Thread: The best place to monitor battery voltage?

Hi Wil and you have made one of the best research discoveries about battery voltage monitoring and I’m being quite serious.

Many people buy these plug-in voltage monitors without knowing they are next to useless because of the very results you have discovered.

To your question first, you need to take a battery voltage reading right at the battery’s terminals.

The easiest and safest way to do this is to get a length of figure 8 speaker wire, this can be as thin as you can find.

You connect the speaker wire to the terminals on the battery to be tested.

Then you need to fit a small in-line fuse ( 1 amp or less ) to the positive speaker wire, as close as possible to the test battery’s terminal.

Now run the speaker wire to your monitor and the job is done and you will now get an accurate idea of the VOLTAGE at the battery.

Now the reason these plug-in devices are a waste of time.

The cigarette power sockets in a vehicles are connected to the battery using thin wire, that runs from the power socket to the fuse panel then in most cases via either to the ignition switch itself or to a relay controlled by the ignition switch then to a main fuse near the cranking battery and finally connected to the cranking battery.

All this wiring route has numerous junctions and other devices running off the same wiring.

Now, not only does the wiring route cause voltage drop but if any device or number of devices are turned on / in use, you have even more voltage drop.

The end result is that the voltage at the power socket is usually 1 volt or more below the voltage at the battery being monitored.

Like most automotive electrical subjects, there is a hell of a lot more to it but this, I hope, answers your question Wil.

Originally Posted by drivesafe

...Like most automotive electrical subjects, there is a hell of a lot more to it but this, I hope, answers your question Wil.

I was hoping you'd be among the responses to my post! :-) Thanks!
That makes perfect sense. Does it matter much what size wire I run to the monitor? I have some 3mm automotive wire hanging around.

I was hoping the monitor would only come on when I have the ignition turned to accessories. I could achieve this with a relay i guess. Or I could also put a manual switch in. If it were running all the time it might discharge the battery.

Given that there's no easy connector on my -ve battery lead, can I run to the nearby earth point instead and still maintain some degree of accuracy? There's a screw thread on the positive lead clamp, so I can easily attach a cable there using an eye terminal or whatever they are.

Thanks for your input! :-)

Wil

Approximate circuit consumtion is 10mA at 12v. I might be adding a multivibrator based buzzer alarm to it, but the non triggered consumption would remain similar i imagine (I am way over my head here by the way)

Hi again Wil, because you will be powering the monitor off the same wires, a thicker wire is better.

Also, as long as the relay’s coil is powered by the ignition circuit and not the monitoring wiring, all should be well.

10 mA is not that much to worry about having to put a relay on to it. Over a couple of weeks you might drain your battery but if you drive your car a couple of times per month I doubt you will flatten your battery. Car clocks etc are likely to draw mA of current.

Originally Posted by Wil2k

Does it matter much what size wire I run to the monitor? )

No. The wire can be as thin as you like. A voltmeter draws very little current so the voltage drop along the wire will be minimal.

Originally Posted by Wil2k

I was hoping the monitor would only come on when I have the ignition turned to accessories. I could achieve this with a relay i guess. Or I could also put a manual switch in. If it were running all the time it might discharge the battery.)

You mention the current draw is 10mA. That won't be an issue if you drive the car occasionally so you could leave it connected and displaying permanently - especially if the display is an LCD.

Originally Posted by Wil2k

Given that there's no easy connector on my -ve battery lead, can I run to the nearby earth point instead and still maintain some degree of accuracy? There's a screw thread on the positive lead clamp, so I can easily attach a cable there using an eye terminal or whatever they are.)

Yes to both.

Originally Posted by Wil2k

Approximate circuit consumtion is 10mA at 12v. I might be adding a multivibrator based buzzer alarm to it, but the non triggered consumption would remain similar i imagine (I am way over my head here by the way)

What is the alarm for? Monitoring battery voltage? If so, I'm sure Jaycar sell kits for those.

Ron (ex-electronics tech)

Funny, I have had a Jaycar LCD voltage monitor for 10 years in 2 cars.

I monitor both batteries by using a 3 way switch batt A - off - batt B. The LCD monitor is positioned on the console of my 92RRC just below the window switches.

Battery A starting , I connected to the power seats (thick) wire which is live all the time and terminates in relays under the passenger seat. Voltage drop is 0.1 volt compared to battery.

Battery B is the 2nd battery and I tapped into the 8G wire going back to the fridge.Voltage drop nil.
Regards Philip A