If you’ve recently installed a dual battery system into your 4WD, you’ve now opened up access to a whole range of 12v modifications that will massively enhance your camping experience. Whether it’s running LED camping lights, a portable fridge, an inverter to charge the kids’ DVD players or any other electrical camping gear, with a dual battery you can power it all without risk of running your main battery so flat that you can’t start your car in the morning.

Many campers install some sort of voltmeter at the same time as a dual battery system so that they can monitor their auxiliary deep cycle battery’s voltage and know how much it has in reserve, and when it’s time to recharge it. A voltmeter can be as simple as a basic LED meter, as complicated as a high-tech digital meter or even just a multimeter that you connect to the battery when you want to check voltage levels. However you monitor your deep cycle auxiliary battery’s voltage, it’s important to understand what it means because it can in fact be quite confusing. What’s worse, is if you let your deep cycle AGM battery drop below a certain point, you can damage it internally to the point where it can’t be recharged again. So how do you know when that point is?

When your engine is running and your alternator is charging, it will typically be putting out somewhere between 13.5V and 14.4V. Back in the days of old school mechanically-controlled engines with alternators that were simply driven off the fanbelt, you knew that if your alternator was putting out between 14.0V and 14.4V that it was healthy and in good condition. If when the engine was revving, it was putting out less than 14.0V it was an indication that the alternator was worn out or damaged in some way. These days it’s a lot more complicated owing to ‘smart’ alternators that don’t always put out a steady rate of charge in an effort to reduce load on engine and thus emissions. That’s where a DC/DC charger comes into play but that’s a topic we’ll discuss at another time.

When you’re parked up at your favourite campsite, and you’ve got all your electrical camping gear setup, you should monitor your auxiliary deep cycle battery voltage every hour or so. On hot days, your battery’s electrical capacity will be diminished since heat has an adverse effect on electrical systems. This is doubly so simply because you’ll be opening your portable fridge more often for a cold drink, causing it to work harder to keep the contents cool!

When you first turn your vehicle off, assuming there’s no load on the electrical system at all (i.e. you’re at home in your driveway and your fridge is off), a healthy auxiliary deep cycle battery should sit somewhere between 12.6 and 12.8V. As you add more load to the system the voltage will instantly decrease, as voltage isn’t strictly the best way to measure overall battery capacity. However it can give a general idea of how much capacity is left in your battery.

To measure proper battery voltage, first remove any load from the battery. That means turning off the camping fridge (it’ll be okay for 10 minutes!), the LED lights, etc. Then let the battery sit for about 10 minutes to let the voltage ‘settle’ before taking a reading.

At 12.4v, your auxiliary deep cycle battery is down to 80% capacity. That’s pretty standard sort of voltage to see when you’re parked up at camp with the fridge running for an hour or two. Most portable fridges draw somewhere between 3 and 5 amps per hour. By 12.0V, however, your battery is already down to 50% capacity, and by 11.8V it’s at 30% capacity. At this stage you need to recharge your deep cycle battery in some way, whether that’s with solar power, with a camping generator or by going for a drive. If you let your battery drop much lower than 11.8V then you risk causing internal damage that will prevent it from recharging properly. If your battery ever drops to 10.5V, then that’s what is generally accepted as 0% capacity and indicates that the battery is dead flat. There’s a very good chance that at that level you won’t be able to recharge your battery again, and it’ll be ready for the scrap pile. Keep an eye on your deep cycle battery’s capacity with a voltmeter and you’ll be able to keep the fridge cold and the lights on when you’re at camp!