Dual Battery Setup

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Written By Jay Clatworthy

Off Grid Camping contributor.

Dual Battery Setup

If you are travelling off grid for any period of time, then you might want to consider a dual battery setup for your vehicle.

A dual battery system, or auxiliary battery setup, is a great way to increase the amount of battery storage you have with you without carrying and charging additional portable batteries or battery boxes.

The number of appliances people are taking camping these days is growing, and as a result, so is the need for 12v electricity storage.

What will happen if you use too much of your car’s battery power?

Your car battery is a great source of 12v power, but if you use too much of your car’s battery power, then you won’t be able to start your car the next day.

No one wants to be stranded in the outback with a flat battery, as you don’t know how long it might be until someone comes along to give you a jump start.

To prevent this from happening, you can install a dual battery setup to give you the extra power you need while making sure you can always start your car when needed. 

There are a lot of different things to explain about dual battery systems, so throughout this article, we’ll start with the very basics and work our way through to more complicated issues.

What is a dual battery setup?

A dual battery setup is a way of setting up your car, so you have increased onboard power available for your camping appliances (fridge, hot water heater, phone charging, etc.), without using and potentially flattening your main car battery. 

It includes an additional battery added to your vehicle, which is charged while you are driving around.

This battery is then used to power all your camping devices and appliances, instead of your primary battery, which needs to be preserved for your vehicle starter motor and computer, etc.

The second battery is not designed to be a ‘backup’ for your primary battery if you happen to draw too much power.

In a dual battery setup, the primary battery will always be designated for car purposes (starter motor, etc.), and the second battery will always be used for your other camping needs (fridges, fans, phones, hot water system).  

But it is not as simple as adding a second battery and connecting it to your main battery. Though this may help to charge your second battery, this does not stop your primary battery from getting drained and becoming flat.

Dual battery systems have special wiring and components then ensure the power flows in the right direction, and can only be used in certain ways.

For example, they prevent the main battery power from being used for the camping appliances and vice versa. 

Where do I mount my second battery?

Some vehicles have enough space for the second battery to be mounted inside the engine bay, but for the majority of vehicles, it will need to be mounted elsewhere.

If you have a ute, then it is easy to find a spot on your tray or under your canopy to mount your second battery and associated equipment.

Some manufacturers have even manufactured systems that can be mounted behind the seats in dual-cab utes. 

However, if you have a wagon-style 4wd or any other vehicle, you will need to find a way to store it in the boot area.

If you do not have space, or don’t want to permanently mount a second battery, there are other things you can do.

Many people use a battery box and store it in the car while camping, then pull it out when the car is in normal use.

You can get wiring and circuits configured to be able to charge your battery box while driving, so have this benefit without having to install a second battery permanently. We discuss this more in another article. 

How do you connect a second battery to your car? 

Now we are going to start getting more technical. As we discuss specific components and parts, you will need to set up a second battery system.

Keep in mind there are many different ways to configure a second battery, with variations depending on your exact use case and your vehicle.

Overall, the main thing a dual battery system is trying to achieve is a way to charge your secondary battery, while eliminating the chance that your primary battery could be drained.

Battery Isolator

A battery isolator is a simple unit that makes sure the charge goes in the directions we want it to, without any unintended side effects or risk to our primary battery. 

In general, these units take electrical current from your vehicle alternator (which charges your primary battery) and feed that current to your primary battery (as your alternator usually would).

Then when it senses that your primary battery is fully charged, it sends the rest of the current to your secondary battery. 

Some are even smart enough to let you use your secondary battery to help jump start your vehicle if the primary battery becomes flat for some other reason.

Smart isolators cost a little bit more than basic isolators, but we recommend them if you can afford the extra dollars, as the benefits really enhance the versatility of your battery system. 

DC to DC Charger

Many modern vehicles are equipped with smart alternators that turn off when the primary battery reaches full capacity.

They are incentivized to do this to help meet fuel consumption and emissions targets, as any time they can switch the alternator off, they can improve fuel consumption as there is less load on the system.

While this is good for the environment and good for your fuel consumption, this can complicate our desire to feed excess energy from the alternator to our second battery.

DC to DC chargers act like a stand alone battery charger, powered by your car’s electrical wiring, and will not be impacted by smart alternators or another vehicle wiring.

They not only guarantee a steady power source for your secondary battery but also charge them more efficiently.

If your secondary battery gets too flat, it may need more than the standard current to bring it back to life, and even when charging, a standard alternator current cannot always get a secondary battery up to its full charge level.

This is because they do not produce enough current. However, these Dc to DC chargers can adjust their flow based on the battery’s depth of discharge and feed it the optimal amount of current to get it fully charged.

Some DC to DC chargers can also be connected to solar panels, so you can receive charges through the day while stationary, which could prove itself to be very useful if you are stationary for a few days. 

The DC to DC chargers are more expensive, but really worth the money if you can afford them.

If all you can afford is a battery isolator, that will perform the required job, but a DC to DC charger would be an asset to your vehicle configuration. 

>>> Check DC-DC Charger Range and Pricing

Can I fit a second battery myself?

Many tech savvy people can and do install their own second battery system.

The parts are available for purchase for the general public, and there is a lot of free information out there that can guide you. 

The first task is to find a position for the battery and install it in a safe, secure way. If batteries rattle around or move, they can be damaged and become useless.

This first task is more mechanical, so if you wanted to, you could safely attempt this task. 

The second and more complicated task is to wire up the battery and components in a way that achieves your desired outcomes and eliminates the potential for harm to your vehicle’s electrical system.

Though 12V electrical systems are unlikely to cause harm to humans, there is a big chance you could cause harm to your vehicle.

Auto-electricians train for years to be able to do this, so I always recommend using a qualified auto electrician to do any vehicle wiring.

Yes, it does cost a bit more, but in my opinion, that cost is more than justified when you think of the potential stress you would experience trying to wire it yourself, the peace of mind knowing it was done properly, and money saved from things working properly and not damaging your vehicle unintentionally. 

What kind of battery do I need to use for my second battery?

There is no restriction on what type of second battery you can install, as long as you have space and a way to mount it.

You can choose whatever battery you want but will need to make sure the charger that is feeding it, is feeding it the right amount of power, in the right way. 

Starting Batteries Vs Deep Cycle Batteries

When you are shopping for a second battery, it is important to understand the distinction between starter batteries and deep cycle batteries.

Batteries for your starter motor are designed to provide a short, large burst of electricity to turn your engine over so the combustion process can kick in and take over.

At the same time, deep cycle batteries are intended to provide current at a lower level over an extended period of time.

So for a secondary battery in your car or caravan for your camping appliances, you want to look for a deep cycle battery. 

Battery Types

Different batteries store energy in different ways. Here is a quick description of the different types of batteries available and some things to consider with each. 

Lead Acid Batteries

Lead-acid batteries are the cheapest battery available and have been around for a long time. They can handle big short bursts of power, making them ideal for starting engines.

They come with some safety risks in that they are filled with liquid acid, so they need to be stored upright and away from people in case the acid were to spill. 

AGM Batteries (Absorbent Glass Matt)

An AGM battery still utilizes acid, but instead of the acid flowing around inside the battery, it is absorbed into glass matts. AGM batteries not only perform better, but they are much safer as they are sealed units, with no risk of spillage.

They can safely be set up in the same area as humans without fear of acid spillage or fumes, and require no maintenance. 

Gel Cell Batteries 

Gel batteries use a gel electrolyte inside as opposed to the led acid types, and this gel is more capable of a great depth of discharge without damaging the battery. The gel is also safe as it won’t spill, like AGM batteries. 

One downside of Gel batteries is that they require a lower charging voltage than lead-acid and AGM batteries, so you need a very specific charging setup for them.

But overall, they tend to be more robust and long lasting than both lead-acid and AGM batteries if set up correctly with the right charging apparatus.

Lithium Iron Batteries 

Lithium Iron Phosphate (LiFePO4) is the very latest, and most expensive battery technology.

They come with numerous advantages over other lead-acid and sealed AGM batteries. They are much lighter, they charge very quickly, they can handle a very regular significant depth of discharge. 

Summary

Hopefully, this article has given you a broader understanding of how to double battery systems work.

I would recommend you have any wiring done by a qualified auto electrician, and at the very least, get their advice on how to accomplish what it is you need.

When you muck around with electricity without knowing what you are doing, you can risk either your own safety or that of the gear you are working on.

If you can afford it, adding a DC to DC charger will have so many configuration options and serve you well on all your travels.

They are a good investment. And as much as a couple of deep cycle Lithium batteries would be incredible, you could also be well served by a deep cycle AGM, which works well, but is very heavy!