Dual Battery Systems

Dual Battery System Upgrade

A dual battery system in a 4×4 is very handy, especially if you like running accessories from your 4×4 when camping and in particular if you run a compressor fridge in the back. The idea of a dual battery system is to have all (or most) of your accessories connected to the second battery (usually a deep cycle). As your accessories use power from the second battery the isolator prevents power from draining from your main starting battery leaving it ready for you to start your vehicle when needed. When you start your vehicle the isolator senses the charge from your alternator and links the 2 batteries together and charges them as you drive.

A big mistake some people make is forgetting that original accessories on your vehicle such as headlights and stereos are still connected to the factory wiring and if left on will still drain your starting battery.

Our Landcruiser came with an Aux battery installed under the bonnet (a standard second starting battery). Linking them together and controlling isolation was a fairly basic voltage sensing relay (VSR). The wiring was rather thin for my liking so it was ripped out quick smart.

We also replaced the second battery with a Century Marine Pro 720 battery- mainly for its starting capacity AND ability to run a few accessories from it such as our aux LED lighting. Even though they are a ‘marine’ battery, they are a perfect choice for 4×4’s due to their strong construction and second set of terminals for connecting accessories to it. As a standalone aux battery they aren’t that great for running high current draw accessories like fridges but as we are installing a 3rd AGM deep cycle battery for that stuff, we think the Marine battery will do a great job up the front!

To replace the existing flimsy VSR we installed a REDARC SBI12 Smart Start dual battery isolator. We reckon that a required feature of any ‘PROPER’ dual battery system is the ability to easily jump-start your vehicle using the second battery if required. (without the need for jumper cables)

The REDARC SBI12 has a convenient over-ride function controlled by a simple push button switch activated from within the vehicles dash. The button and a length of cable cost less than $5 from Jaycar and only took a few minutes to install). When the switch is pressed it links the 2 batteries together manually and allowing me to jump-start from the second battery. For this reason we used 4B&S power cable from the Sbi12 to each battery as the cable needed to be able to safely carry the huge current required when jump starting- plus a heavy gauge cable ensures a good flow of current during charging.

The great features of the REDARC SBI12 apart from the over-ride feature is the compact size making it very easy to mount which is important in modern vehicles with little space to spare. The large terminals also allow for safe use of the larger gauge cable- something that many other isolators seem to overlook. Just remember the bigger the cable the more efficient the charge!

One thing we have noticed with the REDARC SBI12 is the sound it makes when activating or ‘engaging’. Previous isolators we have owned have all made a smooth ‘click’ sound when engaging- the SBI12 makes a noticeable ‘clunk’ sound. I guess that reinforces the quality of the unit knowing that the contacts and relay inside are more than just flimsy metal tabs making a less than ordinary connection like other units. Overall, we are very impressed with SBI12.

Power to the rear!

As full time travellers, we have fairly high demands for battery power. With 3 kids, we also appreciate the importance of a compressor fridge in the back of the car. For this reason we have installed a 3rd battery in the rear wing of the Black Widow storage drawers.

A 100ah AGM Deep Cycle battery. From this battery we will run our Ironman compressor fridge full time. It will also run other various 12v accessories such as extra lighting, chargers etc.

Because of the high demand we have on this battery, the fact that it is an AGM AND the fact its located so far away from the starting battery- we need to make sure it is managed properly and efficiently.

The ONLY way to ensure that your expensive deep cycle batteries are maintained properly is to use a DC-DC charger. Although a dual battery isolator is great in many cases, AGM’s or deep cycle batteries in general, struggle to be charged fully from a cars alternator. They need a special applied combination of volts vs amps to get the last 10% of charge into them. A vehicle’s alternator simply isn’t smart enough to do it (and really, was never designed to do it).

We installed a REDARC BCDC1240 40 AMP in-vehicle charger. This mounts close by the AGM battery and connects to the main battery and ignition in the front via 8mm power cable.

We also installed a 65w Solar Panel on the roof so we still have input power when stationary – the REDARC BCDC1240 also acts as an MPPT solar regulator. We needed to use a 60amp Relay which automatically switches the charging input source between vehicles alternator and solar.

The REDARC BCDC1240 has a proper 3 stage charging algorithm which closely monitors the state of charge of the battery and applies a balanced rate of charge ensuring that the battery is reaching 100% of its capacity. After all, batteries are expensive so we want to get the most from them.


The DC-DC charger is very compact to mount and the chassis of the unit is designed as a heat sink to ensure it doesn’t get too hot.

We crimped AND soldered all the connections to the battery, relay and charger- we reckon this is the best way of ensuring a good quality connection.

The BCDC1240 is rated at 40amp and has plenty of grunt to spare so we can connect our camper via an Anderson plug as we tow and have it charge its battery as we travel. (they are both AGM batteries)

So now, we have plenty of power, top quality gear and in our view one of the most efficient ways of charging and maintaining a vehicle’s battery and power system for high demand use.

The power system of your vehicle is really an area you don’t want to skimp on. It may not be cheap initially but you really need to consider it as an investment. We have learnt from the past that buying cheap versions of things and ‘making do’ with lighter gauge cable and connectors ends up costing more in the long run as you need to replace them and really, nothing can put a price on the inconvenience it causes when they fail (usually when you are relying on them the most).

Our advice is:

  • Speak to a battery guru about your power needs. Get a second opinion and compare their thoughts.
  • Buy a good name battery with a warranty
  • Buy the heaviest gauge cable suitable for you application and try to keep the length as short as possible.
  • Plan your install before drilling holes and cutting cables.
  • Use suitable rated fuses on ALL cables and place them as close to the battery as possible
  • Don’t waste your money on inferior crimp connectors- get your auto electrician to supply and crimp them for you.
  • Ensure you make a good earth connection and remember your charging efficiency is only as good as your poorest connection
  • Choose your isolators and charging units carefully. See what other people are using and read reviews. Check for quality of the posts (terminals), how well sealed are they (from dirt, dust, moisture etc.).
  • Remember, you get what you pay for!


REDARC BCDC1240 DC-DC Charger Brochure