Basic Dual Battery System

The modern 4WD tourer is required to perform duties far beyond it's design and the additional power demands created by fridges, caravans, lighting and entertainment systems may mean a serious upgrade to the vehicle's electrical system is in order. Running a fridge from the standard electrical circuit via the cigarette lighter socket may be fine for the occasional weekend away but if you're 'going bush' then it's time to think about a dual battery system.

The aim of a dual or auxiliary battery system is twofold. The absolute priority is to protect the vehicles starting (cranking) battery from being flattened and thus rendering the car unable to be started. The second aim is to provide an auxiliary 12 volt supply - to power all those accessories that make life in camp so much easier.

The Basics

Auto electrics is a dark science and we're not going to get overly technical here, just a basic outline of some principles, good practise and proven techniques.

The addition of a 12 volt refrigerator is the usual catalyst for the installation of a dual battery system, whether it's sitting in the back of the 4WD or being towed in the caravan behind.

When we add a dual battery system to a vehicle we try and maintain two distinct systems. The primary system contains the starting (cranking) battery and the alternator (which provides battery recharging and will eventually provide charging to both batteries) and all the electronics required to keep the car operational.

Onto to this primary electrical system we are happy to add things like GPS navigation systems, phone chargers, interior lighting and in car entertainment systems. Things that can only operate while the engine is running and the starting battery is being charged.

On the secondary or auxiliary battery we run items such as fridge/freezers, camp lighting and auxiliary power sockets to run radios, computers and inverters.

When in camp we make a distinct point of having no accessories that draw power from the starting battery. The key is turned off and any auxiliary 12 volt power is drawn from the secondary battery.

Everytime we turn the ignition key we want to know the starting battery is fully charged and we can drive away from camp. We do not want to be stranded with a flat battery because we were trying to keep a bag of peas and a lamb chop frozen.

dual battery wiring

Batteries aint Batteries.

Don't scrimp on batteries, cables, connectors and terminations. There are no 12 volt shortcuts and if you think you've discovered one it will let you down and you will buy twice.

We like to use big, 'Deep Cycle' batteries as the auxiliary power source. No battery likes being repeatedly discharged and recharged and they don't enjoy being left in an uncharged state. Deep Cycle batteries handle the constant discharge/recharge cycling far better than the standard engine cranking battery. The trade-off is that Deep Cycle batteries take longer to recharge and don't have the same levels of cranking amps ('grunt') to turn over the engine of a car for long periods.

Modern automotive batteries are constructed from various materials and techniques. The most common are lead-acid batteries which arrive in 'flooded cell' configuration (pull off the caps and top up the water) and 'sealed' configuration (maintenance free). Alternative technologies include AGM Batteries (Absorbed Glass Matt) and GEL Batteries (Gelled Electrolyte Lead-Acid). Expect to pay upwards of $220 for a traditional lead acid battery and far beyond that for the longer lasting AGM and GEL batteries.

Expect a touring lifespan of between eighteen months and two years for a reasonable quality lead-acid 12 volt battery. Lucky owners get three years. The enemies of battery longevity are cycling below the manufacturers recommendation, heat and vibration - constant threats on a touring 4WD. Drain the battery below 50% of it's rated capacity and you reduce it's life. Stick it next to the turbocharger and you reduce it's life. Let it bash around over corrugated roads and the cells break down, creating internal shorts, which reduces the lifespan.

The key - buy large capacity, quality batteries and use and maintain them as per manufacturers recommendation. Secure them so they never move and keep them away from extreme heat sources. Batteries release explosive gases. Ventilate battery enclosures as directed by the manufacturer.


Fuses are the safety valve in a 12 volt dual battery system. Automotive electrical systems carry much larger voltages with much greater current draw than you would assume. Electric and automotive cables can wear through the protective outer casing. Alternately, water is a conductor and can act as a cable, passing electricity between components. When cables wear through and contact the metal of the vehicle or water unites the components of a cars electrical circuit, then a 'short' or short circuit is created and electricity can pass through the system unrestricted. This unhindered flow of current heats up cables and electrical components to the point of overheating, fire or explosion (in the case of batteries).

Fuses are deliberate weak points in the circuit and they give up long before big thick cables and electrical connections do. They break the flow of current and end the malfunction. Fuses are vital. When a fuse blows it is telling you that there is a fault in the electrical circuit. Electrical faults don't fix themselves.

Unlike cables where 'bigger is better', fuses need to be the appropriate size for the job at hand. Install an oversized fuse and you run the risk of burning out the circuit.

Cables, Cables, Cables

Think of 12 volt automative cabling as a pipe that allows water to flow to a tap. Use a larger diameter pipe and more water becomes available at the tap. Electricity works in much the same way. The larger the cable (diameter/thickness) the more electricity arrives at the electrical device. We want all the voltage we can get to make electrical devices run as designed. In a further twist, the further electricity has to run along a cable, the less arrives at the intended destination. Automotive batteries actually have a full charge of 12.6 volts (12 volts is simply more convenient terminology). By the time the voltages works it's way down a ten metre cable to, say a car fridge, it may only deliver 12.2 volts. This is called 'voltage drop'. The larger the cable - the lower the voltage drop, meaning more voltage is available to the accessory.

Correct cable size means we get as much possible voltage arriving at our electrical device, which equates to safer use (no overheated cables creating shorts), better operation of our electrical devices and faster charging of our auxiliary battery.

Other factors affect voltage and voltage drop, including temperature and cable construction but this is intended as a very basic primer and the theory and math involved in automotive electrics can be overwhelming. One note on cable construction: use well insulated (the plastic outer casing) cable made from multi-strand copper. Multi-strand cable is more flexible and more forgiving than solid core cable and bends and flexes with less chance of breakage.

More about cable sizing in the wiring section below.

Terminations and Connections

Terminations are how we 'end' a run of cable. Connections are how we join two terminations. Terminations and connections are potential losses in an electrical circuit. Avoid electrical connections wherever possible, using full length cable runs where you can. A poor connection acts like a cable that is too small for the intended task and voltage is lost through the circuit. Use the best connectors you can afford with the best terminating techniques. In automotive applications we prefer crimped connectors (crimping is the squashing of a terminal around a cable) rather than soldered connectors. Soldering creates rigid joins that tend to break rather than flex.

Battery Isolators

The smart side of charging and managing dual battery systems is handled by a battery isolator. The alternator of the vehicle creates the electricity required to keep the battery(ies) charged. In simple terms an isolator allows the charging of the starting battery to an acceptable voltage level and then opens up the electrical circuit so that the auxiliary battery can be charged.

In it's most basic form it is a switch that requires manual engagement by the operator. The driver starts the car and makes a decision to throw the switch over to charge the second battery. Hopefully his/her judgement is good enough to take into account the condition of the battery, whether or not the headlights/aircon/DVD are turned on and exactly how much charging the starting battery requires. Systems like this are all a bit hit and miss and frequently lead to failure, usually with the operator forgetting to throw the switch over to the auxiliary battery resulting in a hot fridge.

We like using a VSR (Voltage Sensitive Relay). These things are basically solenoids (with a small electronic 'brain') which, are connected and act like a switch or 'gate' between the the starting and auxiliary batteries. The VSR delivers charging current to the starting battery as a priority. When the starting battery has recovered its charge the VSR opens (like a gate) and delivers charging current to both batteries.

When you turn off the car engine, electrical devices like your 4WD fridge continue to draw power from both batteries. Once the VSR senses that the voltage in your starting battery has dropped to 12.5 volts (or thereabouts) it opens the 'gate' and isolates the auxiliary battery from the starting battery. The fridge continues to draw power from the second battery but your cranking battery power is preserved so you can start your vehicle in the morning. It's a clever system, reasonably cheap and most importantly it's reliable and virtually foolproof.

Earthing and Grounding

Electrical circuits require a positive(+) and negative(-) connection to work.

It's called an electrical 'circuit' because it's just that - a circuit. Current is drawn from the positive terminal of the battery and through the electrical device, whether it's a fridge, light globe or 12/240 volt inverter. Power must pass through the device. In many instances the circuit relies on the metal in the vehicle to carry the voltage back to the negative terminal of the battery. The circuit uses the car as a cable. All that is required is that a direct, clean metal path is made through the chassis/frame of the car. While it is possible to connect the negative terminal of the auxiliary battery to the chassis of the car and have the current find it's way back to the total circuit, it's preferable to run a negative cable of the same length back to the negative terminal of the starting battery. Using big, chunky cable on the positive terminal is pointless if we scrimp on the negative side. If you use 13.5mm² cable on the positive side then use 13.5mm² cable on the negative side.

Connecting It All Together

We're assuming your auxiliary battery is securely mounted with appropriate ventilation and protected from extremely hot engine components (like exhausts and turbochargers). Ideally it will be mounted close to the starting battery to enable the shortest run of cable possible to reduce voltage drop.

Mount the battery isolator in the same manner as the auxiliary battery - firmly and securely and away from hot engine components.

Determine the correct size of cable from the chart below. We've taken these figures from RedArc's literature (Australian manufacturers of battery isolators).

Important: All cable sizes referred to here are based on AWG (American Wire Gauge) or mm² (actual area of the wire cross-section). Many automotive cables are labelled misleadingly or without following relevant standards. Some cable manufacturers include the outer insulating layer in their size calculations. When dealing with wiring make sure you have ascertained the correct cable size for the job and that you are actually using the correct size in copper wire and not total cable size.

Notes on using the Cable Sizing Chart:

  • Cable length is for the TOTAL length of positive plus negative cable.
  • If in doubt always opt for the larger size cable.
  • Regardless of the math, we would never use 7.71mm² battery cable in a dual battery system. Use 13.5mm² as an absolute minimum.
  • The Century N70T Deep Cycle battery is rated at 100 amps. When looking at amp ratings don't get confused with CCA or 'cold cranking amps' which describe the amount of starting power the battery can deliver.
  • Figures below are based on a cable temperature (actual copper) of 36°C. Current flow will increase cable temperature and increase resistance which may result in the need for a larger capacity cable.
  • Chart assumes an average voltage drop of 0.36v or 3%.

  1 metre 2 metres 3 metres 4 metres 5 metres 6 metres 7 metres 8 metres 9 metres 10 metres
40 amps 7.71mm² 7.71mm² 7.71mm² 7.71mm² 13.5mm² 13.5mm² 13.5mm² 20.26mm² 20.26mm² 20.26mm²
50 amps 7.71mm² 7.71mm² 7.71mm² 13.5mm² 13.5mm² 20.26mm² 20.26mm² 20.26mm² 26.45mm² 26.45mm²
100 amps 7.71mm² 13.5mm² 20.26mm² 20.26mm² 32.07mm² 32.07mm² 39.55mm² 39.55mm² 49mm² 49mm²
150 amps 7.71mm² 20.26mm² 26.45mm² 39.55mm² 39.55mm² 49mm² 64.15mm² 64.15mm² 83.19mm² 83.19mm²
200 amps 13.5mm² 20.26mm² 32.07mm² 39.55mm² 49mm² 64.15mm² 83.19mm² 83.19mm² 99.27mm² 99.27mm²

Converting Cross Sectional Millimetres (mm²) to AWG (American Wire Guuge) and B&S (Brown and Sharp) - as per Tycab Australia's nearest conversion

METRIC 7.71mm² 13.5mm² 20.26mm² 26.45mm² 32.07mm² 39.55mm² 49mm² 64.15mm² 83.19mm² 99.27mm²
B&S 8 B&S 6 B&S 4 B&S 3 B&S 2 B&S 1 B&S 0 B&S 2/0 B&S 3/0 B&S 4/0 B&S
AWG 8 AWG 6 AWG 4 AWG 3 AWG 2 AWG 1 AWG 0 AWG 2/0 AWG 3/0 AWG 4/0 AWG

Considerations When Wiring Up Your Dual Battery System
  • Use a minimum of 13.5mm² (6B&S) cable to connect the batteries to the isolator.
  • Cables should be run with minimal bends and turns. Care should be taken to avoid hot exhaust and intake components. Increased heat in cables equals voltage drop or burning through the casing.
  • Care must be taken to avoid moving parts such as suspension components and steering columns.
  • Wherever a cable must run over sharp metallic corners be conscious to tie it in such a way as to prevent rubbing. Allow a little slack in the cable run. It's not a guitar string. Cabs, chassis's, tray backs and doors move independently of each other and allowances must be made for flexing.
  • Wherever cables run through metal panels such as doors frames and firewalls, use a rubber grommet to prevent rubbing between the cab and the steel. Even if there is no movement, vibration will ensure that the cable frays against the frame causing the circuit to fail.
  • Secure terminated cables to battery and isolator terminals using a nut, standard washer and a spring washer. Spring washers create friction at the connection preventing cables from coming loose.
  • Use 100Amp fuses at both the starting and auxiliary batteries. Position them at the positive(+) terminals, close to each respective battery. These fuses area designed to protect these big, current-carrying cables in the event they wear through and make contact with the car, creating a short and possible fire.

Manufacturers of Battery Isolators may have specific requirements for their equipment. The points made above are a guide only and should be read in conjunction with manufacturers instructions.

Interested in car fridges? We take a look at portable refrigeration in - Car and 4WD Fridges.

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