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When To Add A Second Battery For Car Audio: Know The 4 Signs

Last Updated on: May 9, 2024

A car’s factory battery tends to run out rapidly; users who enjoy listening to music even with the car engine off know this much. An auxiliary battery in place gives you additional capacity that allows extended operation.


How does a car owner determine when to add a second battery for car audio? If this is the same question you have in mind, please continue reading.

Let’s see if it’s time for you to consider adding a second battery for car audio system improvement.

Benefits Of Installing A Second Battery In A Car

While it doesn’t seem to make sense to have a second battery with the car engine running, the extra battery can provide you with additional power.

You can upgrade your main battery or add a secondary battery to allow you hours of enjoying your favorite music. 


Unintentionally leaving your headlights on for hours will leave you with a dead battery. Therefore, you can take advantage of the reserve power once you have an auxiliary battery or your existing one has a bigger capacity. 

The battery in your car essentially meets one objective, and that is to run the engine with adequate cranking amperage. Your car battery functions as a load once your engine and alternator start running.

Car accessories you use when the engine is off will derive current directly from the battery. Additionally, the alternator provides power to your battery for things like lights, turning signals, and a car audio system.

When Is Adding Second Battery For Car Audio Necessary

A common dilemma among car users is to know when adding second battery for car audio is crucial. A dual battery system with an isolator connects two batteries to your car’s alternator to keep multiple batteries separate.

A battery isolator works as a distribution point in your automobile’s electrical system. It controls the flow of current in vehicles with secondary batteries to prevent an auxiliary battery from draining the primary battery. 

It ensures that a single battery failure won’t render an entire electrical system inoperable. Such a system allows charging both batteries as usual while driving through the day, but these batteries get disconnected once you turn the engine off. 

Anyone wanting to give their gear a boost less the risk of running a dead battery while they’re out will find a dual battery setup essential. 

With an isolator in place, your car’s electrical system won’t source all power from various batteries simultaneously.

But how do you know when it’s beneficial to have a second battery for car stereos. Would it be wise, too, if you already knew how affordable or expensive a car stereo installation cost is? Here are some of the things to look out for to know it is high time to get an extra battery:

1. Dimming Car Lights

When you notice your car lights dimming when listening to music with more bass, it indicates that adding second battery for car audio is already necessary. 

The extra battery ensures that your gear continues to perform nicely with adequate electrical power to keep your car operational and power up your accessories.

If you’re the type who would listen to the car stereo with the engine off, you will end up draining your car audio battery. You will rapidly consume the battery, too, if your car audio system utilizes a powerful amplifier with multiple outputs.

The factory battery that came with your vehicle has limited capacity and can only power your car stereo for about an hour or so without a running engine. Installing a second battery will enhance the audio system and its performance.

2. Noticeable Corrosion Around Your Battery Connection

Corrosion, or the whitish or bluish powdery substance, can signify an overcharged alternator. Such can also happen if you overfill the car battery.

If you notice the corrosion forming on the negative terminal, it indicates an undercharged battery. Alternatively, corrosion on the positive terminal means the alternator is overcharging the battery.

Battery terminal corrosion prevents the engine from starting and may cause problems with the electrical system.

One proof is your car stereo won’t turn on with ignition. This is a tell-tale sign your battery is ailing already!

Once you notice the battery terminals in such a state, getting a car battery replacement is the most practical thing you could do.

3. Malfunction In Powered Features And Accessories

Another telltale sign of a possible need for a second car battery is slow-moving power windows or unusual behavior in some of your vehicle’s powered features.

If you start noticing non-essential electrical functions shutting down progressively, such as the central locking, your battery charge may be getting low. 

In cases where your vehicle only has the starting battery, you have the slightest chance of powering all your gears and still getting your engine running.

A 2nd battery installation might only address the need for more capacity. However, securing a dual-battery system ensures that the starter battery will always have the power it needs.

4. Slow Cranking Of Car Engine

The power of a second battery is essential if you heavily rely on 12-volt lighting, a car fridge, or personal electronics. You still get to enjoy all these comforts without ending up with a dead or slow cranking battery.

While a slow engine crank is not always due to a failing battery, it might still be a possible cause. You need to investigate and find out what causes the slow cranking. 

Typically, you will hear an unusual sound when attempting to start the engine, or it seems unresponsive while taking a while to start, especially during cold mornings.

Aside from a faulty battery, a malfunctioning electrical connection, an issue with the charging system, or a poor starter battery rundown may also cause such a problem.

It’s best to check thoroughly or enlist the help of a mechanic to ensure that all parts get the necessary voltage. You need to figure out if it might be the best time to invest in a second battery.

How to Install Second Battery for Car Audio

Aside from figuring out when a secondary battery is necessary, it would also help to know how to install a second battery for car audio.

Installing Two Matching Batteries

Discarding your existing battery is the most efficient way of getting the most out of your second battery. It allows you the extra capacity your car requires and gives you the best possible result with high-performing audio.

It’s best to utilize two matching batteries having the same brand, age, and group; the opposite effects of unmatched batteries can shorten their lifespans.

When installing two matching batteries, place one of the new batteries in the same place as the old one. Wire the other battery in parallel, and connect the negative terminals of both batteries, doing the same for their positive terminals. A parallel connection allows equal power distribution.

Use heavy gauge battery wire, ensuring that the positive wire comes with an inline fuse. You may also opt to add a fuse to both batteries for extra protection.

Adding A Second Battery To An Existing One

So, if that’s how you install two matching batteries, what about adding a second battery to my car? With such a question in mind, here’s how you can do so:

1. If you choose to install the second battery with the existing one, your first step would be to determine the location. Using an isolator is crucial when adding a deep cycle battery to the existing one to separate the batteries from the electrical connection.

2. Drill holes through the installing tabs of your battery box and into the trunk’s floor to install the new battery. It will give way for the new wire and prevent unnecessary battery movements when the car moves. 

3. Screw the battery tray matching the size of your second battery in your desired location, and route the cables without hitting any components. Cut the wire connecting the alternator to the starter.

4. Install the battery isolator to separate the deep cycle battery from the charging system. Use an extra wire to attach the disconnected alternator wire to one of the isolator’s central terminals or the one with the “A” label. 

You can mount the wire’s bare lead under the isolator terminal as you bolt the other lead to the alternator. Connect a third longer wire to the remaining isolator terminal, and connect it to the positive terminal of the second battery.

5. Lastly, secure the wire clamps by screwing them to the chassis. Ensure that no other components get in the way when you do, then reconnect the two cables to the starter.

When securing the negative terminal to a metal ground like the chassis, it is more practical and recommended to start with the positive wire.

Other Essential Accessories In Secondary Battery Installation

Remember that the batteries are only part of the entire process. You also need copper or gold terminal clamps to prevent voltage drop, power wire not smaller than your ground wire, and fuse. Any problems with these are evident when bad amplifier symptoms start creeping up.

Choose a battery cable with the thickest gauge possible; you might even have to use gauge zero for power and ground. Battery isolators, distribution blocks, and wire connectors are also other items you might need.

Your car will consume more power once you upgrade your car’s audio equipment to aftermarket gear. 

Sometimes, a new amplifier will do, but check to see if upgrading the overall electrical system will do more good. Typically, stock electrical systems utilize alternators with 60 and 120 amps.

Adding a new battery isn’t always the solution since the amount of current sent to the batteries depends on the stock alternator. While costly, an alternator upgrade might be more beneficial.

Frequently Asked Questions

What size should a second battery for car audio be?

There are designated alphanumeric numbers for battery group size. The appropriate battery size depends on your vehicle’s construction, model, and engine type. The best way to determine which size battery group your car requires is to consult the replacement guide.

Should I use the primary or secondary battery when installing a second battery in a car?

Different ratings define a battery’s ability to start an engine. The amp-hour shows you how much energy your battery has. On the other hand, the reserve capacity indicates the duration a fully-charged battery can operate several accessories when the alternator fails.

Check your car’s user manual if you are unsure what battery type to use. Once you figure it out, you may start finding a suitable auxiliary battery to use when installing the second battery in car stereos.

What extra battery is suitable for improving my car’s sound equipment?

Batteries for your vehicle’s sound system have the same function as regular automotive batteries — an aux battery may not power all electronics, just the sound equipment. Different cars need varying battery types that will operate optimally with the vehicle.

Consider the battery life, amp-hours, and the amount of charge for your second battery. The XS Power, for instance, is an entire kit of secondary batteries for cars. The standard secondary types you can use are lithium battery, deep cycle, wet and dry cell, and lead-acid battery.


Technological advances in cars lead to more innovation in batteries. Each type of battery has its advantages and disadvantages, and you should only use the one appropriate for your vehicle.

Voltage, current, and cranking amp are factors you need to consider when replacing, adding, or upgrading your vehicle’s battery.

Unlike the conventional ones, the absorbent glass mat battery is often the best choice for low resistance. Your stock battery can power your gears. However, you can stabilize the power of your amplifiers by getting a spare battery.

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