12v Dual Battery Setup – What You Need to Know

A 12v dual battery setup in our Fortuner has been on the to-do list for a long time now. For one reason or another, we have just kept pushing it back. The time has finally come to make it happen.

There are a heap of things you need to consider when you plan a 12v dual battery setup in your 4wd.

  • How much power are you going to use?
  • How many days are you going to need power before recharging?
  • Do you need solar? What type of battery do you want?
  • Where are you going to mount it?
  • What accessories do you want to run from it? Etc.

I know it already sounds hard but trust me, it’s not. If you have basic wiring knowledge and can read a simple wiring diagram it’s easy. This project is all about installing a simple and easy dual battery set up that can be DIY to keep costs down.

12v dual battery setup, beach camping

How much power will I use?

  • What do you want to power?
  • How long are you going to need to power it for?
  • How much power will it consume?

This will dictate the size of the battery you are going to need. I don’t want to get too technical, everything you power will have an amp draw and this will deduct off of the amps of the auxiliary battery. To keep it simple, you have a 120 amp/hr battery that can be drawn down to 0% and you want to power a fridge that uses 1 amp/hr, the battery will run the fridge for 120 hrs or 5 days. Keep in mind you shouldn’t draw an AGM battery below 50%.

For us, we want to power a fridge, led camp lights and be able to charge our phones.

Fridge = 1.5 amp x 24hrs = 36 amp per day (this goes up and down depending on how hard the fridge is working to stay cold. Daytime vs night time temperatures as an example)
Phone = 1 amp x 2hrs charge = 2 amp per day
Camp lights = 0.8 amp x 3hrs use = 2.4 amp per day

I know math sucks but this is important if you want cold beer. We estimate 40.4 amp per day. We calculated the amps at a higher consumption to give a worst-case usage.

Theoretically, we should get 1.5 days out of a 120 AH battery if we don’t start the car. Realistically for us, we will never sit still for that long anyway. So a 120 AH battery will suit our needs. And as a safeguard, we installed an iDrive Bluetooth battery monitor to be able to keep an eye on it. We also have a solar input option as a safeguard.

12v dual battery setup Idrive BM2 Bluetooth battery monitor, App snap shot

What Type of battery do I need?

  • What type of battery do you need/want, AGM, lithium etc
  • Where and how are you going to mount it?

Simple, one that will keep the beer cold, right? It’s not quite that easy.

We decided we wanted to mount ours in the back of the Fortuner. This meant we needed a totally sealed battery as it was in the cab. Also, the space was tight so it was going to need to be a slimline battery. I wanted a lithium battery but due to cost, we decided with an AGM. The mounting location really dictated the battery size.

How do I charge it?

  • What type of charger will you need?

What type of alternator your car has will determine this, most modern vehicles have smart alternators so this means you will need a BCDC charger. We decided to go with a Redarc BCDC 1225D for a few reasons.

  • The main one being, it’s Australian made and Redarc’s reputation is second to none
  • The size of the unit is tiny. It can be mounted almost anywhere (even in the engine bay if you wish)
  • It is capable of charging multiple types of batteries, even lithium. So, if you change your battery later it will still be compatible
  • It’s simple and easy to wire and use
  • The D in 1225D stands for dual input. This means the unit is capable of charging the battery off of solar input. So if you do like sitting around you can do it worry-free
  • It has green power priority, meaning if you do hook up solar it will use the solar power as a priority.

12v dual battery setup Redarc BCDC 1225D solar battery charger

What else do I need for a 12v dual battery setup?

There are a few more bits and pieces you are going to need to complete a dual battery project-

  • How and where are you mounting the battery? We did ours in the cab with straps and eyebolts. There are plenty of options just work out what’s best for you and your vehicle.
  • Wiring – We intentionally mounted everything as close as we could to minimise the amount we would need. Don’t skimp on quality or gauge either it will haunt you later!
  • Fuses and connections – for the BCDC Redarc recommends you should use 40A MIDI fuses for the power out and in. You will then need to fuse everything else you want to run from the battery. We decided to use a 6 spot fuse block with a negative busbar to make wiring easy.
  • Accessories – We decided to use STEDI power outlets and holders. 1x cig socket for the fridge, 1x cig socket for other (drone batteries, etc), 2x dual USB outlet – 1 at the front and one at the back. Also we added a couple of STEDI flush mount work lights in the tailgate.

How to wire How to wire a BCDC dual battery

Wiring the Redarc BCDC 1225D will depend on what profile you need for your battery. We needed an A profile as we have an AGM battery. So this is what I am going to walk you through. It is important to note hydraulic crimpers will give the best join for the heavier gauge wires.

Black wire – earth to vehicle. Hydraulic crimp join and solder. It’s important to note the battery and BCDC need to be earthed to the vehicle or the BCDC will not recognise there is a battery to charge.

Red wire – Power from the vehicles start battery to the BCDC, 40A MIDI fuse at the start battery. Hydraulic crimp joins and soldered.

Brown wire –Power from BCDC to auxiliary battery. 40A MIDI fused. Hydraulic crimp joins and soldered it is important to note Redarc recommend this be no longer than 1m in length.

Blue wire – To an ignition source, I tapped into vehicle cig socket at the rear of the Fortuner.

Yellow wire – Solar input, capped and not currently in use.

Green wire – Optional LED indicator to show BCDC working (I didn’t wire this).

Orange wire – Not required for profile A.

The BCDC 1225D is really easy to wire, for our install it was 4 wires.

For power out to the accessories I decided to use a fuse block with a negative busbar to keep everything tidy. I used stick-on Velcro to install it in top of the battery out of the way. All of the STEDI power sockets run back to this with double insulated wire.

The STEDI Rock lights in the tailgate were a little painful to get wired just because of location. The results however are amazing. The amount of light they put out is awesome and it is the perfect spread for lighting up the fridge and kitchen.

12v dual battery setup Redarc BCDC 1225D battery charger, fuse block, idrive BM2, 120ah AGM slim battery


Total cost $1440

This is just a guide, you can definitely do a 12v dual battery setup cheaper. I will say I wouldn’t skimp on quality on this project. In my experience cheap electrical gear always comes back to bite you. And can you really put a price on cold beer? I think not.

The end result of our 12v dual battery setup

While the cost was a little more expensive than I wanted, the 12v dual battery setup is perfect.  It was definitely an easy DIY dual battery project that anyone could do. The beer is cold, the devices are charged and I have light when I cook.

As for battery longevity, maybe one day I’ll stay in the same place long enough to be able to let you all know.

It is important to note: I am not a qualified auto electrician. If you need advice please seek professional assistance.

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