Tech: DC-DC battery management

By: Collyn Rivers

You can avoid significant hassle by adding DC-DC battery management to your rig.

Tech: DC-DC battery management
Tech: DC-DC charging
RIGHT FROM THE very earliest days of caravanning, numerous issues have bedevilled those who attempted to use electricity in their vans. The concept was fine – reliable tungsten light globes predate WWI, but even in the early 1920s, caravan-related writing occasionally noted that voltage drop was a problem.

This eased marginally in the 1950s when the car industry realised that 12 volts was a better idea than the previous six volts, but worsened again as caravanners became increasingly electrically ambitious.

The lead acid storage batteries of that era were surprisingly similar to the more basic batteries of today – as were the issues encountered. Also around then was the still general misunderstanding of how batteries were charged and, more recently, what goes wrong when you try to run any largish load 10 metres away from a source of 12 volt power.


A battery is charged by connecting it across a source that has a higher voltage. The rate at which it charges depends on how much higher the applied charging voltage is than the battery that is being charged. Ideally, that voltage needs to be up to 20% higher, but there is a limit as to how high this can be when charging from a car system as the optimum charging voltage is too high for the car’s own electrical needs.

So, as in many such situations, one ends up with a sub-standard compromise. In this case, it’s a vaguely fool-proof system that works well enough for the RV and its battery, but less so for charging auxiliary batteries, and a great deal less so if such batteries are in a caravan half a post code behind. The vehicle’s alternator produces a more or less constant voltage. This, as noted above, is low enough not to damage the vehicle’s electrics, but high enough to charge the starter battery sufficiently for reliable operation. The amount of energy needed to do this is surprisingly small – around enough to feed a sidelight for about 15 minutes – and is typically replaced within a minute or two. (For the many who doubt this, a slender girlfriend of my relative youth could readily hand-crank my vintage 4.5 litre four-cylinder Bentley – and replenish the energy required via a single gin and tonic!)

Rarely acknowledged is that at no time does the alternator know, or care, about the battery’s state of charge. The alternator blindly attempts to churn out a constant voltage. The charging battery’s voltage rises toward that fixed charging voltage, with the charge tapering off as it does. It’s a bit like drinking from a reservoir – you stop when you’ve had enough.

For a charging battery, starter or auxiliary, "enough" tends to be little more than 70-75% of full charge after a few hours’ driving. It will go higher, even to overcharging given long enough, but that’s measured in days, not hours. This virtually precludes overcharging, but does not help to cool the chardonnay.

Some argue that 80% charge is achieved, but even if that were so, this limitation is much more serious than it first appears, and the reason why is rarely appreciated.

The limitation is that a battery below a remaining 20% charge is effectively dead. Further, it may have a life of only months if discharged repeatedly below about 40% remaining charge. Battery makers typically quote 50%. Thus, if a battery is charged only to 80% and discharged to 40%, only 40% of its nominal capacity is available for use. Batteries in caravans may not even reach 65% charge...

The reality is that even if batteries are almost driven into the ground when discharged, the loss of capacity is greater than you’d think, unless they are fully charged.


A second major issue is so-called voltage drop. You’ll get a feel for this when you water the garden: the pressure at the end of a hose is a great deal less than at the tap – the thinner and longer the hose, the lower the pressure at the far end.

Electric cables oppose the flow of current and lose electrical pressure (voltage) in much the same way. Run too thin a cable over any distance and voltage is lost along the way.

With a typical caravan-located fridge, unless the alternator supply cable’s copper core is at least 13.5 sq mm (only a bit less than a pencil) it’s far too small to effectively charge that battery, let alone run the fridge at the same time. Thick cable helps, but some years ago a leading battery maker chided me for stating (in an early edition of Motorhome Electrics & Caravans Too!) that a caravan battery rarely exceeds 70% charge. The company stated that 65% was more likely.

For some two decades, vendors produced voltage converters that were installed adjacent to the distant battery, and changed whatever came down the cable to a higher voltage. These helped a fair bit, but they were converters, not chargers, and produced a similarly constant voltage as does the alternator – and with the same limitations.


All this is now changing, not just due to the RV need but also because of the growing demands placed on the vehicle electrical system.

Today, even low-priced cars have more computing power than had early space craft. Their vehicle electronics are rugged enough, but typically require 13.8 volts to run, and some only 13.5 volts – far too low for charging. And they require that voltage to be stable. Further, adding an auxiliary battery bank may not necessarily cause problems, but is all but certain to be blamed if things go wrong. It may also invalidate the warranty.

The solution, the so-called DC-DC charger, is a box that is perceived by the car’s electrics as simply another piece of add-on kit, like a ‘doop doop’ sound system or a pair of spotlights. This box electrically isolates the auxiliary battery from the alternator, compensates for any (reasonable) voltage drop and converts whatever comes in to the optimum charging voltage and regime for any specific size and type of auxiliary battery.

Whereas many other so-called solutions fix only marginally more problems than they introduce, this one really is a genuine technological breakthrough. In one go, it solves most of the electrical problems that have plagued caravanners for close to a century. In particular, it finally enables caravan batteries to be properly charged, and their fridges to work at a capacity a lot closer to that intended for them.

Some DC-DC units provide these adaptations. More sophisticated versions, such as Redarc’s BMS1215 series, also have inbuilt solar regulation, multi-stage mains battery charging and remote readout. This simplifies adding solar and increases reliability by combining functions. This is because most electrical troubles are caused by poor connections and/or lack of 100% compatibility. Now, many external connections are eliminated, and compatibility is built in. If a vehicle has solar modules, the Redarc unit, for example, will automatically select whichever source (alternator or solar) provides the greater charge.

In my regular technical column in the auto-sparky journal, Automobile, Electrical and Air Conditioning News, I suggested that DC-DC charging solved so many problems in one go that the industry should regard it as the preferred method of connection for any application where an alternator-connected battery needs to be charged fully.

I suggest here in Caravan World that it should also be seen as the routine way of connecting any battery mounted in a caravan, fifth wheel caravan or camper trailer.


Installation is straightforward. The DC-DC unit is mounted as close as reasonably possible to the auxiliary battery/batteries via heavy cable. While these units do correct automatically for voltage drop in the cabling from the alternator, it is better not to exploit this unduly: voltage drop wastes energy (as heat) in that cabling.

Follow the sizes recommended by the unit supplier: typically a minimum of 8 gauge cable (ideally 13.5 sq mm). You will definitely need an Anderson connector as a DC-DC unit is able to draw 30 amps or more.

Right now, the approach is experiencing some opposition on the more rabid RV internet forums, mainly on the basis that a direct connection may charge a flat battery more quickly to 50% or so, but that battery is then unlikely ever to fully charge (and high current units are available anyway). It is also only feasible at all where the alternator operates around 14.4 volts. For trailers in particular, DC-DC charging is the more generally sensible approach.

The units are readily available from companies including ABR, Projects, Sterling, RanOx and Redarc to name a few, and prices range from $350 upward. The more sophisticated units seem costly at first, but far less so if their various functions are taken into account; they are also much simpler to install.

I disclose that I had some minor (unpaid) involvement in the specifications of the Redarc BMS1215 and extensively tested the unit, but have no financial involvement of any kind with that company. The unit has since been returned to that company.

Collyn Rivers is the author and publisher of books on various aspects of RVs. His Motorhome Electrics & Caravans Too! has virtually become the auto-electrical text in Australia, New Zealand and now South Africa.

WORDS Collyn Rivers
Source: Caravan World Nov 2011

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