A voltmeter shows the battery’s voltage when the alternator is not running, which should be 24 volts for most modern electrical systems. Voltage can be compared to a reservoir of water and amps can be compared to the current flowing from it. 24-volt batteries have 12 cells. If one of these has failed or short circuited the voltmeter will read 2 volts lower than normal. This reduction in voltage will not show when the alternator is running; however, the ammeter should display a slightly value because of the additional current flowing into the battery to charge it.