Dual Battery Systems

With an ever increasing number of electrical accessories and devices being used when travelling around Australia, along with more complex vehicle electrical systems than seen in years gone by, having the right battery charging solution has never been more important.


Late model 4×4 vehicles have either Temperature Sensitive Alternators or more commonly the new Variable Output Alternators.

The Variable Output Alternator requires a DC to DC Charger/Isolator to be used when fitting dual batteries.

In this era vehicle, the ECU is looking to achieve less emissions, and one way it can do this is by reducing the drag in the alternator. So, the voltage output varies constantly, sometimes as low as 7 volts. The BC to DC Isolator will accept any input voltage and always outputs 14.1 volts. Constant charge to the auxiliary battery.

The earlier VSR (Voltage Sensitive Relay) Isolator is switched by the voltage rising after start-up. When the voltage is higher than approximately 13.6 volts, the VSR connects the auxiliary battery into the charge circuit. If the voltage drops below 12.3 volts, it disconnects the auxiliary battery, hence if a VSR is used in conjunction with a Variable Output alternator the auxiliary battery is constantly being connected and disconnect – as the voltage steps up and down. The auxiliary battery inevitably goes flat.

The VSR style is quite compatible with older charging systems, providing reliable charging of the aux battery in those era vehicles, rather than using a manual switch, often forgotten. Result is a pair of flat batteries!

With Temperature Sensitive Alternators, the maximum voltage is limited by temperature, but in our experience rarely outputs over 13.6 volts. This is sufficient for an OE battery configuration, but not for auxiliary batteries in 4x4s. In vehicles with Temperature Sensitive alternators you have the option of using a BC to DC Charger/Isolator or the VSR style in conjunction with a Voltage Booster. The Voltage Booster lifts the voltage output by 0.5 volts, hence the auxiliary battery will charge at around 14.1 volts.

We recommend the RedArc BCDC Charger/Isolators, either the 25 amp for up to 200ah batteries and the 40 amp for battery configurations over 200ah.

Dual Battery
  • Multi-stage DC-DC charging from vehicle charging system
  • Charges from Solar input with its own regulator, delivers maximum input from the panels
  • Battery chemistry specific charging profile – AGM, Gel, Lead Acid Calcium & Standard Lead Acid
  • Wide operating range, 9 to 32 volts, so is suitable for 12 or 24 volt applications
  • Fully sealed – water, dust, vibration proof
  • Design, built and tested in Australia

We also fit the Projector 25 amp DC to DC Charger/Isolator for a budget conscious alternative.

Our recommendation for an under bonnet fit is the Optima spiral wound battery, compared to the usual parallel plated battery.

It has greater surface area which leads to greater CCAs (Cold Cranking Amps, good for starting and winching), but more importantly for most applications, more RC (Reserve Capacity) – which is where the power is drawn for fridges and lighting etc.

The other advantage of using the BCDC style, in conjunction with the Optima, is it will have significantly reduced re-charge times, due the Optima battery having significantly less internal resistance (see below). More charge is used to restore the battery, rather than overcoming resistance in the battery.

Our experience is that an Optima will fully re-charge in a couple of hours, whereas with other isolator and battery combinations will take 6 to 7 hours driving and even then, will not reach full charge.

Sometimes in life it’s worth paying a bit more.

Cold Cranking Amps CCA
CCA is the measurement of current, Amps, that the fully charged battery cab deliver in 30 seconds, while maintaining voltage above 7.2 volts, at temperatures of -18C.

It is a rating on high current output for starting.

Reserve Capacity RC
Is the time, in minutes, that a battery can deliver 25 amps until the voltage drops to 10.5 volts, at temperature of 25C.

It is rating the capacity of the battery to run the vehicle in normal conditions & load if the charging system has failed.

Amp Hour or correctly termed Amp Hour at 20hr Ah
Is the storage capacity of the battery and is the current the fully charged battery will deliver over a 20 hour period, before the voltage drops to 10.5V, at temperature of 25C.

It is rating the capacity to have a long slow discharge, often for use as an Auxiliary Battery in 4WD vehicles. Even though a battery may have an Ah rating, it is not necessarily a Deep Cycle Battery, which are designed to have repeated deep discharge or Cycling. Only use Deep Cycle Batteries in Auxiliary applications.

The spiral wound design of the Optima Battery has greater surface area (to accept charge), on thinner plates, where there is less resistance.

In addition, we are often asked to fit auxiliary outputs into the rear cargo areas, power outlets via anderson plugs to the towbar for vans and campers, plus solar inputs at the front and/or rear. There is an endless combination of plugs and wiring solutions for each persons camping or travel setup, and we cater to all.

Reserve Capacity RC
On an increasing number of vehicles there is little or no room under the bonnet to fit an auxiliary battery. In these situations we can fit a Battery Box into the cargo area of wagons, or in the tray or tub on utes. However, on many vehicles we can fit the second battery in a chassis mounted cradle.

The advantage of the under body mounting is it doesn’t take up any useful space and is quite a simple fit. In-tray sounds simpler, but with most vehicles having tray liners, there are always issues with spacers and spacing with the liners.

The last option for an auxiliary power solution is using a portable battery pack, ala Thumper, but these are an expensive option when considering the available amp hours (Ah) being supplied, compared to an Optima or AC Delco Lead Acid Calcium Maintenance Free battery. Amp Hours (Ah), is the measure of energy storage, which directly relates to how long a fridge will run.
For instance, a Thumper with around 80Ah cost is approximately $900, whereas the Optima with 110Ah is under $500 and a AC Delco Calcium Lead Acid with 90Ah cost is just over $200.

All components are available separately.

The last option is RedArc Solar Panels.

  • Comprehensive DIY Instructions
  • Experienced Technicians available for fitting
  • 3 Year Unlimited Km Warranty

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