Fully Charged

Glenn Marshall — 25 February 2021
Getting your auxiliary battery and power management system right means pace of mind when overlanding.

When it comes to selecting the right auxiliary battery for your 4WD or camper, it often comes down to what accessories you’re going to connect to it, how much you are willing to spend, and where you want to install it.

Once you have answered these questions, you need to decide how you are going to charge the auxiliary battery, because the method you use can be impacted by the type of battery chosen as well as the type of alternator your vehicle has.   

With the help of Darran Sexton, R&D Manager for Ironman 4x4, we’ve come up with some options to help you with your setup.


There are several different types of batteries used as auxiliary batteries in a 4WD or house battery in a camper, each with their positives and negatives, so let’s see what they are.

  1. Gel: Once popular with competition 4WDs, gel batteries are higher priced than lead-acid or calcium batteries, while offering low amp hours for a deep cycle battery. However, they do not like the heat of an engine bay, so they can be installed inside the 4WD and in any position as they are fully sealed.  
  2. AGM deep cycle: AGM batteries are priced similarly to a gel battery but offer higher deep cycle amp hours, similar to lithium batteries. AGM batteries don’t like heat so can’t be installed in the engine bay but are safe to install inside your 4WD. This allows them to cope with deeper cycling without affecting their lifespan. They are a heavy battery though, upwards of 30kg depending on their size. AGM batteries have a different charging profile than a lead-acid battery, so you’ll require an AGM setting on your 240V charger or DCDC charger. 
  3. Lead-acid: A deep cycle lead-acid battery will certainly save you some bucks and their all-rounder functionality make them popular with a first-time auxiliary battery setup. Many of these also have a high cold cranking rating, meaning they can be also used as a starter battery/deep cycle battery. If using a simple isolator to manage the charging of your second battery, buying two deep-cycle lead-acid batteries is very economical. This type of battery is hefty, weighing in around 30kg and, while most are maintenance-free, some still require regular check-ups to monitor the electrolyte levels. They can be installed in many places except inside the cabin of a 4WD as they vent flammable gases.

Calcium: While similar in cost and weight to the lead-acid battery, calcium batteries do hold some advantages, such as reduced gas venting, less water usage, higher resistance to corrosion and a lower self-discharge. Being sealed, calcium batteries are maintenance-free but, as they still do vent dangerous gases, they cannot be installed inside the cabin of your 4WD. Calcium batteries are reputed to offer an improved tolerance to heat and corrugations but, having replaced four calcium batteries over two years, I will never buy one again. 

Lithium: The new kid on the block and a real game-changer, albeit expensive. LiFeP04 lithium batteries are three times lighter, charge much faster and discharge longer and slower than the batteries already discussed. They also have more usable power with almost 100 per cent available, compared to only 50 per cent with lead-acid and AGM batteries and discharging to these low levels doesn’t affect its lifespan. These batteries are sealed, so can be installed anywhere, except (at this stage) the hot engine bay. They can also be mounted on their side or their end, meaning they can be hidden away in tight spaces. Having installed lithium batteries in my Prado and my camper, I believe once you try lithium, you’ll never go back. 


The introduction of smart alternators on modern 4WDs meant aftermarket charging methodology had to change as compared to fixed voltage alternators. A fixed voltage alternator produces a very high voltage, more than enough to charge both the cranking battery and the second battery. 

The smart alternator reduces the amount of charge it produces according to the operational conditions of your 4WD, meaning your auxiliary battery will not be recharged correctly.

As Sexton explains: “The smart alternator has a varied output to deal with stringent emission regulations, reduced fuel consumption targets and the vehicle’s operational loads. Due to the smart alternator’s variable output and the vehicle’s OEM battery management system, the auxiliary battery may not charge correctly when the cranking battery is fully charged. The vehicle’s inbuilt battery management system reduces the alternator's output, leaving nothing for the auxiliary battery.”


It is important to select the right battery management solution (BMS) to suit your setup. There’s no point trying to save some money and buying a simple isolator that uses a solenoid if you’re looking at installing a lithium battery, 

for instance. 

By the same token, don’t bust the bank buying an intelligent BMS system if all you want is something to charge the lead-acid battery that only runs the fridge twice a year on camping trips. As Goldilocks once said: “The one in the middle is just perfect.” In this case, it’s a DCDC charger, because this charger has the features to provide a reliable and stable charge to any type of auxiliary battery.

  1. Solenoid battery isolator: An isolator (commonly known as a VSR) is the least intelligent of battery management systems and sits between your cranking battery and auxiliary battery, engaging once the cranking battery reaches 13.4V allowing both batteries to be charged at the same time. Once the cranking battery drops to 12.8V, the isolator disengages, preventing the cranking battery from being drained. To be effective, both batteries need to be of the same chemistry, eg lead-acid to lead-acid or calcium to calcium. An isolator isn’t suitable for gel, AGM or lithium batteries as it cannot distinguish between different charging profiles.
  2. DCDC charger: A DCDC charger is designed to be the perfect battery charger that contains all the features required to maintain the auxiliary battery at its peak condition and extend its life. A DCDC charger ensures that there is always enough charge going into the auxiliary battery to keep it topped up, even when your 4WD has a smart alternator. Some models even have a solar input with an inbuilt MPPT regulator to allow charging of your auxiliary battery via your solar panels. A DCDC charger is also intelligent enough to enable you to select the type of auxiliary battery you have and apply the optimum charging profile. For my money, this is the best BMS solution, especially if it includes a solar input. 
  3. Intelligent BMS: For a comprehensive battery management solution, you only need to look as far as state-of-the-art systems such as the Projecta Intelli RV range, the Redarc RedVision Manager 30 and Enerdrive as examples. These units provide charge and maintain all types of auxiliary batteries via AC, DC and solar inputs and are suitable for your 4WD or camper. You can also connect all your accessories to these units and control them via a touch screen or an app on your mobile phone. 


Installing an auxiliary battery into your 4WD or camper is something that some people are keen to do themselves, and I’ve always been one of them. The one thing I have learnt is that reading the instructions is very important, but also that sometimes the instructions miss a vital piece of information, which can lead to dire circumstances. 

I’d replaced a faulty isolator between the cranking battery and the auxiliary battery with another brand, using the existing wiring as I was about to leave on another trip. Further down the track, while using a 240V charger on my cranking battery that had dropped a cell, the battery sent crazy amps to the isolator and it caught fire. 

Thankfully, I discovered the fire in time and was able to extinguish it with minimal damage in the engine bay. Had the instructions for the original isolator mentioned the need for a fuse between the cranking battery and the isolator, this would never have happened. 

Sexton has the following tips for a DIY install:

  • Securely mount the DCDC charger as close to the auxiliary battery as possible.
  • Mount the DCDC unit in a well-ventilated area away from direct heat sources such as your exhaust system or catalytic converter. 
  • Use the correct size BNS cable to avoid unwanted current losses and suit the current that will flow through it.
  • The biggest cause of issues and failures are usually poor soldering and wiring joins, use a crimping tool and connectors that are dust-proof and waterproof.
  • Install the appropriate fuses as close to the battery as possible so that in the event of an incident the electrics are protected from a short circuit.  
  • Winches must always be wired to the starting battery, not the auxiliary battery.


Regulars Column Nuts and Bolts Auxiliary batteries Power management Systems Offroading