Voltage VS. Amps.......

Why I am on the subject today; someone of you guys that are knowledgeable about electricity can explain this to me please; Why does my old truck have an "amp gauge" but I check the generator with a volt meter and the battery with a volt meter? Why not a "volt gauge" in the dash? How do I measure "amps" going to a gauge as opposed to volts going to a battery? It confuses me still! Thanks again.

Voltage is pressure. Amperage is flow.

Think of a watering hose with a spray nozzle.

With the water turned on, there is pressure in the hose.

When the spray nozzle is opened, you have flow.

Voltage is measured across a resistance. An open circuit will show a full potential voltage, and zero amperage flow. A short circuit(like a piece of wire) will show no voltage drop and maximum amperage flow.

To measure voltage, you put one lead on a ground and the other lead on a test point which has a resistive element in between, like a light bulb filament. Given enough amperage flow from the supply, you will measure the supply voltage across the gauge terminals. In a car, that would be ~12VDC. To measure the current, you must put the meter leads in series with the resistance so all the flow passes through the meter:

batt+ ---> Meter- ---> ammeter gauge ---> Meter+ ---> bulb filament ---> batt-

When the connection is made, the bulb will light up, and the current passing through the ammeter will deflect the gauge, indicating all the current being delivered to the bulb filament(there is a tiny amount of current used by the meter).

My Expedition has a volt gauge in the dash. An altinator pretty much will not do anything if the voltage is too low. A generator will start to charge from almost zero volts. Take a very long time to charge but eventually will. Let us keep this simple. If everthing in you system is working correctly....an amp gauge will show you that work is being done. After you start your machine the amp gauge will show 5 or even 10 amps. of positive charge. The altinator or stator, or generator are working to top off the battery. Same if you start turning on lites, power clutches etc. The gauge shows a negative till the charge system can catch up. When you measure your battery you are saying yup the battery is good. When you measure and you see 14.2 volts you can say yup the whole charging system is ok. A little better than clear as mud.

Older vehicles, tractors, etc. used ammeters because they were cheap. And they did a good job indicating condition rate of charge (or discharge) of the battery.

Newer stuff indeed uses a voltmeter instead of an ammeter as they are cheaper/simpler to connect (voltmeter wiring does need to be sized to handle full alternator output). And if you know how to interpret it, a voltmeter provides an excellent indication about the state of the battery and charging system.

Ideally you would have BOTH an ammeter and a voltmeter in the dash (my old Kenworth does....) as this provides the most complete information about the state of the charging system. But either meter alone does job.

As an Electricity/Electronics Professor I will explain it like this: First an Amp gauge shows amount and direction of electron flow. Electrons are what come out of a battery negative and into the battery positive when being used to start an engine, or run the lights while the engine is stopped. (This is not related to positive or negative ground, a separate issue.) The reason the electrons move out of the battery is voltage. The 12v battery fully stuffed with electrons, will show 12.6 volts. When amp gauges were used (most of our old equipment, and some new) charging systems were using generators. Generators are speed sensitive, and when at idle, do not do very well. Generators have voltage regulators that are mediocre compared with the transistor and electronic controls now exclusively used for that control. Farmers (and city folks as well could see an amp gauge and know Charge is good discharge is bad. Many early control systems had the operator manually switch to generator output based on the amp gauge, and time at speed. A volt meter on the other hand is less clearly interpreted. Charging should be done at about 14.2 volts. This pushes electrons back into the battery filling it up to capacity. As charge rate changes with charging on a generator system, an operator looking at a volt meter will have much more to think about between 12.6 and 14.2 as to whether the battery is getting charged. With an alternator, and electronic regulation, the voltage when charging will be far more constant and thus show either 14.2 or so, or 12.6 or less if the engine is off, or the alternator based system has failed. Generally new equipment has been created with neither one. This is because operators (in 99.99% of the cases) have no clue what either gauge means at all. Amp gauges are like an ATM Either you are depositing (good) or withdrawing (bad) a volt meter just tell you how much you have on hand. I hope this helps a bit. Jim

The often-used analogy is that voltage is to water pressure as current is to water flow. The voltmeter is like a pressure gauge on your water system and tells you the potential available to move electrons through the electrical system, just a pressure gauge that indicates the potential to move water through the system.

An ammeter is like a flow meter and indicates the rate electrons are moving through some point in the system, just like a flow meter tells the rate at which water is moving through the system.

Ammeters were once popular, probably because they're simple to interpret. "+" is good, "-" is bad. But they require either running a heavy gauge wire into the dashboard, or installing a remote shunt in the vehicle. Voltmeters are a bit trickier to interpret, since the difference between "good" and "bad" is only a volt or two. When charging systems were producing less that fifty amps, ammeters made sense. But as charging systems became more powerful, the need for current shunts made the voltmeter a better choice, since it only requires a single, light-gauge wire. Also, it's tricky to detect a slight discharge on a 100 amp ammeter, while a discharge state looks the same on a voltmeter regardless of charging system capacity.

You know folks, every answer including mine was very scientific and lots of information. Simple answer....it fills up the hole in the instrument cluster. Also, every now and then the needle moves back and forth.

Actually if I (retired electrical engineer and past tractor truck n car dealer and farmer) had my "druthers" for the most and best INFORMATION I would have BOTH a Volt and Amp Meter. In the old days (less current requirements) you ran all the current up to the dash and through an In/Out cheap easy simple ammeter. In newer vehicles you may have a voltage "indicator" or volt meter or idiot light and not have to run high current up to and back from the dash. The easy simple OHMS LAW V = I x R explains the relationship between voltage and current.

John T

I do not know why they originally used ammeters, as discussed most people do not understand gauges, so idiot light is most useful now. I would "guess" that people used to be more aware of what was going on, the ammeter would show if the current is going into the battery, and would show a stuck cutout relay or other draw when off. I personally like the voltmeter, but I am used to it. My dad used to say, "take care of the voltage and the current takes care of itself", so I go by that premise.

(quoted from post at 19:23:10 06/26/18) Why I am on the subject today; someone of you guys that are knowledgeable about electricity can explain this to me please; Why does my old truck have an "amp gauge" but I check the generator with a volt meter and the battery with a volt meter? Why not a "volt gauge" in the dash? How do I measure "amps" going to a gauge as opposed to volts going to a battery? It confuses me still! Thanks again.

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A volt meter would also work "in the dash" and some people prefer them over the amp meter. A volt meter can show you the charge state of your battery as the key switch is turned on, before starting the engine. Once the engine is running the volt meter reading in excess of the battery's "full charge level" would indicate charging.

An amp meter shows the amperes going to the battery (charging) or the amperes going from the battery (discharging). However, it cannot indicate the charge level of the battery.

John, I had annual electrical safety training at my former job and answered "What is Ohm's Law?" with V=IxR and was corrected by the instructor. He said it's actually I=V/R. A minor point, I know, but I thought it explains the relationship a little more clearly. Also learned in my time in R&D that Ohm's Law does not apply to a welding circuit. But I can't explain why.

Hey thanks... good to know this.

I will study this....thanks for the help.

Great....thanks!

One more to add. Have a couple of these from Ebay. Get the one you see in the picture, they work very well and are reliable. Just stick it in the socket and leave it 24/7.

You can equate electric to a dam. The height of the dam is the 'potential', or voltage. That potential flowing thru the turbines is greater with the height of the dam. The 'flow' through the turbine produces 'power'. The higher the flow, the greater the power. That flow is like amperage, and the power is the wattage or output of the machine. That might make it a bit easier too understand....

One more to add. Have a couple of these from Ebay. Get the one you see in the picture, they work very well and are reliable. Just stick it in the socket and leave it 24/7. Another photo of that one and the built in type in the bottom of my ashtray. That green color is terrific. Shows up even in daylight.

Ok, for all you 'experts' concern about running all the current up to the dash, into the ammeter and back to the loads, this is a fallacy. The only heavy current for the ammeter needed is the known resistance shunt bar. The tiny wires for the EMF sensing the current can be in the dash, while the shunt bar is anywhere you want.

The best setup would be the shunt bar just in front of the +batt terminal. With the key off, and circuit 30 and 15 not powered, the shunt bar would have no current flow, so no EMF produced across it to deflect the 'ammeter' in the dash. When the key is on, and the 30 bus is energized, there would be a very slight deflection to negative on the ammeter, showing the key loaded currents. AFter the engine is running, and the gen/alt is charging, it will show a slight + indication as the batt is charged.

So, there's no need for all that heavy wiring, just the wire from the batt to the loads needs to pass through the shunt which could be mounted anywhere, preferably near the batt. The current to drive the 'ammeter' remotely is in the microamps.

Well at least he didn't ask about grounding.

Mike, in EE 101 the first basic DC circuit course I took at Purdue University, Professor McGillen walked in the class and said in a loud voice

VIRGINIA EQUALS RHODE ISLAND IE V = R I He said we will never forget that and he was right lol

John T

But if he had I know the answer LOL

John T

Hello banjoman09,

The short answer is: amperage reading is a flow of current to the battery. Voltage reading is the battery state of charge, or potential,

Guido.

(quoted from post at 21:03:40 06/27/18) Hey thanks... good to know this.

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Whut
Janicholson
said..

Its the short and sweet version with out a book being writing on it..
Degree holders in electronics don't get it and will beat you to death with there degree cuzz they are stuck on amps as the holly grail they still don't get it tho...

Volts are the first thang you check and the first thang you need to know... Amps come later if need be what folks make up chit and depend on amps I dunno show me the volts on a loaded circuit...

> John, I had annual electrical safety training at my former job and answered "What is Ohm's Law?" with V=IxR and was corrected by the instructor. He said it's actually I=V/R.

Well, strictly speaking, it's not even a law; it's the definition of resistance. (Newton's Laws of Motion are examples of scientific laws.) So, it might be better to say "R = V/I". Makes no difference, they're all mathematically equivalent.

> Also learned in my time in R&D that Ohm's Law does not apply to a welding circuit. But I can't explain why.

Well, it does, but in practice Ohm's Law only covers resistance and not inductance or capacitance. To properly describe a welding circuit you must include the inductive elements. And because welding is seldom steady state, you need to look at things in the time domain, which complicates matters quite a bit.

thank you....all of this helps me alot.