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Vehicle Dual Battery System

Battery useage indication

Most people look at installing a dual-battery system either because they want to run winches, portable refrigeration, lighting and a mobile telephone charger while camping and travelling in remote areas. 

Vehicle batteries are charged by the vehicle’s alternator whilst the engine is running so when the engine is shut down the charging of the battery ceases.  Running portable 12V appliances will draw power and reduce the power supply stored in the battery.  This often results in flattening the starting battery which can then leave you stranded.  Installation of a dual-battery system will help you overcome this.


Electric winches can drain significant amps from the vehicle battery quicker than an alternator can replace them so a dual-battery system can be set up so you can run the winch off both batteries.

Firstly, you should consider what 12V electrical items you expect to run from the dual-battery system and calculate how much power source (Watt hours) is required to run all the items and how long you want to run the items between recharges.  It is recommended you then factor in about a 25-30% safety margin.  This calculation will assist in deciding the amount of amp hour battery you need.

Power Supply Calculation

What parts are there to dual-battery system?

IBS Dual Battery SystemA dual-battery system consists of the two batteries with one termed as the ‘primary battery’ and the other the ‘auxiliary battery’, battery cradles, wiring for the discharging/charging system which integrates the vehicle’s electrical system and a battery isolator.  An optional visual read-out device can also be placed in the cab of the vehicle for you to check on the battery charging or battery power storage level.

Some larger 4WD vehicles come with two starter batteries that are electronically paralleled for starting the vehicle and this should not be confused with a dual-battery system.  Starter batteries can have CCA (Cold Cranking Amps) marked on them and are called ‘shallow cycle’ batteries.

The size of the cable linking the batteries in a dual battery system should be at least 13.5mm square.

Batteries and what is the best battery for a dual-battery system?

There are two main types of batteries.   When you buy your vehicle from the manufacturer it will likely have a ‘shallow cycle’ single high-current discharge (HCD) battery.  These HCD batteries or cranking batteries provide a brief high amp charge to start the engine. 


Deep-cycle batteries are preferred as the ‘auxiliary’ battery for a dual-battery system as they provide sustained power over long periods of time and are designed for repeat charge and discharge.  These deep-cycle batteries can be larger in size so the one you purchase will obviously need to fit somewhere whether it is in the vehicle engine bay or behind the seat in the vehicle cab.

Auxiliary Vehicle BatteryBatteries come in various sizes and weights.  The number of internal lead plates can vary in number ie: 7, 9, 13, 15, 17 & 19 and thickness so the amp hour capacity also varies.  Battery terminals can also range from standard, standard/wingnut to dual fit.  Deep cell batteries also cost more.

Starting batteries have thinner plates and will accept a greater amount of charge more quickly. Deep cycle batteries have thicker plates so they only accept a small amount of faster charging before the resistance of the plates begin to reduce the amount of charge uptake. So, different batteries reach full charge at different voltages and with a dual battery system you need to consider if your vehicle alternator and battery isolator are capable of charging both batteries.

There are wet cell and dry cell batteries.  Wet cell batteries give off hydrogen gas which is an explosive vapour so you would not want to place this type of battery behind the seat inside a vehicle cab or near inverters, battery chargers or any source of a likely spark.  Care should be taken when handling lead-acid batteries as any acid spill can cause serious burns.

Flooded Batteries

These types of batteries also known as ‘wet cell’ as the lead plates are suspended in wet acid and there are caps on the battery for you to check the electrolyte levels which require periodic topping up.  Wet cell batteries should be housed in a well vented area and kept upright.

Gel-Cell Batteries

Gel-cell batteries are called ‘gel’ batteries as the electrolyte is held in a jelly-like state and the batteries are sealed and spill proof.  A three stage smart charger is required to charge these gel electrolyte batteries.  Gel-cell batteries hold their charge longer than ‘wet cell’ batteries but will suffer permanent damage if over charged.  A rest period will be necessary after rapid charging to make sure the true state of charge is known.

Absorbed Glass Matt (AGM) batteries so far have been considered to be the best or superior for 4WD or RV use especially where the main source of charge is from the vehicle’s alternator as they are able to be recharged faster.  AGM batteries have a positive and negative plate separated by a glass matt and the acid is also a gel.  Reports state AGM batteries are less sensitive to charge rates than the gel-cell batteries.  AGM batteries are sealed, spill proof and maintenance-free. 

For example, ‘Full River’ have a AGM ‘HGL Series’ which is hybrid type battery best for use when charging from the alternator only.  These batteries have thicker plates and more active internal materials so they are easier to charge when flat.

Spiral batteries

Spiral batteries are a type of AGM battery with the lead plates tightly compressed into spiral wound cells.  Spiral batteries are smaller in size, lighter, sealed and maintenance-free.  Spiral batteries are reported to have a high cranking performance, a longer shelf life (when not in use), a higher charge acceptance rate during recharging and more robust to withstand vibrations.  Some spiral batteries can be installed in any position making for more flexible installation.

There is also a battery called Allrounder which is built for starting and deep-cycle requirements.

Good sturdy battery trays or battery cradles are required to secure the battery in the vehicle.  Battery tray joins should be MIG welded and those that are powdercoated will resist abrasion and corrosion.

Battery differences - Standard and Deep CycleBattery life will be extended if you do not run the battery down more than 50% of their capacity.   

It would also be a good idea to know how many hours it will take for your vehicle’s alternator to charge the auxiliary battery.  A standard vehicle alternator is not designed to recharge a deeply discharged battery so it could take up to 5-6 hours to charge an 85 Ah deep cycle.  The vehicle’s alternator can be modified with a specialised charging alternator and a smart multi stage regulator.

A few brand names for batteries are Century Batteries, Trojan Batteries, Exide Endurance Deep Cycle batteries, Optima Batteries, Federal Batteries Deep Cycle Series and Concorde Lifeline AGM deep cycle batteries.

Isolators and what they do

Battery isolators do what the name suggests.  They ‘isolate’ the battery used to start the vehicle leaving it to maintain its charge after you have turned off the vehicle’s engine. 

When you start a vehicle’s engine there is a surge of power to the starter then the alternator starts producing current.  The isolator responds by firstly providing power to the primary battery until it is fully recharged then when the primary battery is full the isolator switches the current over to charge the auxiliary battery.  12V electrical items are then run off the auxiliary battery.

Batteries can be isolated by a manual switch, a fuse or an automatic switch.  With manual switches to turn the isolator on and off, human nature is that you will forget to do so at one time or another.  So that leaves you with a battery management system or automatic system.  Automatic battery management systems link batteries for charging and isolate batteries for discharging.

To avoid any damage to a vehicle’s on board computer or electronic fuel injection (EFI) system select an isolator that has in built spike/surge protection.

The manufacturer of dual battery isolator Smart Start, Redarc Electronics, recommend fuses be fitted in the cable close to the positive terminals of each battery for safety and to reduce the risk of fire.  Redarc website states:-

‘Fitting a fuse to one battery will protect that battery but will not protect the other one and will not prevent the fire risk.  A fuse should also be fitted in the line to the 12V equipment to protect the auxiliary battery in the event of a short on that line.  Please note that if you are wiring the vehicle with an emergency jump start switch via the blue override wire from our Smart Start then the fuses or circuit breakers must be suitably rated. This is due to the high starting currents experienced, which could result in blown fuses.  In this case battery starter cables must be installed between the start battery and the auxiliary battery. The same risks apply as described above therefore if suitable fuses are not obtainable please ensure the cables are well protected’.

Diode isolator

With a diode isolator each battery circuit is completely isolated from the other as diode isolators act as a one-way valve between the batteries.  Current flow is prevented from flowing from one battery to the other so each battery is an independent power source.  These are not used much these days but can be found on older vehicles.

Solenoid isolators

For a good basic ‘parallel’ charging dual battery system it is recommended both batteries be identical (ie: age, size, capacity and design). The batteries are set up to be charged in parallel and once the ignition is turned on the solenoid is automatically switched on and this allows the power to flow between the batteries.  Once you turn off the ignition the solenoid opens thus isolating the main battery.

Then there is Smart solenoids which are essentially a smarter version of continuous duty solenoids.  Smart solenoids have a voltage sensor that allows automatic operation so parallel charging does not commence charging the auxilliary battery until the main battery has reached a predetermined voltage – around 13.5V. 

Once the main battery has reached that state of charge both batteries can equalise.  The batteries remain in parallel, after you turn off the ignition, until the starting battery draws down to a predetermined voltage (around 12.8V) and then the solenoid contact opens and auxilliary battery is disconnected.  This action prevents flattening of the starting battery.

Voltage Sensing Relays (VSRs) are not a ‘parallel’ charging system as such but they operate similar to a smart solenoid via a relay (and not a solenoid) with the two batteries electrically separated. The auxilliary battery is disconnected from charging if the main battery voltage drops below a predetermined level. On the downside, the capacity for a large current draw can tend to be limited given their lighter construction.    

Electronic isolators are the best of all dual battery charging systems. With electronic isolators you can use dissimilar batteries as the electronics are specifically designed to recognise the difference and adjust the rate of charge however, the main battery is always given charge priority.  

There are dual battery specialists who advocate the electronic isolators as, in their opinion, basic parallel charging systems can ‘mask’ the fact that if the main battery is flat or damaged the auxilliary battery will do most of the starting/cranking then when the charge in the auxilliary battery is depleated the vehicle will not start as both batteries are in a discharged state.

If you are a keen 4WD enthusiast make sure the system you select is one that has dust and water resistance features.

Installation of a dual battery system

Installation of a dual battery system is best done by an expert.  It requires connecting into the vehicle’s electrical system and this must take into account the vehicle’s auto electrics.  A certain amount of frustration can also be experienced in anchoring the sturdy battery cradle to accommodate the weight of the deep cycle battery and finding space for it in the vehicle engine bay.  Care also needs to be taken with the installation of the isolator so it will not vibrate too much or melt near exhaust heat.  The type of wiring and cables for a dual-battery system need careful consideration to ensure a high charging rate to the auxiliary battery and to avoid voltage drop.

Names of some of the products for dual-battery systems and suppliers are, Redarc Electronics, ARRID Advanced dual-battery controller, DBS Management System, IBS Management System, Piranha Off Road Products, TJM, ARB and eBay.

If you are technically minded dual battery management kits are available from the above suppliers and eBay.

For further information do a Google Search below -

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