How To Set Up 12V Dual Batteries In A 24 Volt Light Truck

The Truck Has 24V But I Want A 12V Second Battery System. Ummm…

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What do you do when your truck runs a 24 volt system and you want to install dual batteries at 12 volts? Well, it depends on what you want to run off your second battery.

Dual Batteries Isuzu NPS And Wedgetail Camper — Our Isuzu NPS has a 24V system. This makes a dual battery more complicated than normal.

And it depends if you have mostly 12V or 24V appliances. For example, most car fridges operate on 12V and 24V. Some compressors can be specified at 12V or 24V. LED lights will happily run on 12V and 24V. But the majority of other camping gear is 12V.

There are an infinite number of solutions. It all depends on what you want to achieve. How about I show you what I did, then you can have a go at designing your own system.

Terminology

Let’s clear up a few terms before we go any further.

The term “dual batteries” is used interchangeably with:
- Second battery,
- Auxiliary battery,
- House battery,
- Dual battery system,
- Second battery system,
- and combinations of the above terms.
“Common”, “ground”, “earth” all mean the same thing.
“24 volt”, “24V”, “24VDC” all mean the same thing in this article.

First Things First

Every system is different. What do I mean by this? Well for example, we have a slide-on Wedgetail camper. Sometimes it will be on the truck, sometimes it won’t be.

The Wedgetail has its own standalone power system but is configured to accept a power supply from the truck when mounted on the truck. We might as well utilise this, so we need to add a power source from the truck.

The truck will have a fridge/freezer in the rear seat area, an inverter and will need to power an air compressor. The Wedgetail won’t always be on the truck, so the truck needs its own standalone dual battery system.

It’s getting complicated…

Regardless of your set-up, two simple questions will get you started.

1. Usage – Power Out Of System

The first question you need to ask is,”What appliances do I want to run?” This will give you a great starting point. This is what we needed:

Wedgetail Camper

Has a REDARC Battery Management System (BMS). It can accept 240VAC mains power, solar power and 9 – 32 volts DC.
Has a 200W solar panel and a 240VAC mains inlet. It’s only missing a DC supply from the truck.
The BMS deals with the 3 incoming power sources and maintains optimal charge on a 100Ah (amp-hour) lithium battery.
So we need to supply 24V to the Wedgetail’s BMS.
The BMS draws up to 50A (amps).

Truck Cab

We are going to fit out the rear seat area with a freezer, some USB outlets, LED lights and a 350W (watt) inverter.
Fridge draws 7A.
Inverter draws up to 44A.
USB and LEDs draw about 5A.
So we need to supply 12V to the truck cab.
Total amperage is 56A. Let’s round it up to 60A.

Truck – Other Items

We need a large compressor for tyre inflation and maybe some air tools. We chose an ARB Twin Compressor. We also want a portable water pump for refilling our tanks and LED lights in the tray’s storage boxes.
Compressor draws up to 60A.
Pump draws up to 7.5A.
LEDs draw about 2A.
So we need to supply 12V to these other items.
It’s very unlikely compressor and pump will operate at the same time, so let’s call total amperage at 60A.

2. Supply – Power Into System

The next question is, “How do I want to power these appliances?” In our case we have:

Wedgetail Camper

It’s covered off above.

Truck

Will have approximately 150W solar panel, 240VAC mains supply and 24VDC input from truck battery.
Will have a 100Ah gel battery (existing battery from our old 4WD).

So we need to provide all 3 sources of power.

Let’s Get Our Heads Around This

A simple flow chart should make sense of what we’ve discovered so far. We’ve put all the missing bits into one flow chart. Now we’re getting somewhere – we can now see what we need to do.