School me on how an ammeter works

[Richard]

I've got an International. Put new wiring harness & gauges on/in it. The Ammeter is a new thing to me. I get that it's showing something like the system is discharging or charging..... but I'll lengthen the question.

Normally when I have tractor running, ammeter is at 'zero'. If I turn some/all the lights on, it will drop down to say, the first hash mark to the left, under "D" which as I understand means the system is discharging (??)

Question here, what I don't understand is, if the alternator is providing "X" amount of volts and the (LED) lights are energy efficient, why does it go into "D" when they are on? (presuming batteries are good)

Now, to flip it....other day I noticed that my alternator belt was floppy loose (so maybe it WASN'T charging as well as I had thought!)

Anyway, tightened the belt and fixed a flat tire. Took it out to dry run and now, the ammeter is slightly above zero but not yet at the first "positive" hash mark.


I would have thought that you'd discharge a lot of energy to start the tractor, ammeter might show in the negative/discharge section BUT, as the tractor runs and the batteries charge back up, I would expect the dial to slowly move towards 'zero' indicating a full charge.

I guessing there's something right in front of me that I'm missing. (I've done some googling too but so far don't seem to find anything that explains above)

What am I missing?

(side comment, if the tractor sits for multiple days, I usually have to jump start it. I've checked for drains and thus far, can't find any)

 

[WranglerX]

A ammeter is a simple device that indicates current flow..... To work proper it needs to be connected between battery and generator/alternator and all system power must go though it.... If it shows a discharge then generator/alternator is not putting put out enough current to sustain the battery charge and system power demands, and more power is being drawn from battery than generator/alternator can province... IF ammeter is showing a charge them mean the generator/alternation is able to sustain amperage/voltage load of all device turned on and requiring power and generator/alternation is function properly and can sustain vehicles electrical system demands without having to draw on battery.... You have to think of battery as just a starting (storage) energy provider and is used mainly to initially start vehicle..... While generator/alternate is main power supplier for electrical system and ammeter indicated if it doing its job or not by showing charge or discharge....

In more recent times a voltmeter has replaced ammeter as it does better diagnostic of system condition... A nominal reading of about 14 volts is indication generator/alternator is suppling enough voltage/amperage for systems electrical demand (same as charge on ammeter) and if voltage of system is dropping and going down to lower voltage (12.8 or 13 volt or lower) if shows alternator/generator is not functioning properly and can not sustain system power demands and is drawing from battery to sustain the systems power demand.......

Sort of rudimentary, but shows paths of current flow in system using a ammeter vs a volt meter... Done for VW/Dune Buggy community but is typical of almost all electrical system using ammeter...

 

[TerryR]

Normally when I have tractor running, ammeter is at 'zero'. If I turn some/all the lights on, it will drop down to say, the first hash mark to the left, under "D" which as I understand means the system is discharging (??)

Question here, what I don't understand is, if the alternator is providing "X" amount of volts and the (LED) lights are energy efficient, why does it go into "D" when they are on? (presuming batteries are good)
.....

I would have thought that you'd discharge a lot of energy to start the tractor, ammeter might show in the negative/discharge section BUT, as the tractor runs and the batteries charge back up, I would expect the dial to slowly move towards 'zero' indicating a full charge.

Your analysis of what should happen seems correct to me, with one minor exception. I don't think you would see the actual draw of the starter as it's much too high a current to run by the meter, so the starter cable would bypass it. But then you should see the generator/alternator re-charging the battery afterwards.

The behavior you describe, including that the battery discharges is a few days, suggests an issue with the system.

 

[CausticUrbanCoast]

A voltmeter is a very poor replacement for an ammeter, its results are deceptive.

An ammeter uses a shunt in parallel with a power resistor to measure the current flow through the shunt.

A voltmeter simply measures the voltage difference or voltage potential between two points. This is especially problematic with batteries as what it shows you is the voltage potential only, not the capacity or capability to deliver any power.

An ammeter informs you of the capacity, capabilities, and condition of the battery if you understand what it is telling you!

In your initial case, your ammeter is telling you that your battery is either under sized for your tractor or that your alternator is not able to regenerate fast enough to keep up with your system AND charge your battery (this means either your alternator belt is slipping, the alternator is starting to fail, or your battery is failing - providing it is correctly sized).

 

[WranglerX]

Your analysis of what should happen seems correct to me, with one minor exception. I don't think you would see the actual draw of the starter as it's much too high a current to run by the meter, so the starter cable would bypass it. But then you should see the generator/alternator re-charging the battery afterwards.

The behavior you describe, including that the battery discharges is a few days, suggests an issue with the system.

Correct, starter current are no included into ammeter circuits, system usually just indicates battery and generator/alternator functionality (charge/discharge) ...But may show a discharge while starter is being cranked because of amperage used to engage solenoid in being dawn through ammeter...

 

[Henro]

A voltmeter is a very poor replacement for an ammeter, its results are deceptive.

An ammeter uses a shunt in parallel with a power resistor to measure the current flow through the shunt.

A voltmeter simply measures the voltage difference or voltage potential between two points. This is especially problematic with batteries as what it shows you is the voltage potential only, not the capacity or capability to deliver any power.

An ammeter informs you of the capacity, capabilities, and condition of the battery if you understand what it is telling you!

In your initial case, your ammeter is telling you that your battery is either under sized for your tractor or that your alternator is not able to regenerate fast enough to keep up with your system AND charge your battery (this means either your alternator belt is slipping, the alternator is starting to fail, or your battery is failing - providing it is correctly sized).

Using a shunt to produce a voltage proportional to actual current flow is a time proven method. BUT not the only one. Some ammeters use the force resulting from DC current flow through a coil to cause rotation of the needle against a spring to give the indication of amount of current flow.

So one cannot really day there is only one kind of ammeter.

 

[bcp]

I'm wondering what would happen if his ammeter was accidentally wired like a voltmeter.

Bruce

 

[RalphVa]

Been a long time since I've had an ampmeter in a vehicle. As I recall, their indication jumps around a lot. Probably why they cut them out because they cause more alarm than help in many cases. Doesn't sound as though your alternator is charging as it should.

 

[Richard]

I'm hoping that when I tightened it, things will improve. Won't really know until I cut a field and put some time on it
Side comment, the new alternator is larger than original and batteries are now maybe two years old . I have two skinny 12 v batteries in it and new harness.

 

[WranglerX]

And ammeter is just a real sensitive volt meter that measure the voltage differential across a shunt (resistance), the shunt can be internal to meter if measuring small amount of amperage or external for large loads...

Have to object to comments that volt meter are useless, as they give actual condition of electrical system in any given instance, IF you know how to read it....