The smart way to stay in charge

Intelligent DC-DC chargers are the new wave when it comes to designing a dual-battery system that can deal with the so-called ‘smart’ alternators in most modern vehicles. Angus Boswell explores the latest trends.

The last eight years or so, the advent of cars with so-called ‘smart’ or variable voltage alternators has forced owners to revise how they charge a secondary battery when on the go. Not far back, a dual-battery system used to power a fridge and other devices could rely on a simple voltage-sensitive relay (VSR) to reliably charge the cranking and auxiliary battery and separate them when the vehicle was switched off. This is no longer the case.

This is where ‘smart’ DC-DC (or battery to battery) chargers enter the picture. They take the highly variable (and always changing) output from a smart alternator and increase or decrease it to a stable voltage that will ensure that a rapid and safe charge gets to the auxiliary battery, according to a controlled multi-stage charging algorithm. The higher the output from the DC-DC charger, the faster the charge rate, limited of course by the type of battery being charged. In this way, a full 100% charge can be achieved in a shorter time (in some cases up to five times faster), against the typical 80% maximum charge of most previous systems.

Like those older relays, the DC-DC charger monitors the cranking battery, and when the car is turned on and the voltage picks up, it connects the auxiliary battery to allow charging from the alternator, and conversely, isolates it when the car is turned off so the cranking battery cannot be drained by subsequent power draws.

Even if you are using an older vehicle with a fixed-output alternator, usually in the range of 13.8 to 14.4 volts, you will still get a faster and safer charge to your auxiliary battery as the intelligent draw from the DC-DC charger will provide the most appropriate inputs and ensure the battery has a longer life. This is because alternators aren’t good at charging up depleted secondary batteries. They prioritise juicing up the electrical components of a car, with their secondary job being to keep the cranking battery at a decent state of charge. High draws from an alternator add mechanical load, and thus push up fuel consumption.

Battery types and solar

Many quality DC-DC units include sophisticated MPPT (Multi Point Power Tracking) solar controller circuitry, which allows solar panels to be connected. These can be permanently connected so the charger uses both solar and alternator inputs when driving, or they can be plugged in when parked up as an additional charging source.

The advent of new deep-cycle battery chemistries has given impetus to the use of DC-DC chargers, as each type requires a slightly different charging regime. Lead-acid choices have expanded to Gel and Absorbed Glass Mat types, with Calcium another option, and the more expensive Lithium-ion and LiFePO4 (Lithium-Iron Phosphate) now taking over. If you intend to invest in lithium technology, be sure the charger supports this option, as their charging profile is quite different to that of a lead-acid battery. It is worth noting that lead-acid chemistries are notoriously slow to charge, taking up to 10 hours, with AGM batteries requiring a high bulk phase voltage, whereas a lithium battery can take a third (or less) of that time.

Charger sizing

There are two factors at play here: the size of the battery bank you need to charge, and the type of battery selected. The battery bank of course needs to give you a decent amount of off-grid run-time and be able to power all your accessories until they can next be charged, without doing damage to the batteries by running them too low. For lead-acid deep-cycle batteries (Gel and AGM), the charger must have an Amp output of 10 to 20% of battery capacity – so for a 100Ah battery, you need a 20A DC-C charger. A lithium battery needs a charger with 30 to 50% of capacity – so a 100Ah lithium battery will require a 40-50A DC-DC charger.

When it comes to sizing your system for solar, be sure the voltage input from your solar panel array can be handled by the DC-DC charger. Buy for the long-term, especially if you plan to add more power draws, like a second fridge or an inverter. This will call for more battery power, and in turn more solar input to charge the batteries. Most of the 20-25A chargers out there will manage a solar panel wattage of upwards of 150W, with the larger 40-50A chargers able to handle upwards of 400W. Talk to your supplier, and carefully check the manufacturer’s specs.

Construction and safety features

Most DC-DC chargers will have a hard life, located in a hot place under a vehicle’s bonnet or in a dusty rear of a bakkie load bay, where it will be subject to vibrations and regular wetting. You want to choose the most robust unit possible, with the highest dust and waterproof (IP) rating. Also look for a good range of operating temperature (under bonnet temps can be above 50 degrees C), and the ability to handle a wide range of input voltages. It’s worth paying more for a unit with an adequate solar controller able to handle the inputs from your solar array (with some spare capacity), and the ability to charge a range of battery chemistries (particularly if you might upgrade to lithium).

Are the electronics set up to deal with snags like reverse polarity, and under- or over-voltage situations which could damage the batteries? At a practical level, it is well built? Does the unit have deep metal cooling fins to dissipate heat, is the casing sturdy, and are the connecting terminals able to handle heavy-duty connectors suitable for the necessary 13.5mm cabling?

The installation

You can buy the best charger in the business, but if badly installed, it will not perform as expected. Basically, it will be connected between the positive terminals of the cranking and auxiliary batteries, with a negative earth to the vehicle chassis or to the negative terminal on the cranking battery. The best fitment centres know what they are doing, and will have it done in a few hours, presenting you with a bill of R2 000, on average. What should you look out for?

  • Modern engine bays are cramped, so install the secondary battery at the rear of an SUV or at a visible place in a load bay.
  • To reduce voltage drops you want the DC-DC charger close to the secondary battery and close to the power draws (like the fridge, etc).
  • Be sure the DC-DC charger gets adequate airflow and less exposure to the elements.
  • Cables from the cranking battery backwards are long and must be carefully routed, and cinched down, ensuring there is no rubbing or exposure to hot surfaces and no chance of snagging when off-road. Cover them with a protective sheath.
  • Use thick 13.5mm cable (not less) throughout to reduce the voltage drop, as thinner cable heats up with current flow, reduces conductivity and makes the charger work harder.
  • Ensure decent, properly crimped cable ends are used, and properly rated fuses are installed in-line.
  • Sturdy connectors such as the Brad Harrison range are best for connecting the solar panels (inputs) and devices like a fridge (outputs), with appropriate colour-coding to ensure the system is easily understood and fool-proof.
  • Spoil yourself with an in-cabin display, so you can check power inputs and the state of charge of all the batteries. Some manufacturers (Victron included) have good apps so you can monitor via your smartphone.

Cost, warranties & brands

As with most electronic devices, features and quality come at a price. A cheap quick-fix will backfire when the system goes on the blink in the middle of the Kalahari and your frozen meat goes green overnight. Rather dig a little deeper and go for known brands supported by a retailer who will play fair. Warranties against manufacturer defects vary from one year, are on average two years, and the best I have seen is five years.

The cheaper units, before installation costs, will set you back from R2 500 to R3 500. Prices rise smartly to R5 000 to R6 500 for the 20-25A units with more features such as built-in solar controllers, and step up to R8 000 to R9 500 for higher-output 40-50A units which can be used for all battery types, handle bigger solar inputs and are adaptable for 12 and 24V systems.

Brand choice is subjective, but it’s worth trawling through the various support websites to read user testimonies, and to take a critical look at the specifications. The stuff from Australia is excellent, and we know those cobbers have done their field testing in hot conditions – think REDARC, Projecta (4×4 Mega World), and Intervolt (Opposite Lock). Sweden’s CTEK is supported by local battery manufacturer Enertec, while motor distributor Ring Automotive brings in the Ring brand from the UK. The Netherlands produces the Victron Energy brand, though they do not yet produce an all-in-one DC-DC charger. The HCDP and WRND brands are from South Africa, as is National Luna which has two units on offer (a 25A and 40A).

Rest easy with REDARC

4×4 Mega World has since March 2021 been the authorised distributor in Africa for the highly-rated REDARC electrical management product range. This market-leading, innovative Australian brand, headquartered in Adelaide, is at the forefront of testing and pioneering the latest in electronic products used in vehicles that use battery power, be it cars, boats, trucks, buses or mining equipment.

The REDARC product range is extensive, covering a range of vehicles and electrical system requirements. This includes Smart Start SBI battery isolators, 12 volt in-vehicle DC-DC battery chargers, pure sine wave inverters, and now the RedVision range of vehicle management systems. Additionally, they offer a range of vehicle-specific mounting brackets to improve the installation process, making it easier and quicker to wire your very compact DC-DC charger. All the BCDC units are dust-, water-, shock- and vibration-proof to ensure they can withstand the rigours of overlanding.

From the newest BCDC range, the options are for 25A, 40A and 50A capacities. These all feature Smart Start technology, have an integrated MPPT solar regulator, and are suitable for both standard and smart alternator types, in both 12V and 24V configurations. The flagship 50A BCDC 1250D is suitable for the spectrum of battery types, including AGM, Gel, standard lead acid, calcium and lithium (LiFePO4).

*All Redarc products are sold in-store and online by 4×4 Mega World. Go to:

*For more on auxiliary batteries, go to:

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