Dual volt meters

[davepl]

If I understand the way out factory ammeters work, all they do is compare the reference voltage at the harness to the voltage at the main battery junction. If there's more at the harness, it reads as charging, and if there's more at the battery, that shows up as discharging.

With some debating whether an ammeter or voltmeter is better, I thought to myself "both", but what would be even better than both would be two volt meters!

If you had two, one connected at the alternator or dash harness and one connected at the battery you could read both voltages and you'd be able to know what's happening not only with the battery but also with the alternator and the state of charge and where current is flowing.

Taking it a step further, since no one wants two volt gauges, is a single gauge with two needles.

When both needles are high, all is good
When both needles are low, system is broken
When needles are apart, system is either charging or discharging

Ideally the gauge could present the area between the two needles as a span of color, like green for charging and red for discharging.

I'd buy one... but would anyone else? I think it'd tell you a great deal more about the charging system than our current single-needle gauges.

[Jack_71C10]

I'm pretty sure both volt meters would always read the same.
Even though its a battery and an alternator, they are still one circuit.

[davepl]

Originally Posted by Jack_71C10

I'm pretty sure both volt meters would always read the same.
Even though its a battery and an alternator, they are still one circuit.

That's why you need two! They're not the same (which is how ammeters and idiot lights work) because of voltage drops in the circuit.

The two needles would show the biggest voltage spread and which direction it was in, so you'd know at a glance how loaded the system is, etc.

Of course I could try it with a voltmeter, which I think I shall when it cools off a bit!

[dfwright]

The voltage differential which causes the ammeter gauge displacement is so small, that your voltmeters wouldn't be sensitive enough to register the difference. I would think it's probably down in the milli-volt levels. As stated, your volt meters would read the same.

[CCCC]

If you want to test the idea with a single-needle voltmeter, just use a switch to change the read.

[dmjlambert]

An ammeter is a volt meter that has essentially been shorted with a shunt, which is just a piece of wire. In our trucks, the shunt is the 12 GA wire that goes between battery and alternator. To get some good info on the dash you could probably design an Arduino circuit to analyze the voltages at the ammeter and display stuff like voltage of the battery and discharge or charge rate. Perhaps even add another wire at the starter to read the amount of current used for starting. An electronics geek would use something like this.https://www.sparkfun.com/products/12923

[VetteVet]

It's simple enough to test, all you need to do is use a multimeter in conjunction with the trucks voltmeter and compare the voltages between the two at the same connection and then move the multimeter to any other location you wish to read, I suspect that you won't see much difference.

[Andy4639]

Originally Posted by VetteVet

It's simple enough to test, all you need to do is use a multimeter in conjunction with the trucks voltmeter and compare the voltages between the two at the same connection and then move the multimeter to any other location you wish to read, I suspect that you won't see much difference.

The voltage reading should be pretty much the same all over the circuit no matter. If all things are equal. The amp draw per all things is different so that is why you have the differences when running a amp gauge only. It's looking at load on the system where as a volt reading is looking at over all voltage only. If that makes sence!

[davepl]

Originally Posted by dfwright

The voltage differential which causes the ammeter gauge displacement is so small, that your voltmeters wouldn't be sensitive enough to register the difference. I would think it's probably down in the milli-volt levels. As stated, your volt meters would read the same.

That I will have to test. I know the difference is big enough to be amplified and shown on the ammeter gauge as a reading, but you're right, it might still be tiny. Will have to get out my multimeter and see.

I'd need at least a 1V differential to make it useful with two needles, likely.

[VetteVet]

Originally Posted by Andy4639

The voltage reading should be pretty much the same all over the circuit no matter. If all things are equal. The amp draw per all things is different so that is why you have the differences when running a amp gauge only. It's looking at load on the system where as a volt reading is looking at over all voltage only. If that makes sence!

Hello Andy, how are you.
since you quoted me I'll return the favor. As you know there will be voltage drops in the circuit downstream from the alternator that could be as high as 2 volts. Fortunately if the alternator is properly wired as yours and many others on the forums are, the alternator will compensate for that. It's output should always be higher than the battery voltage or it wouldn't be able to charge the battery or maintain the draw on the system. VV

[davepl]

Originally Posted by VetteVet

Hello Andy, how are you.
since you quoted me I'll return the favor. As you know there will be voltage drops in the circuit downstream from the alternator that could be as high as 2 volts. Fortunately if the alternator is properly wired as yours and many others on the forums are, the alternator will compensate for that. It's output should always be higher than the battery voltage or it wouldn't be able to charge the battery or maintain the draw on the system. VV

Aye, and there's the rub - it's that differential that the alternator is trying to make up from it's end that I'd like to capture and display.

Just like hydraulic pressure is the same everywhere in a closed system but varies once you start opening things, electrical voltage (like a pressure) is the same everywhere in a static circuit but varies when there are current drains (loads) open.

You could be 12V at the lighter socket and 15V at the back of the alternator, at least with a 3-wire alternator. But I actually don't know if the differences get anywhere near that large, that's what I need to test yet.

[Bigdav160]

The difference in the two voltmeter readings is just a reflection of the resistance in the circuit between the two points. It doesn't help with diagnosis of the battery or generator.

[Tiger Joe]

FWIW, I installed a 3 gauge autometer pod under the heater controls on the dash.

Its been many years since I put it in so I don't remember where I tapped into power from. Truck has a 1 wire alternator on, and I have 14v at the battery, and stopped with the lights and radio on, I can drop as low as 10v if I have everything on (radio, brake lights on stopped, high beams, and turn signal.)

I know there is nothing wrong with the truck or alternator, because I checked charging with a multimeter several times, was just a poor choice to tap it in on my part!

[davepl]

Originally Posted by Bigdav160

The difference in the two voltmeter readings is just a reflection of the resistance in the circuit between the two points. It doesn't help with diagnosis of the battery or generator.

Please revisit my earlier post; I enumerate a number of scenarios where you can use the delta in voltage to understand what is currently happening in the charging system, the state of the battery, which way current is flowing, and so on.

Besides, the delta in voltage is all the ammeter tells you, so if that's not useful, the ammeter was a pointless gauge. I disagree.

[Andy4639]

VV,
doing great man. Just took my dad who is 91 and bought a 56 Bel Air this past Saturday!

Yea the system is going to make up the drain if everything is working as it should. My 6.0 LS alternator is putting a strong 14.5 out at the volt gauge in the dash even when I have the lights on and AC with fans it never drops below 13 volts.
davepl,
I sorta have what you are wanting to do.

In my dash cluster I swapped over to a volt gauge.

I also have the scan gauge 2 hooked up to the PCM with a volt reading. They are always the same reading as far as I can tell. The dash is a needle and the Scan gauge is digital. They basically read the same.
The dash is tied to the unfused spot in the fuse block so it to me gives me a system reading where the scan gauge reads off the alternator through the PCM.

[davepl]

I went out and tested my crazy theories, here are some numbers. This is with a bunch of load on (AC on medium so no relay, lights all on, etc).

At the lighter: 12.5V
At the battery: 12.85V
At the Aux battery: 12.95V
At the Alternator: 13.5V

Actually makes me think my wire from the alternator needs to be upsized. Almost a full volt of drop.

So, you can indeed find measurable differences, up to a full volt in my case, and that's when things are operating normally.

My thinking then is the closer the two needles are together (which is the same as saying the closer the ammeter is to the middle, really) the less is going on. When they are spread, you're seeing how far the ammeter would be from center, and the order of the needles shows you whether its charge or discharge.

So now that I think it through, I don't think it tells any more than a combination of a voltmeter and an ammeter would, really. Just in a different way. I think it'd be neat, but obviously combination volt and amp meters haven't taken the world by storm, so likely no demand...

[Andy4639]

Originally Posted by davepl

I went out and tested my crazy theories, here are some numbers. This is with a bunch of load on (AC on medium so no relay, lights all on, etc).

At the lighter: 12.5V
At the battery: 12.85V
At the Aux battery: 12.95V
At the Alternator: 13.5V

Actually makes me think my wire from the alternator needs to be upsized. Almost a full volt of drop.

So, you can indeed find measurable differences, up to a full volt in my case, and that's when things are operating normally.

My thinking then is the closer the two needles are together (which is the same as saying the closer the ammeter is to the middle, really) the less is going on. When they are spread, you're seeing how far the ammeter would be from center, and the order of the needles shows you whether its charge or discharge.

So now that I think it through, I don't think it tells any more than a combination of a voltmeter and an ammeter would, really. Just in a different way. I think it'd be neat, but obviously combination volt and amp meters haven't taken the world by storm, so likely no demand...

I'm assuming this was done on a factory setup correct?
Mine isn't factory by no means. I have relays installed on everything that I think puts a heavy load on the system. Lights, AC, windows, and the motor harness has them along with fuel pump.
So my readings would be off from your no doubt.

On a stock harness I can see the drop on the voltage with all the stuff turned on.
The bigger wire would help I'm sure.

[davepl]

My headlights are HID and have transformers, like your relays, so a lot of my draw is up front and not in the cab itself too.

In my other vehicle when I wired up my LS motor I ran it as a 3-wire but I'm still tempted to cut the harness and run the voltage sense line to the factory location in the car, not just to the ECM harness. But it all works now so not going to mess with it.