Informational Guide

How To Add A Second Battery To A Car

Not only will we discuss and show you how to add a second battery to a car, but we also look at other reasons it can be useful.

If you are thinking of adding the best car audio battery to improve your stereo experience or you just need more power, we have some factors you should consider. Not only will we discuss how to add a second battery to a car, but we also examine if you need to add a second car battery for accessories.

When you put a second battery in a car, both of them operate as an isolated system. One battery is used for the starter system, while the auxiliary battery is meant for the accessories.

However, the car alternator is intended for charging both batteries. In the best dual-battery setup, you would have an isolator, special wiring, and a monitor where you can view both conditions of the battery with ease.

Reasons Why You Would Add a Second Battery?

Is it worth adding a second battery to a car? In most situations, the standard battery is enough for what you need.

However, if you have high-powered stereo equipment or you use accessories that draw a lot of voltage, you should consider hooking up a second battery in your car. You can mount the extra car battery inside the vehicle for installation ease.

Before you figure out how to install a second battery for car audio systems, think about the pros and cons of using a dual-battery system.


  • Powers up accessories, such as a fridge, radio, and lights without the car engine on
  • Won't kill your starter battery
  • Peace of mind if the primary battery fails
  • Increases power when a winch is needed


  • Adds weight to the vehicle
  • Extra cost involved

Things To Consider When Adding a Second Battery To Your Car

What are some things to think about before connecting a second battery to my car?

Mounting Location

If you have some extra room under the hood, this is the best location for the second battery. However, you can mount it inside the car as long as it is vented to the outside. You don't want to deal with the vapors of a second battery in the vehicle.

Battery Type

The deep cycle battery is cheaper than other options but also suffers from a reduced lifespan if not charged properly. If you choose this type, make sure you use a deep cycle battery charger. Otherwise, consider an AGM battery, which doesn't let out hazardous gases, or the lithium battery for a longer life.

Battery Isolator

With a battery isolator, you can charge both of the batteries automatically without running out the starting battery. You should not set up your dual battery configuration without this vital piece of equipment.

Battery Size And Weight

You need to make sure you have the appropriate battery for your needs. Check the battery size versus the location where you are mounting it. Additionally, consider the weight as an AGM battery can weigh 70 pounds or more.

Usable Capacity

You won't want to use your battery beyond 50% of the rated capacity, or you could damage its lifespan. So, if you are looking at a 75 amp-hour battery, keep in mind that it only contains 37.5 usable amp-hours.


You do need more than an additional battery when you learn how to add second battery for car audio. You will also need to budget for the battery isolator, power inverter, heavy-gauge DC wiring, and a charger to complete the setup right.

How To Install A Second Battery In Your Car

How To Add a Second Battery To Your Car?

You only need to follow some simple steps when you want to learn how to install a second battery in your car. Whether you are installing a second battery for car audio or you need to know how to connect a second battery to a car for accessories, our step-by-step guide will help.

  1. 1
    Start by mounting the second battery to your vehicle. Make sure you will have enough wire to connect it.
  2. 2
    Disconnect the starter battery.
  3. 3
    Mount the relay close to the main battery but far enough away from any hot components.
  4. 4
    Cut all the cables to the appropriate length.
  5. 5
    Fit the terminals to the cable by stripping away some of the insulation and crimping it in place.
  6. 6
    Use heat shrink over the terminal to ensure a waterproof seal.
  7. 7
    Ground the black wire from the relay.
  8. 8
    Connect your main battery to the relay.
  9. 9
    Ground the second battery.
  10. 10
    Attach the relay to a mounting plate. Do not attach positive cables to both batteries.
  11. 11
    Connect the negative leads first, starting with the main battery and moving to the secondary.
  12. 12
    Test the system with your multimeter.

If you need visual guidance connecting, hooking up, or setting up a second car battery, consider watching this video.

People Also Ask (FAQs)

Can you run two batteries with one alternator?

It isn't a problem to run two batteries from the same alternator. The alternator is protected from the excessive drain and won't be affected if two batteries are using it.

How many 12-volt batteries can you run in parallel?

There's no technical limit to how many batteries can be connected. However, batteries will not have exactly the same amount of terminal voltage, so for each pair that is connected, one is going to end up charging the other to some degree.

What's better: an extra battery or capacitor?

The capacitor can provide the same amount of power with less space. However, the battery solves electrical issues since it can handle a longer period of power production versus the capacitor.

Can a sound system mess up an alternator?

If you are pulling a lot of voltage, it can put a strain on the alternator. To resolve this condition, simply upgrade to a heavy-duty alternator that can handle the additional juice.

Will two amps drain my battery?

The amplifiers shouldn't drain your car battery as long as the engine is running. However, if you use your stereo with the engine off, it's likely that you are going to kill the battery quickly.

What happens when two different batteries are connected in parallel?

You should never connect different batteries in parallel. If you still choose to do this, the higher-voltage battery is going to discharge into the other, creating equal voltage levels.


Now that you know how to set up a second battery in a car, it's time to take control of your automotive electronic needs. Whether you are running a high-powered stereo system or you need to use accessories in your car while camping, the second battery can be invaluable.