A dual battery system is one of the first necessities to cross any Overlander’s mind. It not only means having a backup battery in case of failure, but being able to run electrical appliances without the risk of running out of energy to start the car the next morning. Well… as long as it’s done properly.

Getting this right was an utmost necessity to us. After all we intend to drive and live in Vanda for 2 to 3 years and want to rely on our fridge to keep our food cool and our starting battery to crank our engine up.

Here is how we went about getting our dual battery system set up.

Choosing the batteries


Good batteries can be quite expensive and we were not keen to save money by buying cheap ones. So we were very pleased when Marshall Batteries approached us saying they would support our project providing whatever we needed! They even sent a technician to come and see the car, listen to what our requirements were so they would ensure nothing less than the perfect gear would be fitted to Vanda.

In the end we got a Marshal Premium X66HAGM as the main battery and a 120 Ah AGM deep cycle battery as our auxiliary battery. The X66HAGM is a true 4WD battery, which means it can withstand the excessive vibrations we will most certainly go through while driving around Australia. You might be thinking: “Any battery would resist vibrations”. Well, a couple of years back our cheap battery cracked due to corrugations on our way back from Shark Bay and not only did we ended up having to find someone to jump start our car, but battery acid leaked inside the engine bay and if I hadn’t noticed it in time part of the front end of the car would have corroded.

For the auxiliary all we needed was a deep cycle AGM of around 120 Ah. And that was exactly what we got. If you are wondering what AGM and deep cycle mean, AGM (absorbent glass mat batteries) are lighter, more efficient and last longer than flooded lead acid batteries. They also don’t require venting, which makes them safer for use inside the cabin. Deep cycle means the battery is designed to be discharged slowly during long periods of time, which is what you need to keep your fridge running overnight for example. Normal car batteries are meant to provide super high currents during short periods, just long enough to start the engine, and will have a short lifespan if used for deep cycle applications.


Connecting the batteries together

Yes, there are several different ways to do this. But all of them should have the same basic goal. It should only connect both batteries together when the engine is on and disconnect them as soon as you turn it off. This will prevent you from drawing energy from both batteries while camped and consequently having no charge left to start your engine.

The simplest and cheapest way to achieve this other than manually connecting and disconnecting the batteries would be using a VSR – or voltage sensitive relay. It’s nothing more than a switch capable of identifying the increase in voltage generated by your car’s alternator when the engine is on, and then connect both batteries.

We decided to go a step further and get a proper battery charger. They are meant to regulate the charging pattern so the auxiliary battery can be charged more efficiently, avoid overcharging and maximising its life. We initially bought a CTEK D250S Dual, but were unhappy with its performance and ended up replacing it with the Australian made and very sturdy Redarc BCDC1225.

It is an MPPT solar regulator also, which means it can be hooked up directly to a solar array and it will charge your auxiliary battery efficiently whenever there is some sunlight available. So if you are intending to eventually add a solar panel to your set up it might make sense to consider a Redarc BCDC1225 or similar over a VSR plus a solar regulator.

Battery Management System


There are some very good BMS available on the market, which will tell you everything you need to know about your battery’s health. So buying one is definitely a great addition to your dual battery system. Unfortunately they are also expensive and we can’t afford it. So instead we bought a cheap watt meter from eBay and use it to get the information we need about the batteries.

For little over 10 dollars it will not only measure amps and volts but store the usage over time. So you can know exactly how much energy you’ve used overnight, for example. However they only measure the current flowing one way, so to keep track of both charge and discharge you will need two of them and subtract the numbers. Honestly, for 20 bucks they are worth it. I find it brilliant way to see how our solar panel is doing, how much battery we have left or if there is anything using too much energy.

So far our dual battery system hasn’t failed us. We run our fridge constantly, charge our phones, laptops, crockpot, and other appliances while still having no problems starting the car. Without it we would certainly live a lot less comfortably.