What voltage is required as the minimum to start the APU on both the APU and the main battery?

What voltage is required as the minimum to start the APU on both the APU and the main battery?

• A. 18 Volts
• B. 20 Volts
• C. 22 Volts
• D. 24 Volts

Answer: (C)

The key idea is that the APU starting system must have enough electrical power to spin the starter motor and energize the start control circuits. The minimum voltage required to reliably initiate a start from either source—APU power or the main battery—is 22 volts. If the voltage is below that, the starter won’t engage properly and the APU won’t light off, even though the main battery may be present or the APU battery is connected. So 22 volts is the threshold that ensures the starter can operate and the control electronics are energized. The other numbers are either too low to guarantee a reliable start or simply reflect a normal operating value rather than the minimum needed to begin starting.

The key idea is that the APU starting system must have enough electrical power to spin the starter motor and energize the start control circuits. The minimum voltage required to reliably initiate a start from either source—APU power or the main battery—is 22 volts. If the voltage is below that, the starter won’t engage properly and the APU won’t light off, even though the main battery may be present or the APU battery is connected.

So 22 volts is the threshold that ensures the starter can operate and the control electronics are energized. The other numbers are either too low to guarantee a reliable start or simply reflect a normal operating value rather than the minimum needed to begin starting.