The 4×4 travel market has no shortage of hot debates and shootout tests. Whether it be vehicles in a comparison, or tyres, suspension, fridges, winches, or even camping pots for that matter, people want to know what works “best”.

However, there’s one subject that many overland travellers are generally keen to steer clear of, and that’s the topic of batteries and split-charging systems.

So why is the topic so controversial?

Well, it has a lot to do with varying vehicle types, alternators, preferred battery locations, power needs, and of course, different driving times / distances. There are so many variables to consider that it’s almost impossible to say: “This dual-battery system is better than that one”.

Nonetheless, sometime ago, we published a feature detailing why it took National Luna so long to develop a DC-DC dual-battery system, after our age-old solenoid kit was already 22 years in service. The reason for our delay came down to one reason: We weren’t convinced that DC-DC systems were outperforming solenoid switches.

With this in mind, we laboratory tested various dual-battery kits, along with various battery types, to determine which setup performed “best”. However, rather than deliver a conclusive result, the tests highlighted just how complex the subject really is.

The following graphs and images depict exactly what we found in terms of when a solenoid system works best, and when a DC-DC system should be the preferred choice.


The above mentioned tests concluded that in certain instances, if your vehicle’s alternator output delivers a constant voltage of 13.9V or more, then a 20A to 25A DC-DC system will actually slow down the recharge process. In fact, above 13.9V, the solenoid system radically out performs (within the first few hours of driving) any 20A to 25A DC-DC charger on the market, regardless of what type of battery you use.  



As shown in the graph above, the solenoid system rapidly outperforms a 25A DC-DC setup within the first 3-hours of driving. You’ll also notice that at this voltage (13.9V), the solenoid system doesn’t quite make it to full capacity (98% charged).

However, in saying that, it will also take the DC-DC system more than 5-hours (driving time) to fully recharge the battery. This explains why so many dual-battery systems work well within the first year or so, but as time goes by, the system becomes less efficient. The reason for this is because the performance of the battery is declining with each partially recharged state. This is especially important for wet-cell batteries.

That said, it’s strongly recommend that an Intelligent Maintenance Charger be used (after your trip) no matter what type of system you use. The mistake many of us make is to believe that a few hours drive home will do a sufficient job at recharging the auxiliary battery, when in fact, the battery still needs a maintenance charge in order to reach a full (storage friendly) state.


The graph below highlights the obvious advantage of a DC-DC setup once the alternator’s output is as low as 13.5V. Although the solenoid switch is still outperforming the DC-DC system within the first 2-hours, the bigger issue is that the solenoid noticeably fails to fully recharge the battery (even after 8-hours), and the best it can do is an 86% recharge state. Once you hit the 2-hour mark, the DC-DC system starts to drastically outperform the solenoid switch.



To further complicate things, the tests showed that your chosen battery type (wet-cell, deep-cycle, AGM and lithium) played a fundamental role on how well the DC-DC system worked. Put another way: If your DC-DC charger is unable to change its charging profile to suit certain auxiliary battery types, it could potentially ruin your battery.

What’s more, because DC-DC systems are heat sensitive, once engine-bay temperatures reach 50ºC+ (a very normal under-bonnet temperature), many DC-DC units shut down. Which is why it’s often best to install your auxiliary battery and DC-DC system in the boot of your vehicle – away from heat. Incidentally, solenoid switches don’t have the same problem.

Both the Solenoid (Left) and DC-DC dual-battery systems are available in Power Pack form. The Grey Box (Middle) featuring the solenoid switch, while National Luna’s new Green Box (Right) offers a DC-DC charger with solar input. 


  • If your vehicle’s alternator delivers a constant voltage output of 13.9V or more, a solenoid switch is the preferred choice.
  • If your vehicle’s alternator delivers a constant voltage output that’s less than 13.9V, a DC-DC charger will do a better job at restoring the auxiliary battery’s (near) full capacity.
  • No matter what dual-battery system you install, it’s advisable that an Intelligent Maintenance Charger be used to service the auxiliary battery and restore it to full capacity after each road trip.
  •  Aside from the 13.9V threshold, the tests do highlight a growing need for DC-DC chargers within the modern-vehicle market and their increasing use of variable voltage alternators.

Naturally, the results of these tests will significantly change if a 40A DC-DC charger were used. On that note, stay tuned for a future post where we’ll compare the recharge performance of our 25A DC-DC charger, versus our newly launched 40A DC-DC system.


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