Workshop: Black Box – Part 2


Words Martin Pretorius

In Part One of this article we investigated the different types of automotive batteries, and figured out the ideal combination of batteries to support both your vehicle’s operating demands and your need to keep something frosty in the fridge.

This instalment will look at the supporting systems needed to make a dual-battery system work, the best way to charge those batteries, strategies to conserve whatever battery power you have, and revisit alternative charging techniques.

Get connected

Our last discussion about battery cycling shows you’ll need a deep-cycle battery to power the fridge and a few campsite lights, in addition to the starter battery already on duty in your 4×4. You’ve also calculated the energy storage capacity you need from the secondary battery by applying some simple mathematics, and your friendly retailer furnished you with a good quality unit from a reputable brand.

How to stay in charge

How best to install that battery? Finding a secure spot and physically mounting it is tricky enough, but connecting it to your vehicle’s electrical system seems even more complicated than you imagined: it’s a bit more involved than simply laying extra cables to the alternator and an earth point somewhere on the body (or to the original battery’s terminals). Fortunately, there’s a multitude of specialised accessories to help you get the most from a dual-battery system, with modern control electronics simplifying battery power management even further for novice and expert alike.

Technology is your friend

The simplest way to keep the starter battery separate from the deep-cycle battery is by means of a simple kill switch. Twist it one way to complete the circuit, twist it the other to break the connection. But this method is a little crude: we all enjoy life’s little conveniences, especially if it can automate the dual-battery system’s operation. This is why the old-fashioned solenoid-type isolator was created, a design which still remains the most affordable way to disconnect your vehicle’s native electrical system from the secondary battery when the engine isn’t running.

Solenoid isolators separate the main (starting) battery from the secondary battery system by opening or closing an internal electrical contact, and are activated by an electric input (usually power from the ignition system’s main power supply).

When the vehicle is turned off, the connection between the starting and the secondary battery is severed. The starter battery stands alone, ready to be used only by the vehicle’s control electronics and starter, while the secondary battery gets on with the business of powering any assortment of fridges, inverters and lights – and get drained in the process. This means that the engine will still start in the morning, even if your breakfast bacon has defrosted somewhat overnight.

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