What Is A DC To DC Charger?

In this Off Grid Power Geek breakdown, we’ll tell you exactly what a DC to DC charger is and how it should be used, so that you can see if it’s something you need or not.

We’ll look at why DC DC chargers were developed and the problems they solve.

And we’ll explore how the newest models incorporate MPPTs for solar charging, thus providing a multi-faceted charging solution that ekes out as much power as you can get from driving your tow vehicle and the sun.

what is a dc to dc charger

Introducing ‘What Is A DC To DC Charger?’

Let’s take a look, then, at what a DC to DC charger is and what it’s used for.

Expressed most simply, a DC to DC charger is a device that allows you to safely and efficiently charge your auxiliary/house battery from your tow vehicle battery.

DC, by the way, means direct current. It refers to the type of electricity that is produced by batteries. So the DC power of the vehicle battery is being used to provide charge in the form of a DC current to the campervan/caravan/motorhome battery.

Why Do I Need A DC DC charger?

You may be thinking: can’t I just connect the batteries in parallel to have the same effect.

Ah, would that were so!

In the past, people did this but it’s highly energy inefficient and in some cases, simply won’t work any more. Modern engine bays don’t have room for the auxiliary battery any more, and they’d need to be next to each other for this system to work properly. But even if you could, wiring in parallel isn’t possible any more.

With a DC DC charger, you’ll charge your auxiliary battery far better than wiring in parallel. It gives your auxiliary much more charge at the higher amperage and voltage that it needs to charge fully. As a result, it charges considerably faster.

This lengthens the lifespan of the auxiliary battery. That’s because if a lead-acid battery doesn’t charge fully, it suffers from sulfation and the memory effect means that the potential charge it can store is reduced. Therefore, it’ll deliver less and less power over time.

So even if you have plenty of power provided from solar, the fuller charge received from your alternator is a significant benefit to you.

A ‘Smarter’ Charger?

Many modern chargers are termed ‘Smart’ chargers. That’s because they don’t simply pump a specific voltage and amperage of charge into a battery. Rather, they tailor the charge to exactly what the battery needs at that moment.

Most smart chargers have multiple stages in the charging process, the most simple of these have 3 stages – Boost, Absorption and Float stages.

In that case, a Battery to Battery charger must be a Smarter charge. Because not only does it provide this multi-stage, smart charging, it’s clever enough to provide this charge from the lower voltage and amperage received from the alternator and convert it to the precise charge needed by the auxiliary battery.

Auxiliary batteries want a nice, high voltage and amperage to charge fully, and that’s what a DC to DC charger will provide.

Not only that, many DC DC chargers have a solar charge controller built in, meaning they’ll also efficiently convert solar energy input into charging the house battery as well. More on that later.

Off Grid Living

DC to DC chargers can be so efficient in converting the electrical energy produced by the alternator that some users find that their caravan/motorhome/campervan electrical setup can allow them to live completely off-grid, particularly if they also have solar panels.

If so, they’re getting plenty of energy both from any driving they do and from their solar panels, and the DC DC charger plays an essential role in maximising their energy input to charge the auxiliary battery.

Of course, that depends on the person not using too much energy on-car electrical components like radio, air-conditioning, lights and charging electronic devices.

It also depends on the size of their house battery setup. For example, one 100Ah or 120Ah lead-acid battery (even a good one, like this Superbatt leisure battery) or even the best Gel batteries will probably not be enough to live completely off-grid. But two or three 100Ah Lithium batteries (we’ve done a breakdown of the best Lithium leisure batteries) (which can be discharged up to 90%), can be fully charged with enough driving time (when the alternator is working).

Can’t I Use A Split Charge Relay / Isolator?

DC to DC charging will maximise the current available in the battery system to best manage both starter and auxiliary batteries. Whereas an isolator sends power where it sees fit.

Isolators / split charge relays – including Voltage Sensitive Relays [VSRs] – simply don’t work well with modern smart alternators. These alternators are extremely efficient and are engineered to limit the power to exactly the amount needed to charge the starter battery and no more. There’s simply no additional power that can be used to charge the auxiliary battery.

DC DC Charger and Solar Power

In the past, charging from a vehicle alternator and charging from solar power were two different processes, often one was backup for the other, depending on whether you drove more or less, and on the amount of sunshine you were getting at that time.

People would use an isolator or Battery to Battery charger to charge the house battery from the alternator and a standalone solar charge controller to charge the house battery from solar panels.

A DC DC charger with inbuilt solar charge controller (the best one is the CTEK D250SE) allows you to do far, far more efficient charging. You can connect your solar panels directly to the charger, so you’ll charge your battery even when the engine is off.

With the B2B charger, charging from alternator and from solar are complementary processes. For example, if your system isn’t getting enough power from solar, a DC to DC charger can allow you to get the power you need from the vehicle battery.

And if you’re not doing so much driving, solar power can lighten your vehicle’s alternator load.

So the DC to DC charger/solar charge controller allows you to charge your auxiliary battery whether you’re spending a lot of time driving or if you want to stay at a campsite for several days with no driving. It gives you the best possibility to maximise your power storage.

The solar charge controllers on B2B chargers are not simple solar regulators either, they’re MPPTs (Maximum Power Point Trackers). These controllers provide far more efficient and safe charge than more basic solar regulators.

How does a DC to DC charger work?

It works by isolating your 12V auxiliary battery system from your vehicle alternator. As a result, the vehicle battery system sees the auxiliary battery system as something like a set of lights.

Due to this action, the DC DC charger boosts the voltage of charge to the house battery system from as low as 9 volts to 14.4V or perhaps more, as needed. The Amperage is also boosted, and this maximises the charge delivered to that system, making it a much more efficient and complete charge. And, as we said earlier, the charge delivered is a multi-stage process, always delivering the right type of charge for the 12V battery for its current state of charge.

What size of DC DC charger do I need?

Ideally, you’ll want to work out what electrical appliances you use, and how much power you’ll need over time. Then you can see how much amperage charging would be needed.

As a rough guide, if your auxiliary battery bank is from 70-150Ah, you’ll want a 30A DC DC charger to be most safe. And if it’s 150-220Ah, you’d likely want a 50A model, to be sure of efficient and healthy charging.

Could a B2B charger result in my vehicle starter battery failing?

Simply, no. The DC DC charger always prioritises charging the starter battery first.

Benefits of a DC to DC charger

  • You’ll comfortably get your auxiliary battery 100% full after a day of driving.
  • Better for the environment – uses the power produced from the alternator to charge the house battery and maximises power from solar power inputs.
  • Ideal for off-grid living, allows best use of energy produced by driving and from solar power.
  • Saves money and gives you more freedom – spend only as long as you want to at campsites with electrical hookup, you’ll get enough power just from driving.
  • Great for UK weather (and other not so sunny countries!) – peace of mind knowing that you’ll get enough charge from driving to your next destination, regardless of how much sun your solar panels get.

If you’re interested in a typical mains charger, here are reviews of the best leisure battery chargers UK.

Summing Up ‘What Is A DC To DC Charger?’

Hopefully, now you understand DC to DC chargers and what they can bring to benefit your lifestyle as a campervan, caravan or motorhome owner.

They’re an investment but it’s an investment that will pay off multiple times over, both in terms of money and in terms of improving your lifestyle. The fact that a DC to DC charger allows you to charge your auxiliary battery very efficiently from your alternator, means you’re getting so much more power to that system than you otherwise would. So your expense for other charging needs will be less, and you can spend less time connected to electrical hook up charge.

And with the inbuilt MPPT, the B2B charger will make the best use of both energy inputs to maximise charge. More full charge more often is exactly what your 12V battery needs, so it’ll also last longer and work better for longer (more on that in How Long Does A Leisure Battery Last).

You asked ‘What is a DC to DC charger? Well, now you know.

DC to DC Charging (or B2B or battery to battery charging)