Voltmeter question

[DiscoDavis]

Have VDO voltmeter in my last blank spot on the dash. I pulled 12v feed from the next gauge over, the water temp. Was fine for months, when it was first hooked up I found it to be a bit off on the actual voltage, so adjusted the little screw slot on the side to get it to read correctly. All fine

Since then, I ran a wire directly from the gauge, to the fuse box on the aux battery as always hot (on a 5A fuse). Same issue, was a bit off, adjusted a few times to get it roughly right again. Was kind of wanting to have it display the aux battery voltage, but also both battery voltages together as they are hooked together when vehicle runs. What is weird is that it is not accurate when running vs sitting (and vice versa).

Multimeter: Both batteries read ~13.9V @ idle, ~12.9V engine off

Gauge: after adjust, reads ~13.9V @ idle, ~13.2V engine off...

Should I be doing all adjustments with as little electrical load on the batteries as possible? IE headlamps off, gauge lights off etc. It was pretty accurate when wired to 12V feed from temp gauge, I just wanted it to be on all the time vs having to switch on the glow plugs and ign circuit just to see what battery volt was.

Doesn't need to be perfect, all I care about is knowing I have enough volt to start car, run winch, run all the halogens etc.

 

[RBBailey]

My new VDO was showing approximately 1.3 volts too high versus three other meters I tested at the same spot.

So I'm curious to hear if you come to some conclusion on this.

 

[nas90tdi]

A automotive volt gauge tends to not give you the accuracy of a good RMS voltmeter. It just isn't really designed to be that precise. You are fighting a losing battle trying to get it to line up perfectly with a voltmeter.

And, yes. If you are going to adjust the gauge, this is done with no load. Load on the battery will vary as amp load fluctuates with accessories on. Especially things like glow plugs. As the temperature of these things change, so does the resistance involved. So, voltage/amp draw fluctuates along with it.

 

[Napalm00]

Ask Adam Santa about this

 

[RDavisinVA]

Your description was pretty unclear, but having the voltmeter on all the time will drain the battery if that's how you are wired.
You can use a relay to transfer power to the voltmeter when the key is switched on to avoid having the gauge connected all the time.

 

[jymmiejamz]

When you say "a bit off" do you mean low? You could have a voltage drop issue that you have adjusted out for running, but that is effectively just causing the gauge to read low. I would put one lead of your DVOM on the positive terminal of the battery and the other lead on the positive terminal of the gauge and see what the reading is. Ideally you should see 0V-.5V. Anything higher and you likely have a wiring issue. You can do the same on the ground side.

 

[DiscoDavis]

Thanks for the feedback. Dug a bit more into it,

Added a switch to toggle the gauge on and off, did a basic check with a multimeter to see voltage at the gauge leads, and from right next to the battery, those match. Now the gauge is more or less adjusted to match the voltage within ~0.1V margin, so driving around it reads just at ~14V where a multimeter would say 13.9V. Will adjust some more (with minimal load)

It seems a bit slow to register voltage drop compared to when it pulled power from the gauge next to it, IE when I used to cycle the glow plug circuit on, you could watch the needle drop down, then return slowly as the circuit clicked off. As it is pulling feed from the aux battery (they are still keyed in parallel while operating), it seems to be a bit dampened. The toggle switch helps to double check that it is even registering a change sometimes.

Primary battery has
>Starter lead
>alternator lead
>Winch power
>Roof lights power

Aux battery
>12 slot blue sea fuse box
>>voltmeter pulls power from the fuse box, good sized wire, grounded to gauge cluster, on a 5A fuse

Batteries are keyed together with a solenoid and a toggle switch controlling that solenoid, so when separate the voltmeter only reads the one aux battery.

 

[DiscoDavis]

Your description was pretty unclear, but having the voltmeter on all the time will drain the battery if that's how you are wired.
You can use a relay to transfer power to the voltmeter when the key is switched on to avoid having the gauge connected all the time.

Now have it on a switch, wanted it to be dual purpose so you could power it without anything else, see what say a fridge or a few light strips were doing to the battery in a camp or stopped setting.