The BATT RLY 2 indicator extinguishes to indicate parallel operation when which condition is met?

• A. External DC power is removed
• B. START PROTECTION OFF
• C. DC external power is applied with START PROTECTION ON
• D. The battery is disconnected

Answer: (C)

Parallel operation happens when the aircraft’s main DC bus is being powered by both the external DC source and the battery in a controlled way, with the Start Protection logic engaged to ensure safe interconnection. The BATT RLY 2 indicator shows the status of the battery relay that ties the battery into that bus. When you apply DC external power and have Start Protection ON, the control logic places the system into parallel mode and the BATT RLY 2 relay is de-energized, causing its indicator to extinguish. This extinguished state signals that the external source is powering the bus in parallel with the battery, rather than isolating the battery or leaving it disconnected. If external power isn’t present, if Start Protection is OFF, or if the battery is disconnected, you wouldn’t be in the proper parallel configuration, so the indicator would not extinguish under those conditions.