Captain Tightarse pinched Brenno’s 80 Series for a quick overnighter the other day, and all was going well until we got a phone call about 8am the next morning. It was old Captain himself, having weasled his way onto a local farmer’s phone (the bugger didn’t want to spend the 50c at the payphone). “Yeah boys, better send out a 4WD with some jumper leads. This bloody piece of junk has a flat battery!”
Turns out the boofhead’s been cranking the stereo all night, and had run the crank battery down to 11.8V. With the 80 back in the shed we figured we’d lob in and add a few easy-to-read battery gauges to the dash so whoever’s driving can see at a glance whether the batteries are charging and what they’re doing around camp. Naturally, connecting them straight to the batteries didn’t sound like our style, so we’ve ‘4WD Actioned’ up this easy gauge install to make it all fool-proof. After all, He Who Shall Not Spend Money’s been hanging around the shed again…
The range of volt gauges out there is mind-boggling. They start at about a fiver each for these digital fellas, right up to a couple of hundred clamshells for slick, high-tech ones. For less than a schooner each we aren’t expecting them to be the most accurate bits of gear in the world, but even if they’re a couple of tenths of a volt out, it’ll still show whether the batteries are charging or in need of some extra juice.
The basic idea is this. You can hook the gauges straight up to the battery, but you miss out on being able to automatically turn them on and off with your ignition. It’s a much better idea to use a mini relay, to pickup power from the battery you want to monitor and output to the gauge. It’s done in exactly the same way as you’d wire up a set of spotties, and has the added benefit of giving a true reading of the battery. If you pick up off the 12v wiring behind your stereo, for instance, every time you crank the volume to hear Beryl’s tips for watering your petunias you’ll compromise the accuracy of the gauge. Running new wiring also by-passes any chance of old dodgy connections that would otherwise interfere with the readout.
Just follow this simple diagram above. Run fused power from your battery into post 30 on the relay. Use a multimeter or a test light to find a wire behind your stereo that has accessories power, and run that into post 86 – that’s your trigger, and that’s what tells the relay to output the voltage readout when you turn the key. Post 85 goes to a GOOD earth (more on that in a moment) and post 87 you run into the positive side of your battery gauge. Earth out the negative side of the gauge, and you’ve now got your gauge working.
We always remember the advice a crusty old sparky gave us years ago – “95% of 12v problems come from a bad earth”. That’s especially true when you’re trying to monitor battery voltage. A poor quality earth will instantly and dramatically reduce the readout on your volt gauge, making it wildly inaccurate. The earth needs to be clean, secure and in a protected spot. Trust us, chasing earth issues later is a pointless waste of time.
Just because we like things really easy, here’s a quick tip. So far, your wiring turns the volt gauge on when the ignition is turned on. But what if you’re around camp and you want a quick instant look at how your batteries are doing without having to fumble around with the keys? Run a single cable from between the fuse and post 30 on the relay, into one side of the switch. Run another single cable from the other side of the switch into the positive side of your voltmeter. Now, if you hold the button down the voltmeter will instantly and temporarily display the battery voltage, even with the ignition off.
Wiring the volt gauge for the second battery is the same as the first, except you obviously pick up fused power from the other battery. A 5A fuse is plenty enough, and how you mount the gauges is vehicle-specific. We ditched the in-dash ash-tray and used a couple of dabs of clear silicon to mount them nice and flush. Now there’s no more excuses for flat batteries!
WHAT IT COSTS
Pair of cheap digital voltmeter gauges: $5.95ea, total $11.90
Pair of mini 12v relays: $8.99ea, total $17.98
Micro momentary switch: $6.99
Misc wiring: $free (from the shed)
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