Whether you’re free-camping in the Australian outback or spending a week at a coastal caravan park, having a dependable auxiliary battery is one of the most important parts of your power setup. Your second battery keeps essential 12V systems running — from fridges and lights to water pumps and USB chargers — without draining your vehicle’s starter battery.

In this guide, we’ll break down how to choose the right auxiliary battery for your caravan, camper, RV or 4×4, explain the differences between battery types, and help you figure out what size battery you actually need.

Why You Need an Auxiliary Battery?

Your auxiliary battery serves as a dedicated power source for your camping gear. Unlike your starter battery, which just gets your vehicle going, an auxiliary battery is designed to deliver slow, steady power for extended periods.

This means:

• Running a 12V fridge all night

• Powering lights, pumps and devices while you camp

• Using a small 12V inverter for laptops or portable Starlink systems

• Running accessories without risking a flat starter battery

Many travellers want to wander away from powered sites and enjoy true freedom — and that’s where a quality auxiliary battery earns its keep. 

What type of Battery do I need?

1. You’ll often see Lithium batteries branded as either Lithium-ION or LiFEPO4 (Lithium Iron Phosphate), I’ve written a quick summary on the differences between the two here. Lithium batteries are known for their high energy density, which means they can store a large amount of energy in a relatively small and lightweight package. They are commonly used in portable electronics, electric vehicles, renewable energy systems, and camping equipment due to their long cycle life, lightweight design, and ability to provide reliable and efficient power. However, it’s important to handle and use lithium batteries safely, as they can be sensitive to overcharging, overheating, and physical damage.
   

   Pros	Cons
   Lightweight	Expensive
   High capacity	Extra considerations when installing
   More efficient
   Longer lifespan

2. AGM (Absorbent Glass Mat) battery is a type of lead acid battery. AGM batteries contain a fiberglass mat soaked in electrolyte, which is placed between the battery’s positive and negative plates.
   AGM batteries are known for their maintenance-free operation, as they do not require regular watering or acid level checks like traditional flooded lead acid batteries. They are also highly resistant to vibration, making them suitable for rugged applications, including camping, boating, and off-grid power systems. 
   

   Pros	Cons
   More budget friendly than Lithium	Very Heavy
   Low maintenance	Shorter Lifespan
   Readily available	Only ~50% of the rated capacity is

What Size Battery do I Need?

First you need to determine what you will be running and the current draw of those devices or accessories. You can use the calculations I provided in our Inverter article to work out the current draw of your setup. 

Once you’ve figured that out you will be able to work out the battery size you will need. Depending on your set-up you might find you will need more than one battery to supply everything.
Most people with a simple set-up (fridge, few lights, small inverter) will start with a 120ah and work from there. 

You will also need to consider if you have the means to recharge your battery during a trip. Will a DC-DC Charger or Solar panel recharge it or will you need extra capacity until you can get home and recharge everything?
In my car I have a 120ah Lithium Battery, a DC-DC charger to recharge when i drive and a fixed 160W solar panel to recharge when I’m stationary. This easily runs my 75L fridge full time as well as a few camp lights.  

Remember, for Lithium the rated capacity is considered the total amount of useable energy. If you have a 100ah Lithium battery, you can use 100ah or energy. 
For AGM, you need to cut that in half. For a 100ah Battery you will have 50ah of useable energy. 

How Long Will a Battery Last?

How long a battery will power your gear depends on:

• Battery type and size

• What you’re running (e.g., fridge vs lights)

• Your ability to recharge via vehicle or solar

For example, a good quality 120 Ah lithium battery can easily run a compressor fridge for 24–48 hours on its own, depending on temperature and usage.

Quick Tips for Choosing the Right Battery

– Always size for your real usage, not just rated Ah
– Lithium is costlier, but gives far more usable energy
– AGM is fine if your power demand is low or occasional
– Add solar or DC-DC charging for extended trips
– If budget allows — go lithium for long-term peace of mind

Conclusion