Updated 15 June 2016

Dc-dc charging charges boat, cabins, camper trailer, caravan and motor home batteries faster and deeper. Collyn Rivers explains how and why.Cartoon showing battery run over by a tractor. Man saying "Hmm.. that's definitely flat!"

Pic: original source unknown.

A battery charges by applying a voltage (pressure) across it that is higher than the voltage it already has across it. The higher that voltage difference the quicker a battery charges. Until 2000 or so alternators charged at a constant more or less fixed voltage. Many did so until 2010 or so. A few still do.

As a battery charges its voltage rises but if the charging voltage remains the same the, difference between that and that of the charging battery constantly falls. As charging depends on that voltage difference, if charging from a fixed voltage – the charging rate constantly falls. Because of this few auxiliary batteries so-charged exceed 80% of full charge. Many in caravans never exceed 65% – and may take many hours to reach even that.

Dc-dc charging – how starter batteries are charged

A vehicle’s starter motor is designed to work with a 70%-80% charged starter battery. These batteries absorbs charge very quickly. Further, the energy required to start a cold engine is tiny. It is less than 2% of that battery’s capacity and is typically replaced within two minutes. The original alternator charging system thus works well for the original electrical needs. It does not, however, work well for charging RV or other auxiliary batteries. Nor, due to voltage drop along the cabling from the alternator to that caravan battery, does it work well for running fridges in caravans. It is cheap, rugged and simple: just fine for starter battery charging – but not for typical RV use.

Dc-dc charging

Dc-dc alternator charging, developed around 2004, overcomes the above problems.

It works fast and deeply yet safely. It accepts whatever voltage is available (as long as it is above 12.7 volts – see below re bc-dc charging) is available from the alternator and converts it to that optimally required for battery charging. In effect it bulk charges at whatever constant current the greatest the battery can safely accept. It constantly increases charging voltage as battery voltage rises – enabling charging fully, deeply and rapidly. It is a far from new technology. It has been used for charging telephone exchange batteries (from 230 volts) for decades, and was adapted to operate for vehicle use from a dc source – see Battery charging and battery chargers.

Dc-dc charging truly scores where batteries are more than a metre or so from the alternator. Even with heavy cable, inevitable voltage drop prejudices charging, and fridge operation. Whilst heavy interconnecting cable is still necessary, a dc-dc charging unit installed close to the associated ensures the required voltage. It extends battery life and can transform fridge performance (particularly of three-way fridges whilst driving).

Dc-dc charging – voltage sensing relays

 These systems require a voltage sensing relay. This relay senses starter battery voltage and directs alternator charging to that alone for two to three minutes after engine starting (when that battery is typically at 13.6 volts whilst under that dedicated charge). It only then allows the alternator to charge the auxiliary battery. It also isolates the starter battery if the RV system voltage drops below about 12.6 volts. Some dc-dc chargers have this relay facility inbuilt. 

Bc-dc charging

Until 2000 or so, vehicle alternators put out a more or less constant 14.2-14.4 volts. Some post-2000 vehicles are fitted with temperature controlled regulators that charge at 14.1-14.2 volts when the engine is cold, then reducing to about 13.2 volts. This became more common from late 2009 and even more so post-2013. Both of the above types of alternator (essentially all that never drop below 12.7 volts when driving) should work just fine from dc-dc alternator chargers.

A third type of alternator – Variable Voltage Alternators – was introduced around 2013. These are controlled by the engine’s central computer system to vary voltage from as high as 15.4 volts to as low as 12.3 volts – and, with some, to zero at times.

Bc-dc charging – regenerative braking

These alternators are commonly used with vehicles that have regenerative braking. They assist braking by increasing alternator voltage to force charge the vehicle’s main starter battery. To enable this, the main vehicle battery is normally only 80% charged. Braking boosts this to 100% – enabling the vehicle to draw all the electrical energy required from that extra 20% battery charge alone. During this time, the alternator’s output is either totally cut, or its voltage reduced far too low for charging. This also results in the voltage sensing relay opening – and thus precluding charging any auxiliary battery for two or so minutes after braking has ceased.

This development necessitates the introduction of a modified form of dc-dc charging (often known as bc-dc). With these, the charger senses the main starter battery voltage, and also vehicle’s ignition voltage. These units are covered in my recently added article http://caravanandmotorhomebooks.com/smart-alternator-problems-with-rvs/  This article also shows how to know what type of alternator is fitted to your vehicle.

Bc-dc units are made by companies including Redarc and Sterling Products. Some companies now produce all such products in that bc-dc form. Those for use with Variable Voltage Alternators are in Low Voltage form. The above Link explains this.

Much of the installation is similar to that below but there is no voltage sensing relay – and a signal  lead needs to be taken to the ignition switch. The makers give full details.

Installing dc-dc charging

Dc-dc charging ensures a battery is alternator-charged safely, deeply and fast. To do this the charging unit must be close to the main energy drawing load and battery that drives it. That load is typically a fridge or fridge freezer and that’s where the dc-dc unit and battery needs locating.

Whilst dc-dc charging ensures optimum charging voltage, you still need to connect the unit via adequate sized cable. This typically requires at least 10 square mm (ideally 13.5 square mm) all the way from the source to the dc-dc charging unit. This is made clear by the units’ makers – but not always sufficiently stressed.

Most such units have inbuilt protection to ensure they only start charging the auxiliary battery once the starter battery is recharged after engine starting. This typically takes two to three minutes. If inbuilt protection is not provided it is necessary (excepting for bc-dc units) to include a voltage sensing relay to ensure starter battery charge priority.

Some dc-dc charging units also have an inbuilt solar regulator and/or a mains battery charger. The sketch below shows a typical such installation.

Installation may vary in minor detail from brand to brand and type to type. The unit shown will charge an auxiliary battery from a 12 volt alternator at up to 40 amps.


redarc bcdc install 4 COL


How to install a typical dc-dc or bcdc charger. In this case the voltage sensing function that safeguards starter battery voltage is inbuilt. Pic: courtesy Redarc. 

Programming dc-dc charging

Different battery types require different voltage/current settings so dc-dc chargers have programs for standard lead-acid, sealed lead acid, gel cell, AGM batteries etc. No programming is needed for the alternator voltage – the systems accept whatever it is.

Lithium (LiFePO4) batteries have different needs – Redarc has a range of LFP units specifically for this purpose but stresses they should really be used only with LiFePO4 batteries recommended by Redarc.


Redarc Dc-Dc Battery charger

The specialised Redarc LFP series of dc-dc battery chargers for (specific) LiFePO4 batteries. Pic: courtesy of Redarc.

Issues with dc-dc charging 

Forum queries indicate that it is not generally realised that a basic dc-dc alternator charger will not work well (or not at all) with a variable voltage alternator. Doing so may also damage the auxiliary battery/s. Forum responses indicate that even fewer have any knowledge of the cause. Further, some forum posts are very critical of dc-dc charging. These however are mostly from vendors whose sales of existing charging products are threatened by it. This website and associated books accepts no advertising, nor payment for any product photographs, or mentions. It is totally unbiased.

A minor downside of dc-dc charging is that initial charging of a deeply discharged battery is limited to the capacity of the dc-dc charger. (rather than the alternator). Dc-dc chargers under 20 amps will take longer to bring a deeply discharged battery to 40% or so of charge. From thereon however charging is hugely faster. The CTEK Smartpass dc-dc charger (below) overcomes this by using alternator charging alone until the battery reaches the point where dc-dc charging takes over.

CTEK 'Smartpass' dc-dc charging.

 Pic: The CTEK ‘Smartpass’ dc-dc charger. Pic: courtesy of CTEK. 

This limitation is far less of an issue with the increasingly common 30-50 amp units. With these, the charging limitations are alternator output, and (mainly for lead acid deep cycle batteries) the maximum safe charge the batteries can absorb. This is far less an issue with gel cells and AGMs – and barely at all with LiFePO4 lithium-ion.

Further information:

If you like this article you will truly benefit from Collyn’s books. All have similar technical accuracy in a down to earth plain English style. I cover, in depth, batteries and battery charging (particularly dc-dc charging) in my Caravan & Motorhome Electrics. I cover solar, in the RV area, in my (now third edition) Solar that Really Works!  I cover solar  for larger homes and properties in my (now second edition) Solar Success. I likewise cover camper trailers are covered in my The Camper Trailer Book.  The totally new Caravan & Motorhome Book covers innumerable RV issues in depth. Information about me is at Bio.

See also the many and constantly increasing number of constantly updated articles on this website. Click here to access the Articles’ index. For solar (for homes and properties see also our associated website successfulsolarbooks.com – it is in process of major updating – but still useful.)

Do please also add a Link from this article to any related forum query. Doing so assists others.