Dead Easy Digital Battery Monitor!

Once upon a time it used to be really hard to accurately monitor battery voltage. You needed an LCD or LED panel meter – and then, to make the meter work, you needed a separate isolated power supply. That is, the battery that powered the meter couldn’t also be the voltage source you were monitoring. And in turn, that meant overcoming a lot of complexity to produce a working system.

But now you can forget all of that stuff.

The wonders of low prices and high technology mean that you can buy an LED digital panel meter that accurately and rapidly displays voltages from 7 - 15V... just by connecting the leads of the meter to whatever battery you want to measure! And even better, such a meter now costs you under AUD$20 – delivered right to your letter box!

Uses

So what use is this type of meter then?

Well, if you’re charging a battery, you can read actual battery voltage, switching off the charger when the battery reaches the right voltage level. That allows you to use very cheap chargers (like we covered at Dirt Cheap DIY Battery Charger and Zero Cost Trickle Charger) without the risk of overcharging.

If you’re into running your car sound system long and hard with the engine switched off, you can see exactly how healthy your battery is. That way, when you turn the key, the engine will still crank...

You also can remote-mount the meter near a big car sound amplifier, so that you see the real-time voltage drop in the power cabling to the amp.

If you’ve got a 12V electric bike, you can monitor exactly what juice you’ve got left in your battery – much better than the ‘3 LED fuel gauge’ approach used on many electric bikes. (Or if the bikes uses multiple 12V batteries, you can monitor the health of each battery.)

Or you can mount the meter in the car cabin, so letting you use the display as a normal car instrument to check on alternator and battery health – much more accurate than a traditional analog voltmeter.

If you run a 4WD with twin batteries, you can run two of these meters and always see exactly what battery levels are. And, talking about this sort of application, if you’re into car camping, where the car battery runs lights and maybe a portable fridge, you can use the meter to tell you exactly when it’s time to switch off for the night.

Finally, you don’t need to run the meter all the time. By putting a simple switch or pushbutton in the circuit, you can turn on the meter only when you want to.

The Meter

We bought the meter from VirtualVillage.com through their eBay shop. (Do a search under the ‘stores’ category of eBay.) The cost of the meter was just AUD$10.99 (‘buy it now”) with international postage another AUD$6.99 – a total of under AUD$18!

As described above, the meter is powered from the voltage source it is measuring. In addition, it will work across the range of 7-15V (ie it has an inbuilt voltage regulator to power the panel) and it draws little current – only 42 milliamps at 7V. It has a rapid update rate and displays the voltage to two decimal places (eg 12.34 volts). This resolution is particularly good if you want to see a trend – is the battery voltage rising or falling?

So how accurate is the meter? We compared readings with an expensive Fluke multimeter displaying to three decimal places and found the LED meter very good – for most uses, sufficiently accurate straight out of its box. Furthermore, there is an adjustment pot (arrowed) on the back of the display and by a very slight adjustment we were able to make the LED meter agree with the Fluke over the full 7-15V range.

Note that the LED display is not visible in direct sunlight, so this needs to be kept in mind when making mounting decisions.

Wiring

There’s almost nothing to the wiring. Just plug the wiring harness into the socket on the back of the display and then connect the red wire to positive and the black wire to negative (ground).

If you want the meter to come on with the ignition switch, connect the meter to a power supply energised only when the key is on – eg the radio or cigarette lighter wiring.

If you want the display to be manually switched on and off, put a switch in the circuit – eg a ‘normally off’ momentary pushbutton switch.

Conclusion

The good thing about this project is how simple it is. Mount the meter and connect two wires and you’re done... it’s that easy.

And it might be easy, but the real-world usefulness is fantastic!