At some stage everyone’s deep cycle batteries go flat and fridges go warm. Here’s a easy-to-follow guide to pin-pointing the problem.

If you’ve ever gone for something in the camping fridge and realised it’s either not on, or the error light is flashing, or the temperature’s reading 18°C, you’re not alone. Exactly 98% of campers have issues with their dual batteries at some stage or another, and the other 2% are telling porkies. It doesn’t matter if you’ve DIY’d your 12v setup or paid someone mega-bucks to do it right, corrugations, vibrations, dust, water and mud have a way of introducing electrical gremlins to any 4WD. Before you throw your fridge off a cliff, grab a multimeter and have a poke around, because we promise you that nine of out ten dual battery setup problems are an easy fix!

First things first – let’s check that the alternator is charging properly, because they hate mud. Set your multimeter to measure voltage (the V sign) and have a quick probe of your 4WD battery under the bonnet to make sure all the settings are right and that it is reading something between 13.2V and 14.4V with the vehicle idling. Anything less, and there’s a good chance the alternator is the issue and your deep cycle batteries aren’t getting proper charge. Note that lots of modern 4WDs have ‘smart’ alternators, which only charge when required, so switch on spotlights, air con, the stereo and the like to see if it’s charging or not. If you aren’t getting those voltage levels, trace the big fat cabling from your positive battery down to the positive post on the alternator, to see what the alternator is putting out and rule out a broken or corroded main charging cable. If the volts come up with the revs, it could also be as simple as a loose alternator belt, so turn the vehicle off and check the belt’s tension.

Do the simple things first. Grab some sockets and spanners and check every single fastener, AGM battery clamp and connector from the front of the vehicle to the back. Be careful though – if a connector is loose it will be incredibly hot because of the resistance that occurs when it breaks contact, so gently feel each connector first before giving it a wiggle. Pay particular attention to battery clamps – if they have been over-tightened they can develop cracks that can be invisible at first glance.

Okay, let’s establish that the camping fridge is getting good power now. Take the probes of your multimeter and put em into the two prongs of the 12v fridge plug – doesn’t really matter which way, if you get em around the wrong way it’ll just measure minus voltage instead of voltage but you’ll still get a reading. With the vehicle running you should be getting pretty close to the same reading you got off the 4WD starter battery – if it’s a volt or so less your issue could well be the wiring to the fridge being undersized and causing voltage drop. If it’s substantially less, like down at 11 or even 10V, then your auxiliary battery hasn’t been charging – however you need a load (i.e. the camping fridge) running to properly check voltage drop – what you’re more likely looking for here is a complete break in the power cabling going to your auxiliary deep cycle battery.

It’s pretty rare for a 4WD battery isolator to develop a fault of its own, but it does happen every now and then if you’ve been hammering through mud and dust. With the vehicle running, check first that the input side of the isolator is getting with 0.2V of the charge that the alternator is putting into the starting battery. Make sure you’re picking up a good clean earth with your negative probe or you’ll get falsely low readings. If it’s getting the right voltage into it, then probe the output side to see what voltage it’s putting out. The exact voltage will be dependent on the dual battery isolator – basic voltage sensing relays will put out roughly the same voltage as is input, smart chargers like DC/DC chargers will cycle between full charge for batteries that are low, and lower maintenance ‘float’ charges for fuller camping batteries.

Most of the time a flat auxiliary battery is caused by a break in the wiring somewhere. You can’t rule out the fact the battery is buggered, especially if it has been run flatter than about 10.8V, but the next step is to trace the wiring from the alternator to the auxiliary battery to make sure the voltage and voltage is flowing freely. To do this you use a trick not many people know about – you poke through the wiring with the sharp ends of the multimeter probes at various spots. This allows you to test that the electricity is indeed flowing without having to cut back the insulation, instead leaving only a tiny hole. Make sure you wiggle the connector right into the wiring. If you have no power where you’re checking, keep going up the loom towards the alternator, probing both sides of every single connector until you find the break.

Heat shrink is a great habit to get into, but it can actually hide problems and make diagnosis harder. The less stress you put on a connector when you’re securing it in position, the less chance there will be of a break in the connector. We’ve had soldered and heatshrunk connectors actually crack through the solder, causing an intermittent fault that was a bit of a pain to diagnose. Give each connector a good wiggle while probing just downstream of the heatshrink to check for a break in the circuit. This should give you a good overview of the flow of electricity all the way back to your deep cycle camping battery – or at least where to look if that’s not the case!