Amp vs volt gauge

Lot of y'all have mentioned using a shunt.

Anyone want to elaborate?

Dusterbd13-michael said:

Lot of y'all have mentioned using a shunt.

Anyone want to elaborate?

a shunt is a piece of known-resistance wire that can handle the full amperage of the system. You measure voltage on both sides of the shunt and V=IR. since R is known, V is measured (the difference between the two sides of the shunt), you can calculate how many amps are going through the shunt. 

That way the amps go through the shunt, not the gauge. Best part is that you can use anything as a shunt, as long as it's resistance is known and constant. A large piece of wire like a battery cable usually works. 

So, why couldn't an amp gauge be hooked between alternator output and positive battery terminal? That way the stock wiring takes the load, and the gauge just reads it. Or does it not work thatway?

It could, if the voltage difference was large enough to be accurately measured by your meter. It depends on the size and length of the alt-batt wire. 

Dusterbd13-michael said:

So, why couldn't an amp gauge be hooked between alternator output and positive battery terminal? That way the stock wiring takes the load, and the gauge just reads it. Or does it not work thatway?

That's exactly how an ammeter works.  The issue is that an ammeter is a series instrument so the wire that goes from the alternator to the battery has to pass through the gauge so you have to run a large high current carrying wire into and out of the cockpit.  There are marine and industrial ammeters that mount the shunt (the part of the gauge that has to be in series) remotely but I'm not aware of any automotive ones. I suspect that's because a voltmeter is just as useful for telling you that you have a problem as an ammeter.  Sure an ammeter might be helpful for diagnosis or maybe even let you know you have a problem sooner but a voltmeter combined with a charge indicator light (or perhaps more accurately, a not charge indicator light) is just as effective at indicating that there's an issue.

Dusterbd13-michael said:

So, why couldn't an amp gauge be hooked between alternator output and positive battery terminal? That way the stock wiring takes the load, and the gauge just reads it. Or does it not work thatway?

If your electrical load is taken from the battery, the only thing you are learning from that is how much the alternator is producing.  If your alternator can create 40 amps at idle, but your car is consuming 60 amps, you are discharging the battery, but the ammeter shows you are charging.  That's why a voltmeter is better in a car.  More load out of the battery than in shows a drop in voltage.

I was pretty convinced before this thread that the ammeters in my gauge box were useless, and should be scrapped unless very cool vintage. This thread reinforced the safty/functionality of the volt gauge over the ammeter. 

But i learned stuff!

In reply to Curtis :

Magneto is self generating.   No gauges   Usually have two for safety

An ammeter will give an indication of the state of charge of the battery.

 A voltmeter does not.

I am wondering what the output of those GM amp clamps is, and how easily a voltmeter could be recalibrated in amps.  WAY better than a shunt, probably.

The amp clamps on modern cars and trucks are just used in their battery life remaining calculation, which is generally wrong.

My final word: If ammeters work better than voltmeters, why is the last vehicle that used one from the factory built in the 70s?

I'll drive a car from here to Cleveland and back with a bad battery. What I want to know is what my alternator is doing :) = ammeter.

In reply to Streetwiseguy :

Nobody said that.  They do different things.  Voltmeter was probably cheaper.   Now we don't even have that.

I've got a hand held ammeter that works by inductance. I suspect the same sort of device could be made into a dash gauge. Ammeters are better as diagnostic tools, a volt meter is a better choice for the dash, but a simple light is really all that is needed.