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Why you need an in-vehicle battery charger

There are endless ways to customise the setup in your vehicle to ensure it will perform how you want it to and where you want it to. And just like any purchase you make for your touring vehicle, it is important to get the right gear that will allow you to do what you want to do, as well as support the additional electrical accessories you want to run from your vehicle or RV.

This is where a dual or auxiliary battery system can help provide extra capacity for running these items and ensure the vehicle will still start when you are ready to move on.

Things have changed so much in the last decade when it comes to vehicle technology, battery technology, and dual battery options, that it is no longer just a case of whacking a second battery and isolator in and heading off on your adventure.

In this guide to vehicle battery chargers, we’ll run through basic considerations for your vehicle set-up as well as the types of battery charging technology available today.

What is a Dual Battery System? 

dual battery system, allows you to reliably run electrical equipment once the vehicle is shut down and ensure that the vehicle can still be started again afterwards. This allows for the secondary battery to run electrical equipment and accessories and ensure the standard battery is used for its intended purpose; starting the vehicle.

Power is stored in the auxiliary battery for the electrical items you want to use when the engine is not running, providing extra capacity for an extended run time of equipment. Essentially, any devices you intend to run only while the engine is running should be connected to the start battery, and anything you want to keep running when the engine is not should be connected to the auxiliary battery.


There are three main things to consider before you even get to the shopfront that will make your selections clearer cut for your dual battery set-up.

  1. Type of vehicle and alternator
  2. Accessories to be run from the electrical system
  3. Desired length of time spent off-grid

Type of vehicle and alternator

Most vehicles that predate 2010, have fixed voltage alternators. The alternator is the power supply for your vehicle when the engine is running and provide power to vehicle electrical consumables while running such as lights, stereo, and clocks. A fixed voltage alternator has a high enough voltage to successfully charge a secondary battery in the vehicle to a usable level for leisure or auxiliary use.

Fixed voltage alternators are becoming less common on new vehicles as reduced fuel consumption targets and more stringent emissions standards are adopted by manufacturers.

Nowadays, smart alternators are used in modern vehicles with the aim of increasing fuel efficiencies and reducing engine emissions.

The smart alternator system allows the vehicle to control the output voltage from the alternator based on vehicle operating conditions to reduce electrical load and in turn the mechanical load on the engine by the alternator.

The smart alternator does not consider the secondary batteries state of charge, chemistry type, or location in the vehicle, which renders it unsuccessful at charging a secondary battery system to a usable level.

However, the alternator has never been designed to be a designated battery charger, so even fixed voltage alternator secondary battery charging methods can be improved.

In addition, most four-wheel drives, around 2010 and earlier, have enough room under the hood for an auxiliary battery system. For vehicles made after 2010, other sections of the vehicle can be used for mounting the auxiliary battery or batteries when there is not enough room under the hood or you require extra battery capacity. For example, this could be inside the wagon area of a vehicle, or in a ute tub or tray, or even on the vehicle chassis under the body.

Electrical accessories

When considering what you want to run from your auxiliary battery system, it is as much about being efficient with power as it is storing or generating lots of power. With advances in fridge and lighting technology there’s no need to have excessively power-hungry devices just to keep the beer cold and the camp site lit. 

More efficient loads can drastically reduce your power consumption, meaning less weight in batteries you need to carry, and less money spent on solar panels to try and extend your stay. The main thing is to write down everything you wish to run and see if it is achievable with the size battery system you can fit.

To work out your power consumption:

  1. Write down the consumption of each of the loads in amps
  2. Multiply each load by the length of time you want it to run in hours, commonly in a 24 hour period so you know how much you will use each day. This gives you the Ah consumption of each device
  3. Add all of your Ah results together


  • Fridge – Draws 5A and the compressor will run for 8 hours over a 24hr period on average (most fridges actually give you an average Ah draw in their specs) = 40Ah
  • LED Lights – Draws 3A and they run for 4 hours each night = 12Ah
  • Phone Charger – Draws 1A and runs for 3 hours each day = 3Ah
  • Camera Charger – Draws 1A and runs for 4 hours each day = 4Ah

In this example, the total loads consume 59Ah per day. Then double this consumption to ensure the battery is only discharged to 50%, extending its life and ensuring you have some power in reserve should you need it. In this example, you will need a 120Ah battery which in most four-wheel drives and campers is easily achievable.

Length of free-camping

The next challenge is to replace this power in the battery, ready for the next day. That is where going for a drive, or breaking out the solar panel comes in. If the dual battery system is set up right, a good few hours’ drive should replace most, if not all, of this consumed power. Either that, or a quality 150W solar panel will likely recover the same amount of power over the period of a nice sunny day.

Whichever vehicle you drive, whether you tow a trailer or pitch a tent, it pays to have invested in the right gear for your touring setup. It will mean your trips are more enjoyable when everything performs as you expect it to.

In the next blog, we’ll cover the different types of dual battery charger technology available and what will be best suited for your needs.

Take the REDARC Hilux product tour to find out what products are best for your overlanding, power management and towing needs.