What does a negative reading on an ammeter indicate?

A negative reading on an ammeter indicates that the current is flowing from the battery to the aircraft's electrical systems, meaning the battery is discharging. This typically happens when the electrical load in the aircraft exceeds the output of the alternator, causing the battery to supply additional power.

In contrast, a positive reading would suggest that the alternator is charging the battery, which generally indicates that the system is functioning normally. Therefore, a negative reading confirms that the battery is actively providing power to support the aircraft's systems, making it essential for the pilot to monitor this closely to ensure that the battery does not become overly discharged during flight.