What does a negative reading on the ammeter indicate?

A negative reading on the ammeter indicates that the battery is discharging. This means that the electrical system is drawing more current from the battery than is being supplied by the alternator or generator, leading to a net loss of charge in the battery. Essentially, the aircraft's electrical equipment is using battery power to operate, which can occur during engine startup, when the electrical load exceeds the output of the charging system, or if there is an issue with the alternator.

Understanding the role of the ammeter is crucial for monitoring the aircraft's electrical performance. A positive reading would mean the charging system is providing more current than the aircraft is using, which is typical during normal operation when the engine is running and the alternator is functioning properly. If the alternator were not functioning or there was an electrical short, the system might behave differently, leading to different readings on the ammeter.