What does a voltmeter read after starting the engine?

The voltmeter is a device used to measure the electrical potential difference (voltage) in a system. After starting the engine, the voltmeter typically reads the alternator's output voltage, which is higher than the battery voltage due to the alternator's function of charging the battery while providing power to the electrical system of the vehicle.

When the engine is running, the alternator begins generating electricity and typically produces a voltage ranging from about 13.5 to 14.5 volts, which is necessary to both power the vehicle's electrical systems and recharge the battery. If the voltmeter were to read battery voltage after starting the engine, that would indicate that the alternator is not functioning correctly, as the battery voltage alone (around 12.6 volts when fully charged) would not suffice to power any significant load in the vehicle.

Thus, the correct understanding is that the voltmeter reflects the alternator voltage, which signifies that the electrical system is operating correctly and the battery is being charged; therefore, the reading indicating battery voltage would not accurately reflect the state of the system after the engine has started.