DCDC Chargers | A Guide To Everything You Need To Know

A DCDC charger is a smart way to ensure your auxiliary deep cycle battery or accessory battery receives the correct level of charge and ensures your battery reaches its maximum charge capacity in a safe, fast and regulated way.

It does this by taking power from the alternator while the vehicle is running and optimizing the charging to get the best capacity of charge into your auxiliary battery.

A DCDC charger is integral to your dual battery setup. A quality one will often include a solar regulator too. This allows you to connect a solar panel directly to the charger to continue charging and maintain your battery even whilst the engine is off. This means you don’t have to buy more equipment such as external or inline solar regulators to add solar charging to your dual battery setup.

Here is a typical in-vehicle dual battery setup including a solar panel:

Here is the vehicle solution with the addition of a camper trailer/caravan setup. The trailer/caravan has a larger 40 amp charger to cope with the additional current required to charge and maintain 2 auxiliary batteries:

A DCDC charger is required when charging lithium LiFep04 (Lithium Iron Phosphate) batteries as they require specific charging profiles that a traditional fused cable setup cannot provide.

Because the majority new vehicles on the market are equipped with “smart” alternators that have low outputs, it is very difficult to charge any secondary batteries that are fitted.

What is a Smart alternator and why are they being used nowadays?

Traditionally, the alternator on vehicles had a fixed voltage output which was sufficient to charge both the starting (cranking) battery and a basic auxiliary battery too.

Most current new vehicles have “smart” alternator that is either low output (usually < 13.7-13.8 Volts) or load sensitive. The reason these have been adopted by the automotive industry is as a result of stringent emissions regulations and the constant quest to minimise fuel consumption.

Whilst they are more economical to run, are sufficient to charge your factory battery and run the cars factory electrical systems, they lack the output to charge a second auxiliary battery.

A DCDC charger is intelligent enough to overcome these issues and allow you to charge and optimize your auxiliary battery using the factory alternator.

How does a DC to DC Charger work?

Essentially, it takes whatever output is available from the alternator and converts this to a higher rate of amperage. It will charge the auxiliary battery at whatever current the auxiliary battery can take.

Depending on the rating of the charger, that can be up to 50amps in the case of the Redarc BCDC1250D. This allows fast replenishment of the auxiliary battery’s charge.

Quality DCDC chargers such as the Redarc BCDC 1225D and CTEK D250SE are a 3-stage charger. What this means is that as the battery charges, different levels are used. 

Stage 1: Boost- this delivers maximum voltage to a low battery in the initial charging.

Stage 2: Absorption- this is a continual charge that is maintained until the battery current demand reaches less than 4 amps.

Stage 3: Float- this is essentially a lower charge that maintains the battery at around 13.3 volts or essentially fully charged.

Where a DCDC charger really excels is when the auxiliary deep cycle battery is a long way from the alternator, i.e. in the tray of a utility or the back of a 4WD wagon. Auxiliary batteries are often fitted in battery boxes too. 

 As long as heavy gauge cables are used, the voltage drop is minimised and locating the charger as close to the auxiliary battery as possible ensures the optimum voltage is delivered.

Having the DCDC charger close to the main energy load also helps. This is often large electrical items such as 12 volt in car fridge freezers.

Why do I need a DC-DC Charger?

In a nutshell, your DCDC charger will charge your auxiliary battery faster, to a far greater level and safely regardless of your vehicle’s alternator type.

If you run a modern vehicle fitted with a “smart” or low output alternator, a DCDC charger is crucial for optimized charging and battery maintenance.

If you run an older vehicle with a fixed output alternator, the addition of a DCDC charger will significantly improve your auxiliary battery charge and maintenance over a traditional parallel wired dual battery setup. This is because an alternator was never designed as a designated battery charger, especially for a dual battery setup.

With older vehicles, the traditional parallel wiring of a second battery was often easier as there was usually room in the engine bay to mount an auxiliary battery. The auxiliary battery needed to be close to the main starter battery for this setup to work.

Modern engine bays tend to be very crowded and locating an auxiliary battery can be impossible or at best, resulting in using a smaller than ideal battery in order to make it fit so a  charger that can be located anywhere allows far more battery placement options.

There is vehicle specific BCDC mounting brackets available if you need to mount your charger under the bonnet.

Due to the programmes in a quality charger, it will optimize to suit the type of battery you have, whether it be Lead Acid, AGM or Lithium. If you have or are considering a lithium battery, then you absolutely have to run a DCDC charger.

The smart charging stages will also increase the battery life and efficiency, both when running accessories or not.

Now you have a good understanding of DCDC chargers- what to buy?

There are quite a few brands of chargers, and with a variety of prices. Sometimes the cheapest is not the best. The more affordable ones often sacrifice the solar regulator and often are rated at only 20 amps or lower so it is important to do your research and ensure that the DC to DC charger you are buying will actually do what you require.

You do not want to be a few months down the line and realise that your dual battery setup isn’t cutting it when you are out enjoying your time off grid.

We have years of experience with different chargers, MPPT solar regulators including installing them. Based on our collective experience, we have selected the most feature packed, robust and reputable dual battery solutions for you.

These include DCDC chargers, AGM batteries, Lithium batteries as well as the very best in 12v electrical accessories.


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