Why do we still need ammeters?
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Because ammeters measure the flow of current (positive or negative) they can immediately show when the alternator is no longer providing power and the battery is discharging instead. In a voltmeter, like most modern automobiles have, you would have to wait to see the charge level on the battery drop, and by that point, several minutes (or longer) may have passed. In an aircraft, where it may be important to quickly shutdown electrical devices and conserve battery power, every moment counts and the ability to see flow shift from positive to negative is very helpful.