Voltmeter or Ammeter, which is more useful?

Assuming that my understanding of the function of both of these gauges is correct...
  • Ammeter will show current usage, i.e. with A/C and headlights on, current draw will increase resulting in a higher drain on the alternator and resulting in engine out put shifting from turning the wheels to turning the alternator
  • Voltmeter shows either voltage on the battery or output of alternator depending on running state of the engine.
Seem to remember reading that ammeters were more important when charging was provided by a generator rather than an alternator but i could be mistaken.

also why do ammeters show positive and negative figures.

All questions that would not need to be asked if I had grown up around cars that had more dials than a speedo and fuel gauge.

Jack Houpe

GT40s Supporter
Voltmeter, it shows potential energy, amp-meter shows kinetic, you can have a dead cell in your battery and a amp meter will only show that your trying to charge the battery. With a volt meter you will see the potential of the battery before the alternator starts charging and that it is charging after its on line.

Dimi Terleckyj

Lifetime Supporter
Hi Ant

An ammeter has positive and negative sides to show whether the current is flowing either into the battery or being drawn out of it.

Normally you should see the current flowing into the battery charging it all the time.
If it shows negative current then the total load on the system exceeds what the alternator can provide and the battery is making up the shortfall and draining.
This can only be sustained for so long till the system crashes.

A voltmeter is more useful in day to day driving to show the battery voltage and when the engine is running the charge voltage.

Normally when you start the car the voltage will be at its highest as the battery is being charged at its maximum and after a period of driving you should see the voltage drop to about 13 - 13.5 volts.
Initial charge voltage after starting will be about 14.5 - 15 volts.

If it gets lower than 13 while driving you need to be careful;

Thanks for the quick responses, i was just going to call it a night but just had a thought.

An ammeter would also have to be fed with high current capable cable and also would have to be rated to handle more than the stated output of the alternator.

Not sure i fancy running that gauge of cable upto the dash and back (ammeter is wired in series, where voltmeter would be parallel).

Think that i have not only filled a hole in my knowledge but also decided on a voltmeter rather than ammeter, thanks chaps

Bill Hara

Old Hand
GT40s Supporter
I had an ammeter to go in my dash but was discouraged by the thought of a large cable going from the battery out the back though the cabin and up to the dash. Decided the voltmeter was not only easier but gave the more relevant information.

You can get round the heavy current wiring to the ameter by using a 'shunt' type. These have a remote shunt sensor (resistance) close to the alternator, the reading s gained by detecting the voltage change across the shunt, this needs only light load wire.


Randy V

Lifetime Supporter
Ammeters / Amp meters have all been long discontinued from any production car due to the potential source of fire. I know I won't use one and I have (had?) a box full of them that came with various gauge sets that I've installed in my own cars and customer cars. Maybe the auto manufacturers were not savvy enough to figure out this shunt type spoken of above. I know I don't quite get it yet...
From the ETB website:

10. What is best: Ammeter or Voltmeter?

Ammeters measure electrical current (in Amperes) flowing through a circuit and therefore are wired in series with the load. They are useful when using a dynamo based charging system, as they can warn the driver (for example) of negative charging conditions. However, because ammeters are connected in series with the circuit being measured, it requires that current flow (both positive and negative) passes through the ammeter itself. This means that heavy duty cables are required to connect to the ammeter in order to withstand the large electrical current being passed through it.

If using an alternator based electrical system, the electrical current flow tends to higher than that of a dynamo, and therefore the greater the current flow the larger the cables need to be. Modern alternators tend to be connected to an ignition warning light that illuminates should the charging current become too low and thus reducing the need to constantly monitor the charging current. However they are a useful, instant method of measuring current drain incurred from various loads (e.g. headlights).

Voltmeters measure electrical "pressure" and are connected across a circuit. This means that wiring is very simple and standard automotive wire required for the other gauges can be used. Voltmeters will provide you with the current status of the car battery, as well as indicating charging problems (by a gradual voltage drop indicated on the gauge).

In general, if you are concerned about electrical wiring, ETB would recommend a voltmeter rather than an ammeter.
I would definitely second all of the notes the gents mentioned about using the voltmeter rather than an ammeter.
I once installed an ammeter in a car I had in the 60's, and everything was fine for a while until the car would not start, ammeter showed plenty of charge, almost 60 amps at times.
The problem was the alternator which had a bad field coil and although it was putting out lots of current, it couldn't overcome the voltage (As Brett mentioned Pressure) stored in the battery. The result was an 11 volt charging voltage that a voltmeter would have shown.
Out went the ammeter and a lot of wiring that I discovered was cooked (8 Guage to boot).
One question you had was the fact that ammeters have a negative side, this would show a reversal of current flow such as lots of draw with insufficient current coming from the alternator. A voltmeter will also show this as a low voltage condition.
Since the voltmeter measures system potential it can be installed in any 12V supplied area in the system, and will measure the potential (pressure) as long as the source has no resistance such as an ignition coil supply etc.
Good luck
Thanks to everybody who has taken the time to answer. A voltmeter is now my informed choice of gauge and i will update my other electrical related thread with that.

Thanks again