How To Charge An Auxiliary Battery & How Long It Takes

Whether you plan to upgrade your 4x4 to take the family camping in comfort or just want to camp off-grid, an auxiliary battery is the perfect place to start building your 12V power setup.

A dual battery lets you run 12v appliances like a fridge or extra lights without worrying about a flat starter battery. Let’s explore how auxiliary batteries work, how to charge them, and how long it takes to charge a second battery.

How To Charge An Auxiliary Battery

The most common way to charge an auxiliary battery is by using DC power from a vehicle, but newer systems have technology that enables easy charging from AC power at a campsite or at home. Let’s take a look at the types of battery charging that’s available:

Battery isolator (fixed voltage alternator)

A battery isolator is the ‘OG’ of battery chargers and acts as a ‘gatekeeper’ between the auxiliary battery and the starter battery. When the car is stopped, if the auxiliary battery is not isolated, it will take power from the vehicle's starter battery, which poses a risk of leaving the starter battery flat. The isolator disconnects the auxiliary battery when the ignition is off, so it can’t continue to draw power.

An isolator does not modify the current or voltage, it simply allows power to flow or cuts it off. This means it’s only suitable for vehicles with fixed voltage alternators, usually manufactured before 2010.

DC-DC Charger (smart alternator/solar)

A DC-DC charger charges an auxiliary battery by optimising DC power from the vehicle, or if the charger has a solar input, from solar power. A DC-DC charger will optimise the voltage and current to match the precise charging profile of the battery, future-proofing for a change of auxiliary battery type down the road.

Portable auxilliary battery with AC input

REDARC’s GoBlock is a portable auxilliary battery (not fixed to your vehicle) with an in-built charging system that can accept AC or DC power. This means you can fully charge the battery (just like a giant power bank) at home, then top it up during your trip by connecting it to the vehicle.

Battery manager with AC input

A battery manager is an all-in-one solution that provides data and monitoring of the power system. REDARC’s Manager30 & Manager Alpha also facilitates charging from DC power, solar power and AC power. The Manager30 prioritises the most efficient charging source to ensure your battery gets the most efficient charging at all times.

Types of REDARC DC-DC chargers

There are several different types of DC-DC dual battery chargers available from REDARC, but the three best for charging an auxiliary battery fixed to your vehicle are the BCDC Core, the BCDC Classic or the BCDC Alpha.

BCDC Core

The BCDC Core is a compact DC-DC charger that is designed to be conveniently installed in-cabin. As the BCDC Core is not weatherproof, the DC charger is designed to be installed in-cabin, in a dry and protected area, away from any weather conditions. The BCDC Core does not have an external heatsink like the BCDC Classic but rather relies on passive cooling and built-in thermal management to regulate its temperature.

BCDC Classic

The BCDC Classic is made for under-the-bonnet installation. It has active cooling with an in-built fan and is engineered to withstand high temperatures in the engine bay. The BCDC Classic has an input to your vehicle's alternator and also supports solar charging, which allows you to set up solar charging if you plan to camp off-grid.

BCDC Alpha

The BCDC Alpha is a more efficient and easy-to-use DC-DC charger. The BCDC Alpha has Bluetooth connectivity to monitor the charge status, input sources and battery health from your smartphone. The BCDC Alpha50 (50A) has the fastest charge rates out of the three DC-DC chargers.

Each of these DC-DC charger options will charge any type of auxiliary battery. Which is best for your setup will depend on where you mount it, how much you want to invest and your long-term plans for your camping power system.

Factors To Keep In Mind When Choosing An Auxiliary Battery

When you are choosing an auxiliary battery for your setup, there are several factors to consider, such as:

Battery Type: AGM, Standard Lead-Acid, calcium or Lithium.
The size of your battery: Compare your power requirements against the capacity of the auxiliary battery you are considering.
Mounting Location: Where you plan to mount your auxiliary battery - typically under the bonnet or in-cabin, adjacent to the auxiliary battery where practical to minimise voltage drop.
Cable Installation: Ensure that your cabling is the right gauge and size and is installed correctly to reduce energy loss through heat, which can lead to system faults.
Ventilation and Heat Management: Consider whether there is enough ventilation so that heat does not build up within the dual battery system. This is crucial for AGM and standard Lead-Acid batteries.

Once you’ve selected your auxiliary battery and dual battery charger, you can DIY install or have an auto electrician install it for you. Make sure you read the product manual for your DC-DC charger and understand what the different lights and notifications mean so you can track the state of charge.

How Long Does It Take To Charge Your Auxiliary Battery?

There are many factors to consider when trying to estimate how long it will take to charge your auxiliary battery. Some of these factors include:

The DC-DC charger size: Some range from 25A-50A.
The capacity (Ah) of the battery: 100Ah-200Ah.
The type of alternator you have and the output of the alternator (some can be 150A+, and some can be between 90A-120A.
The chemistry of the battery and its charge acceptance rate.

With these factors in mind, let’s have a look at the difference in charge time between the different types of battery chemistry, so you can pick the best auxiliary battery chemistry for your setup.

Battery Chemistry	Maximum State of Discharge	Charge Time	Charge Acceptance Rate
Standard Lead-Acid	50% Discharge	5 to 6 hours (50% to 100%)	15-20A for a 100Ah Battery
AGM	50% Discharge	4-5 hours (50% to 100%)	20-30A for a 100Ah Battery
Lithium (LiFeP04)	80% Discharge	2 to 3 hours (50% to 100%)	50-100A for a 100Ah Battery

Depending on your battery's chemistry, you will need to drive between 2-6 hours to charge the battery from DC power. If you are taking a shorter trip, you may want to consider solar charging or a GoBlock, which charges from AC power at home before you leave for your trip.

Get A Reliable Auxiliary Battery You Can Trust From REDARC

For over 45 years, REDARC has been designing and engineering top-quality electronics for your rig, and our lithium deep-cycle auxiliary batteries are no exception. Built for reliability and performance, they deliver the trusted power you need. Our dual battery chargers ensure your auxiliary battery receives the right voltage and current so you get the most out of your dual battery system.

For help installing any of your REDARC gear, head over to our REDNETWORK for our trusted installers near you. For any additional help or support please feel free to contact us today!

FAQ

Can you charge an auxiliary battery?

An auxiliary battery can be charged with DC power using a DC-DC dual battery charger. A DC-DC charger is the best option to charge your auxiliary battery as it can:

Charge your auxiliary battery
Regulates the voltage and current supplied to the battery
Isolates the auxiliary battery from the starter battery to avoid draining it when the car is idle.

What happens if an auxiliary battery dies?

Completely discharging an auxiliary battery can have drastic effects on its lifespan. If you fully discharge a lead-acid and AGM auxiliary battery, they may suffer permanent capacity loss due to sulfation, making them unusable. While it is not recommended to completely discharge a REDARC lithium battery, these are more resilient due to an in-built battery management system that shuts down the battery until it is recharged.