Learning, Wiring,

How to Wire Multiple Batteries for Car Audio (Guide)

Are your car headlights dim while listing to bass-heavy music? If so, that is a clear indication you don’t have enough battery power. Running a high-performance car audio system needs a large amount of current. So, it might be time to install an extra battery for your car. Installing a second car battery requires some calculation and proper execution. For instance, depending on the total amp requirement, you might have to upgrade the alternator. That is why today, I’m planning to teach you to wire multiple batteries for car audio.

In general, to install an extra car audio battery, follow these steps.

  • Buy a new battery that matches the old battery’s brand, group, and age.
  • Cross-check the amperage values of the two-car batteries and alternator. If needed, upgrade the alternator.
  • Pick a place for the new battery and fix it in that position.
  • Connect the old battery’s positive cable to the new battery’s positive terminal. Connect the negative cable too.
  • Finally, properly ground both batteries.

That’s it. Now you can run the car audio system without any issues.

Before You Get Started

Connecting multiple batteries to your car is not a difficult task. But, there are a few factors that you need to inspect before starting this process. So, I’ll try to explain them to you here.

  • Reserve Capacity of the Battery
  • Alternator Amp Output

Reserve Capacity of the Battery

Reserve capacity refers to the number of minutes that a fully charged battery is able to sustain a 25 amp load until it gets discharged to 10.5V. This reserve capacity directly affects the operating time. Therefore, higher reserve capacity increases the time that you can listen to a high-performance audio system while the engine is off.

Keep in mind: Above amps and voltage values should be measured at 80°F.

Second Battery or Higher Reserve Capacity?

Buying a new battery that has a higher reserve capacity is one way to solve the lack of power issue on your car. All you need to do is replace the stock battery with a new battery that has a larger reserve capacity.

However, first, check the size of the new battery. It might not fit into the old battery space. If everything works smoothly, you won’t need a second car audio battery.

Alternator Amp Output

Depending on the type of the alternator, the amp output might vary from 40 amps to 120 amps. However, if you are looking for a current of 300 amp, the high amp alternators are the best.

To pick a suitable alternator, first, calculate the total amp requirement. Take the total for electrical components such as driving lights, brake lights, electric fans, high-performance stereo systems, etc. Add 10% to the above amp total and take that value as the final value.

After considering the alternator amp output and total amp requirement, you’ll be able to determine the amount of power output that is needed by the alternator to keep the battery charge. So, if you are adding an extra battery, you might have to upgrade the alternator.

How to Install a Second Car Battery for Car Audio? (Step-by-Step Guide)

After you upgrade the alternator, now you are ready to install the second battery for your car. Follow these steps properly to get the best results.

Step 1 – Buy the Right New Battery

Lil Rob showing the two batteries of his car
Video | Lil Rob

First and foremost, buy a new battery that matches the old battery’s brand, group, and age. When you connect two similar batteries, both batteries will charge and supply at an equal rate. So, you don’t have to worry about one battery getting overworked.

Keep in mind: However, if you cannot find a matching battery with your old one, you might have to buy two new batteries.

Step 2 – Switch Off the Engine

Then, switch off the engine. Never try to connect the two batteries while the engine is on.

Step 3 – Set Up the New Battery

Next, find a good position for your second battery and set it up there. If your car hood has enough space, you can put it under the hood close to the original battery. Or you can set it up under the rear hood.

Step 4 – Connect the Two Batteries

Connecting two batteries is a little tricky. Here, you might have to decide two things.

  • Size of the cable
  • Connection Type

Size of the Cable

When choosing a suitable battery cable, you should consider voltage drop. Depending on the length of the connection, the voltage drop might vary. So, try to keep it under 3%.

Tip: Usually, 4-gauge or 6-gauge wires are excellent for a 12V battery.

Connection Type

There are two types of connections; series and parallel. Here are some of the differences between these two connections.

  • A series connection connects all the components end to end.
  • A parallel connection connects the components across each other.
  • A series connection has higher resistance, and a parallel connection has less.

Lil Rob connecting negative and positive battery wire
Video | Lil Rob

For this demonstration, we are going to use the parallel wiring method. So, connect the positive sides of the two batteries using a heavy gauge wire.

connecting the positive sides of the two batteries
Video | Lil Rob

And do the same for the negative sides.

old and new battery wire diagram

Install a fuse for both batteries. Then, connect the battery output wires to the amplifier.

Step 5 – Ground Both Batteries

To prevent any damage or electrocution, remember to ground both batteries properly. Now your second battery for car audio is ready to use. (1)

What Should I do If I Cannot Find a Similar Battery?

For this kind of situation, there are two solutions. The first solution is completely removing the old battery and buying two matching batteries.

Or keep the old battery and wire both batteries in a way that they do not interact with each other. That means you’ll have to provide a separate connection for each battery.

How Long My Car Battery Last When the Engine is Off?

To calculate the operating time of the battery, you’ll need the following equation.

Operating Time = (10 × RC) ÷ Load

RC – Reserve Capacity

Load – Required Wattage for Car

If the RC = 75 minutes and Load = 150 watt,

Operating Time = (10 × 75) ÷ 150 = 5 hours

That means you’ll be able to run the car audio system for 5 hours while the engine is off.

Do You Need an Extra Battery for Car Audio?

It is always better to understand the circumstances in which the car requires more power. So, here are some circumstances you might encounter.

For a High-Performance Car Audio System

If you are planning to operate large amplifiers and other sound components, the stock battery power might not be enough for your car.

To Listen to Car Audio System While Engine is Switched Off

If you are running the car audio system while the engine power is off, you’ll need a second car battery. Because the engine is off, the alternator will not charge the battery. So, having a second battery will do wonders for you. Besides, it will provide current for other electrical components too. If you enjoy outdoor activities such as camping, traveling, and tailgating, having a second battery will come in handy. You can provide power to televisions, radios, electrical coolers, and other items. (2)

Wrapping Up

If you are looking for an easy and budget-friendly way to increase the amp value of your car, follow these two steps.

  • Buy a new battery that is similar to the old battery.
  • Then, wire the batteries in a parallel circuit.

However, if you cannot find a similar battery, you might have to consider other options; buying two new batteries or wiring the car batteries separately.

Take a look at some of our related articles below.

(1) electrocution – https://www.ncbi.nlm.nih.gov/pmc/articles/
(2) outdoor activities – https://www.mayoclinic.org/diseases-conditions/coronavirus/in-depth/safe-activities-during-covid19/art-20489385

Video References

Provo Beast Audio Installation

Outdoor Tradesman

Lil Rob

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About Sam Orlovsky

AvatarCertifications: B.E.E.
Education: University Of Denver - Electric Engineering
Lives In: Denver Colorado

Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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