How to Charge an Auxiliary Battery

There’s an overwhelming amount of information out there on how to best charge your auxiliary battery. There are options for every set-up and budget.

There are 3 main ways to charge a secondary battery. They can be used on their own, or combined to provide consistent and accurate charging, getting the most life out of the battery.

• Alternator - Power generated while running the engine
• Solar - Power generated from the sun
• Mains or Shore - Using the electricity available at home, a marina or a holiday park

The engine main battery is often at full charge as it gets a constant charge every time the engine is running. This means there is often power generated by the alternator that isn’t needed and essentially goes to waste. This is the perfect opportunity for free power for the secondary battery.

Tapping into the alternator and main battery system allows us to take the excess alternator power and send it off to the secondary batteries. There needs to be a ‘controller’ of some sort between here otherwise the second battery will constantly draw off the main battery and leave it flat. There are a few different components that can control this charge, including VSR’s (Voltage Sensitive Relay) or Battery Isolators and DC-DC chargers.

The cleanest and greenest way to put power into your battery bank, solar panels are a great way to get ‘free’ power without the need to move, run a generator, or even be near civilization. A solar panel catches light from the sun's rays (even on cloudy days) and converts that into electrical energy. That energy is then fed down through the wires and can be used for charging a battery. As with power from the alternator, a controller needs to be used to regulate the charge and protect the batteries.

Again, there are many types of chargers for this, known as solar charge regulators. These will vary in function, quality and price. The most common are PWM or MPPT. DC-DC chargers often have built-in solar regulators, making them a great all-in-one unit to charge while running the engine and from solar panels.

If holiday parks or marinas are your thing, why not take full advantage of the power supplied and use it to run large domestic size appliances without the worry of killing your batteries? Most large motorhomes, caravans and launches have pre-wired mains power that simply needs to be plugged into a power outlet. A 240v outlet can then be used to run a standard battery charger. You'll need a cable to to do this. Most marinas in New Zealand also require boats to have an Electrical Warrant of Fitness before connecting to shore power.

Now, let's look at how each unit operates and what set-up will suit your needs:

This is the type of charger that many people may be familiar with already. If you’ve ever had a flat battery in your car and needed to charge it, this is the type of charger you would have used. These units simply plug into a regular 240v socket and attach to the battery terminals with crocodile clips. They are relatively cheap compared to other charging methods and require no set up or electrical knowledge.

Used for charging a second battery off the engine’s alternator, these units go by a few different names but essentially do the same thing. A battery isolator or relay goes in between the main engine battery and the auxiliary battery. When the main battery reaches a certain voltage (usually around 13.6v), the relay engages and diverts the extra power from the alternator to charge the second battery. This charge is quite crude for modern deep cycle batteries, as it simply provides as much charge as possible while the engine is running. Without various charging stages, these units may reduce the life of an auxiliary battery that needs more accurate voltages (eg. AGM or lithium).

As the name suggests, these units receive power generated by a solar panel, regulate it, then send it off to charge a secondary battery. The installation of every solar regulator is essentially the same and relatively simple: two solar wires go into the regulator and two wires exit the regulator for the auxiliary battery.

For those on a budget, a PWM type solar regulator is a very small, simple unit that will do an acceptable job of providing and controlling power to a second battery. Like with VSRs and isolators, PWM solar controllers will provide as much power as possible to get the battery to full charge. As it approaches capacity, the controller will back off to prevent overcharging.

The latest in the world of solar charge regulators are MPPT models. These intelligent chargers are known to be up to 25% more efficient than their PWM counterparts, which makes a big difference in the speed and quality of charge the battery receives. With no extra steps involved in installing these, it is highly recommended to invest in a MPPT solar regulator to get the most out of your batteries.

Solar charge regulators are rated by their maximum output: higher amp controllers will charge batteries faster and are more suited for large battery banks. Purchasing a larger charger than you currently need will allow space for more solar panels without needing to upgrade the regulator as well. If you want to run a lithium battery bank, ensure the controller has a compatible charging mode.

By this point, you may be thinking, “Is there a way I can combine these to charge from both the alternator and solar panels?” The answer is YES, you certainly can!

DC-DC chargers far exceed the quality and efficiency of VSRs and battery isolators. They feature multi-stage charging to ensure the maximum life of the battery and are often able to provide more current to charge faster. For caravanners, a DC-DC charger is ideal for overcoming the voltage drop caused by the distance between the vehicle's battery and the auxiliary battery stored in the caravan.

Modern units also include a proper MPPT solar regulator that takes care of the solar panel input as well. As with a standalone solar charge controller, the DC-DC’s charger is highly efficient and provides accurate charging in multiple stages. They really are the ultimate all in one charger.

Do you have any questions?

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We provide general information on products, not personal advice.  Always seek the help of a relevant tradesperson if you have a technical query