Everything you need to know to install a reliable dual battery system in your 4WD, uteor caravan - from choosing the right components to connecting your first Andersonplug.
This article acts as a guide - always consult a licensed professional Auto Electrician
⏱ 12 min read - Last Updated: 2026
Why Every 4WD Needs a Dual Battery System
If you've ever driven off-grid in Australia - whether you're heading to Cape York, crossing the Nullarbor or camping in the Simpson Desert - you'll know that running a fridge, charging devices, and operating lights off your starter battery is a recipe for disaster. A flat battery 200km from the nearest town is not a situation you want to be in.
A dual battery system solves this completely. It separates your vehicle's starter (cranking) battery from a secondary auxiliary battery that powers all your accessories. The two batteries are managed by a charging device that ensures your starter battery is always protected and your auxiliary battery receives a full, correctly regulated charge from your alternator and solar panels.
In this guide, we'll walk you through every component you need, how to size your system correctly, and how to wire it safely.
What You'll Need - The Core Components
Before you start, assemble all your components. A complete dual battery system consists of:
Auxiliary Battery- AGM, lithium or lead-acid. For serious camping, lithium (LiFePO4) is increasingly popular due to lighter weight and deeper discharge capability.
MPPT Solar Charge Controller- If running solar panels. The Victron Smartsolar MPPT range is the industry standard in Australia for good reason - Bluetooth monitoring, maximum efficiency.
DCDC Battery-to-Battery Charger- Charges the auxiliary from the alternator while driving. Essential for modern vehicles with smart alternators. REDARC and Victron Orion DCDC chargers are the top choices.
Heavy Gauge Twin Core Cable- 4 B&S or 6 B&S twin core is standard for most builds. Cable gauge must match your expected maximum current draw.
Copper Cable Lugs- Crimped compression fittings connecting cable ends to battery terminals, busbars and fuse holders.
Battery Terminal Upgrades- Two Up Unilugs or similar to enable multiple cable connections at the battery post.
Fuse Protection- Midi fuse or MEGA fuse within 300mm of each battery's positive terminal. Non-negotiable for fire safety.
Anderson Plugs- The standard 12V connector in Australia. Used for connecting accessories, solar panels and inter-battery links.
Busbar or Fuse Block- Distributes power to multiple accessories from a single battery connection point.
Step-by-Step Installation Guide
1 -Plan Your Layout
Decide where your auxiliary battery will live - most builds use the engine bay (with an appropriate AGM battery (although worth noting that Lithium is not recommended in the engine bay), under the rear floor, or in a canopy/tray battery box. Measure your cable runs and add 20% for routing. Sketch your circuit before buying anything. Identify where your fuse holders, charge controller and Anderson plug entry points will be located.
2 -Size Your Cable Correctly
This is where most DIY builders make mistakes. Use the cable sizing table below as a guide - the longer the cable run and the higher the current, the heavier the cable gauge you need. For most 4WD builds with runs of 2–4 metres and currents up to 50A, 6 B&S (or 13mm²) twin core is the minimum. For REDARC BCDC or Victron DCDC charger installations, 4 B&S is standard. Under-sizing cable causes voltage drop, heat and fire risk.
3 -Install Battery Terminal Upgrades
Your factory battery terminals only have one lug connection point. You'll need to connect at minimum: the DCDC charger cable, the fuse holder, and possibly solar and accessories. Two Up Unilugs (positive and negative pair) give you multiple clean lug connection points on each terminal post. Install these first, torquing to the battery manufacturer's specification.
4 -Install Fuse Holders Within 300mm of Each Battery
Australian Standards (AS/NZS 3000) require fuse protection within 300mm of each battery's positive terminal. For most dual battery builds, a Midi fuse (30–60 amp rated to your DCDC charger output) on the run between the two batteries is standard. Larger installations may use MEGA fuses. Never run unfused positive cable longer than 300mm.
5 -Crimp and Install Copper Cable Lugs
Cut your cable to length, strip back 12–15mm of insulation on each end, and select the correct lug for your cable size and terminal stud diameter. Use a quality hydraulic crimp tool for a proper cold-weld crimp. A good crimp is smooth, without cracks or flare. Always tin-plated copper lugs for corrosion resistance. Slide heat shrink onto the cable before crimping, then shrink after.
6 -Mount and Connect the DCDC Charger
Mount your REDARC BCDC or Victron Orion DCDC charger in a well-ventilated location, away from heat sources. Connect: positive input from starter battery (via fuse), negative input to chassis earth, positive output to auxiliary battery (via fuse), negative output to auxiliary battery negative. Set charging profile to match your auxiliary battery type (AGM, lithium, etc.).
7 -Install the MPPT Solar Charge Controller
Mount your Victron Smartsolar MPPT in a location accessible for Bluetooth monitoring. Connect solar panels to the PV input first, then connect battery output terminals to your auxiliary battery. Set the charge voltage profile via the VictronConnect app on your phone. Configure battery type, absorption voltage, float voltage and temperature compensation if using a temp sensor.
8 -Install Anderson Plugs and Accessories
Anderson 50A plugs are the standard connector for linking your auxiliary battery system to a fridge, lights, air compressor and other accessories. Terminate each accessory lead with a correctly wired Anderson plug — brown/red to positive, blue/black to negative. Label each plug. Install a flush-mount Anderson bracket for your main accessory entry point.
9 -Test Before Buttoning Up
With everything connected but before securing cable management, test the system. Start the vehicle and confirm DCDC charger is actively charging the auxiliary (check indicator LEDs or voltage with a multimeter). Test each Anderson plug output. Check solar input is registering. Use a Smart Shunt or voltmeter to verify auxiliary battery voltage. Only then secure cable runs with split tube and secure mounting.
Pro Tip -
Always install a battery monitor (like the Victron Smart Shunt 300A or BMV-712) on your auxiliary battery. Voltage alone is a poor indicator of state-of-charge (SOC) - a proper coulomb-counting shunt gives you accurate % charge remaining, time remaining, and historical data via Bluetooth. It's the best $90 you'll spend on your whole system.
Safety Warning -
Always disconnect the negative terminal of both batteries before working on any wiring. Never work on live 12V circuits. Fuse all positive runs within 300mm of the battery. Use the correct gauge cable for the current load.
**It is recommend to always consult a licensed auto electrician**
Cable Sizing Quick Reference
Cable ( B & S / M M ² ) - Max Current ( Continuous )
8 B&S / 8mm²Up to 30A Short accessory runs, small DCDC 8 B&S Twin Core
6 B&S / 13mm²Up to 50A Most DCDC chargers, standard dual battery 6 B&S Twin Core
4 B&S / 19mm²Up to 80A High-current DCDC, canopy boards 4 B&S Twin Core
2 B&S / 25mm²Up to 100A High-output alternators, large lithium setups 2 B&S Cable
0 B&S / 53mm²Up to 160A Winch feeds, large inverters 0 B&S Red/Black Cable
Common Mistakes to Avoid
Under-sizing cable- The most common and most dangerous mistake. Always go one size up if you're between sizes.
Missing fuse protection- Every positive cable leaving a battery terminal needs a fuse within 300mm.
Using a VSR (voltage-sensing relay) with modern vehicles- Smart alternators in Euro 5/6 vehicles require a DCDC charger, not a basic VSR. A VSR will not charge a lithium battery correctly.
Poor crimps- Avoid compression fittings tightened with pliers. Use a proper hydraulic ratchet crimp tool.
No battery monitor- You cannot reliably know your auxiliary battery SOC from voltage alone under load.
Incorrect Anderson plug polarity- Brown/red = positive; blue/black = negative. Reverse polarity can damage or destroy connected accessories.
Frequently Asked Questions
WHAT SIZE SOLAR PANEL DO I NEED?
A 120–200W panel is suitable for most single-fridge camping setups. A 40Ah fridge running 24 hours uses approximately 30–50Ah per day depending on ambient temperature. Match your panel wattage and MPPT controller to comfortably replenish what you use during daylight hours, with margin for overcast days.
CAN I USE A LITHIUM BATTERY WITH MY EXISTING MPPT CONTROLLER?
Most modern Victron Smartsolar MPPT controllers support lithium (LiFePO4) charging profiles, configurable via the VictronConnect app. Check your controller's specifications and set the correct charge profile before connecting a lithium battery. Incorrect charge voltage can damage lithium cells.
DO I NEED AN AUTO ELECTRICIAN?
While dual battery installations are a popular DIY project, always consult a licensed auto electrician if you're unsure at any step. Incorrectly installed 12V systems are a vehicle fire risk. In some states, modifications to vehicle electrical systems may need to be declared to your insurer.
Products Mentioned In This Article
VICTRON Smartsolar MPPT 100/30 - Click to Shop
4 B&S Twin Core Cable (per metre) - Click to Shop
Two Up Unilugs Battery Terminal POS & NEG - Click to Shop
Female Pass-Through Terminal 300A - Click to Shop
Genuine Anderson Plugs Bulk Pack (10) - Click to Shop
Midi Fuse 40amp - Click to Shop
VICTRON Smart Shunt 300A - Click to Shop
REDARC BCDCN1225 Core Charger - Click to Shop
Cheers -
The JTS 12 Volt Team!