What does measuring battery voltage with the engine running help determine?

Measuring battery voltage with the engine running primarily helps determine the alternator output. When the engine is operational, the alternator should generate electricity to charge the battery and power the vehicle's electrical systems. The standard battery voltage when the engine is running should typically be between 13.5 to 14.5 volts. If the voltage is within this range, it indicates that the alternator is functioning properly and supplying adequate power. Conversely, a voltage reading that is too low may suggest that the alternator is not charging the battery effectively, which could lead to battery drain and electrical system failures. This measurement is a critical part of diagnosing charging system issues within a vehicle.