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 Dual battery systems                      



I have been an auto electrician for 20 years and have done several trips around Australia.  This is where I have seen the need for a simple dual battery system that actually works. I will begin by outlining the requirements that any dual battery system would need to meet. I will give you an overview of the other systems on the market, including their pros and cons. I will then introduce you to the Ultimate dual battery system and let "you" compare the difference.  

  • You need to be able to start yourself, in case you flatten your main battery.
  • You are going to connect a constant power drain to your battery eg, fridge etc.
  • You may want to run lights, radio, CD players and two way radios while stopped.
  • You always need to have a fully charged battery in case you accidentally flatten your main battery.

Basically there are three types of dual battery systems. I will list the pros and cons of each system as I go through them.

A battery switch, that switches between battery 1 and battery 2.

This system works well although it is not fool proof because you have to remember to operate the switch to allow each battery to charge. You will have to select a battery for starting and you also need to remember to isolate your auxiliary battery to stop both batteries from being drained causing you to have two flat batteries.

A diode system, this system uses diodes to isolate the two batteries, yet still allowing both batteries to charge at once.

The main problem with this system although good in theory, is that all the current to run your car from you cars alternator, is running through diode’s, all diodes have a voltage drop across them so effectively you're putting a resistance in the main battery lead from your alternator. The other downside is that you can’t start off your auxiliary battery if your main battery is ever flat.

A solenoid system, this uses an electric solenoid (normally controlled by an electronic box) to turn on and off the second battery for charging and starting.

This is proberly the most commonly used system because like the battery switch system, you can start off your auxiliary battery if needed. The problems with this system are you are connecting a flat battery behind a charged battery which, because of the sensing nature of an alternator, only allows the flat battery to receive a small amount of current to recharge it. It also has the accessories running off the auxiliary battery and starts off the main battery. If by chance you accidentally leave something on overnight like an interior light or the radio you can have two flat batteries. These systems also normally makes sure the main battery is up to about 13.5 volts before connecting the auxiliary battery to be charged, inadvertently this reduces the amount of current the alternator is producing to charge the auxiliary battery.

The Ultimate dual battery system

This system is a hybrid of all the systems listed above. It takes only the advantages of each system and combining them into one simple system. The main feature of the Ultimate dual battery system is its simplicity and effectiveness to meet all the requirements needed in a dual battery system, without any disadvantages or any of the bells and whistles (electronic boxes to fail).

 The Ultimate dual battery system has a switch included that only needs to be turned on if your main battery is flat. This effectively jump starts your car. Turn the switch on, start you vehicle and turn the switch straight off again, that’s it. Due to the internal circuits with the switch left in the off position (where it should be all the time), the auxiliary battery will always be fully charged and can not be flattened and your main battery can be quickly recharged. In this system all your accessories run off your main battery, this is so any power drained from the battery can be quickly replaced because your alternator can sense how flat your battery is and supply the right amount of current needed to quickly recharge it.

System features

This system can be installed by “almost anyone” in around 10 to 15 minutes it is the easiest system to install by far, saving any installation costs. You simply connect the correct, clearly marked lead to the appropriate positive battery terminal.

Your auxiliary battery will always be fully charged and can never be flattened while the switch is in the off position.

You only need to operate the switch if you cars main battery is flat.

You can use the existing power outlets supplied as standard in your vehicle.

The switch is mounted under the bonnet. We recommend not to put down your bonnet until you have turned the switch back to the off position after starting. This way you can never flatten you auxiliary battery.

The emergency start switch has a 12volts intermittent rating of 1000 amps for 15 seconds and continuous rating of 175 amps.

The whole system is sealed and insulated. It is suitable for boats as well as cars because only marine grade products are used.

The system comes in three convenient lengths to fit any vehicle or application.

            Small                                    Medium                                    Long                            With negative lead



Pricing in AUD and includes GST. Postage not included

Small system,      approx 1 metre between battery positive terminals          $185.00      with negative lead   $210.00

Medium system, approx 1.5 metres between battery positive terminals      $195.00     with negative lead    $225.00

Long system,       approx 2.3 metres between battery positive terminals      $205.00     with negative lead    $240.00


Contact details



For any enquiries or purchases please contact us at




               Email                    ultimate@dcsi.net.au



               Telephone              0419 44 88 53



               Address                Ultimate Wiring

                                            370 Caldermeade Road


                                            Victoria 3984                                                                                                                         




The Ultimate Dual battery system

The small Dual battery system with a negative lead


All leads come clearly marked for fool proof installation