running the car stereo for an extended time, then adding a second battery is a good idea.

What Is a Battery Isolator, and Do I Need One for My Car Audio Setup?

A battery isolator is a device that separates your primary battery from the extra battery you’re adding for car audio.

It ensures that the extra battery only gets charged when the vehicle is running and prevents it from draining the primary battery. You’ll need a battery isolator if you’re adding a second battery to your car audio setup.

How Do I Determine the Size and Capacity of the Second Battery I Need for My Car Audio Setup?

The size and capacity of the second battery you need would depend on your car’s audio setup’s power requirements and the amount of time you’re planning to run it.

A general rule of thumb is to choose a battery with a capacity twice the size of your car audio system’s power requirements.

Can I Use My Car Battery as the Second Battery for My Car Audio Setup?

You can use your vehicle’s car battery as the second battery for your car audio setup, but it’s not the best option.

The car battery is designed to start the engine and power the vehicle’s electrical systems, not to provide consistent power to an audio system for a long time, especially with the engine turned off.