ammeter is an instrument that measures and reports the flow
of electric current at a point in a circuit. Ammeters can
be used to diagnose faults in a vehicle's electrical system.
The function of an ammeter is built into a multimeter, which is an indispensable
tool for checking circuitry.
Ammeters can also be attached to the dashboard of, for example,
classic or performance cars, to indicate to the driver if
the battery is being charged (by the alternator) or is discharging. There are two ways to
do this, depending on how the instrument is designed to be
wired. The first is to connect the ammeter in series with
the wire leading from the battery that feeds everything in
the car except the starter. The second, and more common, is
when the ammeter has an external shunt. The shunt is a low
resistance that takes most of the current and the ammeter
is wired in parallel with this so that only a fraction of
the current from the battery flows through it. Since the resistances
of the shunt and meter are known, the meter is able to display
the total current flowing from the battery when running, and
hence whether it is charging or discharging. If the alternator
is charging the battery the ammeter will read positive, if
the battery is discharging it will read negative.
more information on this topic