Dual Battery System: A Beginner’s Guide

The ultimate guide to dual battery systems

If you need your rig to power gear such as a portable fridge, safety lights, or radios while your fourby is parked, you’re going to need a dual battery system.

Don’t run the risk of losing power to your gear, and ensure your vehicle is always ready to start up with the installation of a 4WD dual battery system.

What is a Dual Battery System?

A dual battery system is a vehicle battery system that uses a secondary battery in addition to a vehicle’s starter battery. This secondary battery is used as a power source to auxiliary gear and accessories.

How Does Dual Battery System Work?

In a dual battery system, both batteries function as two isolated systems. While your vehicle’s engine is running, your starter battery works with your alternator to power your vehicle and its electronics.

While your vehicle is turned off, your secondary battery powers all gear and accessories attached to your vehicle; meaning that you can run your portable fridge, lights, and inverters without having to keep your engine running. 

This is achieved thanks to the function of an isolator that disconnects your starter battery from your secondary battery, meaning that you only draw power from one battery at a time.

Keeping each battery functioning independent of each other is crucial in ensuring that your starter battery does not become drained and prevent your fourby from starting up. The battery isolator will also ensure that both batteries charge properly. 

Be aware that some 4×4 dual battery isolators can allow your starter and secondary batteries to work together, boosting starting power to your vehicle should the starting battery suddenly drop in voltage. This is not the case with all isolators, however.

Your auxiliary battery will most likely be a deep cycle battery. This is because deep cycle batteries are built to be repeatedly discharged and recharged without being damaged or shortening their lifespan.

By using a deep cycle battery to power your gear you can rest easy knowing you’ll have a consistent power supply and won’t risk damaging your battery by running it dry. 

The Pros and Cons of Installing a Dual Battery System

If you’re unsure if a dual battery system is right for you, consider these pros and cons.


  • Allows your rig to power a fridge, radio, lights, inverters, and whatever else you need while you have the ignition off. 
  • No need for concern in draining your starter battery.
  • Increases power available if you need to use your winch.
  • Able to charge your secondary battery with solar panels.
  • Peace of mind in case of primary battery failure.


  • Added weight to your rig due to an extra battery box.
  • The potential cost might be too high if you’re on a tight budget.

What is the Best Dual Battery System for Me?

No two rigs are the same, nor are two 4WD enthusiasts. Thankfully, the experts at Total 4×4 have over 30 years of experience and can help you pick the best dual battery system for your 4WD. 

We highly recommend gear made by TJM and REDARC. Both Australian born and bred, these manufacturers provide all the electronic components needed for a dual battery system setup. The REDARC Battery Isolator is a smart isolator that is an easy recommendation for those of you thinking of installing your first dual battery system. 

Whether you’re upgrading your dual battery system or installing one for the first time, TJM and REDARC are excellent choices and leaders in the market with good dual battery systems reviews from 4WD owners.

Once you have the rest of your battery kit, it’s time to choose a secondary battery.

Types of auxiliary batteries

Lead Deep Cycle

Suitable under the bonnet, or in an open tray or tub. Cheap, but are known to suffer from a reduced lifespan if not fully discharged and recharged each use. 


The popular choice among any rig, AGM batteries are suitable anywhere as they do not let out any hazardous gases while charging. This type of battery is better suited to partial use and recharge, rather than fully discharging and recharging in a use cycle.


Boasting reduced weight and increased capacity, lithium batteries are the most durable battery when used in a partial use cycle. However, due to heat restrictions these cannot be fitted under the bonnet of a vehicle.

Installation Basics

If you’re considering installing your dual battery system yourself, have a look at our brief step-by-step guide to see if you’re up to the task. 

  1. Mount the second battery on your rig, ensuring you have enough wire to make the necessary connections to your second battery.
  2. Disconnect your starter battery.
  3. Mount the Voltage Sensitive Relay (VSR). Place the VSR close to the main battery and away from any hot or moving parts. 
  4. Cut cables to length.
  5. Fit lugs to the bare ends of cable by stripping away 15mm of insulation and crimping them in place. Slide the heat shrink over the lug and apply heat to provide a waterproof seal.
  6. Use the black wire attached to the VSR to earth the relay.
  7. Connect the main battery to the VSR.
  8. Earth your second battery.
  9. Attach the VSR to the mounting plate. Positive cables may not be attached to both batteries.
  10. Attach the negative leads, first to the starter battery and then to the second battery.
  11. Test your system is working with a multimeter.

At Total 4×4 we offer a DIY 4×4 Dual Battery System Kit to give you everything you need to get started including a comprehensive installation guide.

How to Keep Your System Running Smoothly

Like any component in your vehicle, your 4×4 dual battery system will suffer from wear and tear, especially if you’re heading off-road and facing the elements.

To keep your charging system running smoothly, regular maintenance is required. 

Before you head out, check your system for any damage to the battery, DC charger, wires, and battery tray. Keeping your system in good condition is the best way to keep things running smoothly.

If you want to know more about the best dual battery systems or dual battery system prices, don’t hesitate to get in touch with the experts at Total 4×4 to see how we can improve your rig today.

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