Most drivers can read and understand most of the gauges on the dashboard of their vehicles. The fuel and temperature gauges, as well as the speedometer and odometer, are relatively easy to understand. An exception for many may be the voltmeter or ammeter, both of which are used to display the state of your car battery. Once you understand exactly what they measure, you can also read your car's battery gauges.
Look at the voltmeter on the dashboard of your vehicle. Cars operate on a 12-volt electrical system. A fully charged battery should read about 12.5 volts when the key is in the ignition and turned partially, but the car's engine is not running.
Look at the voltmeter while the engine is running. Readings of 14 to 14.5 volts on the gauge when as a good sign. This is typical of a properly operating charging system. If the voltage reading drops below 12.5 volts, the charging system is not operating properly, and the battery is having to provide current, meaning it will eventually go dead.
Find the ammeter on the dashboard. The ammeter gauge should read a little greater than zero when the engine is on and the system is functioning properly. This means that the battery is fully charged and the charging system is providing the current. A negative reading (below zero) means the battery is providing current, and unless the problem is corrected, the battery will go dead.
The ammeter may show an abnormally high reading briefly after starting the engine and the lights and other devices are being powered. It should return to normal quickly. Get your charging system checked out if the voltage reading goes above 15 volts. This is an indication of a malfunctioning voltage regulator and overcharging. Overcharging can damage the battery or electrical system.