Which split-charging system is best?

When you mention dual-battery selection, you often touch on the subject of split-charging systems, specifically solenoid systems versus DC-to-DC chargers. What do you need to know before you buy?

The debate on which system is best (solenoid or DC-to-DC) has been raging for years. Unfortunately, the subject is shrouded in unsubstantiated opinions and an obvious lack of research. To date, there have been no tests that prove that one system is better than the other. One possible reason for this is that there are so many variables that such a definitive statement could never be made. What works in one type of vehicle, may not work as well in another. The other problem is the unchallenged belief that if something is more expensive, it must automatically perform better. This argument is further supported by the fitment industry, in which many 4x4 “experts” recommend expensive DC-to-DC chargers because the profit margins are higher; and they also make the mistake of assuming that the more expensive option is automatically better. National Luna has conducted a series of laboratory tests (on a dozen products and options) to determine how each system performs, but the answer is far from conclusive. The biggest problem with determining which split-charging system works best is that the results are based entirely on other influencing factors, such as:

What battery type is used (Drive Out #106) The output characteristics of the alternator Your typical driving time

Each of these variables has a significant impact on how the split-charging system works; therefore, any statement that says one setup is better than the other would be riddled with generalisations. Let’s look at a typical example: If the alternator’s output is 13,7V or more, a solenoid system will generally recharge faster than most DC-to-DC systems. However, over time, the DC-to-DC system will catch up and restore 100% battery power. But these results will greatly vary according to what type of battery is used. What’s more, if the supply voltage is below 13,5V, the results will differ, and (once again) be heavily influenced by the type of battery. Unfortunately, the above information leads to more questions than answers, and doesn’t help much when it comes to buying a split-charging system. In addition, most vehicle owners don’t know the specific output of their vehicle’s alternator. So, instead of running around in speculative circles, let’s start with things that you do know – like your budget and your typical driving time.

BUDGET

Obviously, this is where all battery considerations start, and your budget relates to two things: the price of the split-charging system, and the price of the battery. As mentioned in a previous column, all the research and data National Luna has gathered on split-charging systems points to one undeniable fact: The performance of the system is predominately based on the type of battery you choose to buy. It does no good to spend R10 000 on a split-charging system only to wire it up to an incorrectly-matched battery. That said, the desired application also influences your choice of battery. National Luna’s advice has always been buy the best battery that you can afford! For most people, this will be a Dry Cell, EV Traction battery. This type of battery is considerably more expensive than a flooded (wet-cell) deep-cycle battery, but the performance variation is vast – it would be like comparing the off-road abilities of a Land Cruiser 70 to those of a RAV4.

DRIVING TIME

These days, alternator voltages can vary from one vehicle to the next; but in the interests of simplicity, we can compare the performance of a popular 20-amp DC-to-DC charger to that of a solenoid setup, using a common-case scenario in which:

A 140-amp dry-cell battery is used The vehicle’s alternator delivers a typical output voltage of 13,7V The driving time is six hours or less

The graph below shows the recharge rates of a solenoid-based system compared to a 20-amp DC-to-DC system. The battery was discharged by 110 Ah (close to 80%). You’ll see that (initially) the solenoid system delivers a much faster recharge rate than the DC-to-DC setup. But over time, the advantages of the solenoid system are reduced, and the DC-to-DC charger starts to take the lead due to a higher final output voltage. From a marketing perspective, the DC-to-DC charger is punted as a superior setup (with higher total amps being returned), but from a practical (usability) perspective, in this case, the solenoid system dramatically outperforms the 20 A DC-to-DC charger in the first six hours – where it really matters. Realistically, most of us don’t drive for more than six hours every day while we’re on holiday, in which case the solenoid system will generally restore more charge to the battery than a 20-amp DC-to-DC charger. Of course, DC-to-DC chargers with higher current ratings are available, but they tend to come at a significantly higher price. In summary: If the alternator voltage is lower than 13,5V, the DC-to-DC system becomes favourable, but consequently, as the alternator voltage increases, the performance gain of the solenoid system becomes more apparent. One way to compare these two systems is to think of the race between the rabbit and the tortoise. The tortoise may win overall, but that’s only when a longdistance comparison is made. For shorter distances, the rabbit gets there faster. Coming back to the question of which system is best. Again, that will depend on your budget and vehicle-setup. But in the opinion of National Luna (based on the example above), your best bet in terms of performance and value for money would be the fitment of an AGM (dry cell) EV traction battery, a low-cost (mechanically bulletproof) solenoid system, and a high-quality 220V maintenance charger. That setup will ensure the best possible recharge rate, while guaranteeing maximum battery life and performance over time. This brings us to our final point: no matter what split-charging system you use, we highly recommend the use, and frequent practice, of connecting your auxiliary battery to a 220V maintenance charger when not in use. Unless you’re driving for very long periods (more than 18 hours), it’s highly unlikely that your auxiliary battery will ever be 100% recharged – regardless of which split-charging system you use.