During APU start, which battery powers the starter and which powers the ECU?

During APU start, which battery powers the starter and which powers the ECU?

• A. APU Battery powers the starter and Main Battery powers the ECU
• B. Main Battery powers the starter and APU Battery powers the ECU
• C. Both batteries power both components
• D. External power powers both

Answer: (A)

When starting the APU, power needs are kept separate to protect both the start sequence and the control systems. The starter draws a high, short-duration current, so the dedicated APU battery supplies the starter to spool the APU without pulling down the rest of the electrical system. The electronic control unit, which manages the start process and monitors sensors, runs off the main battery to ensure a stable, continuous supply for the avionics and control logic. This split design helps prevent voltage dips from the starter surge from upsetting the ECU, while keeping the control electronics powered and able to command the start sequence.

When starting the APU, power needs are kept separate to protect both the start sequence and the control systems. The starter draws a high, short-duration current, so the dedicated APU battery supplies the starter to spool the APU without pulling down the rest of the electrical system. The electronic control unit, which manages the start process and monitors sensors, runs off the main battery to ensure a stable, continuous supply for the avionics and control logic. This split design helps prevent voltage dips from the starter surge from upsetting the ECU, while keeping the control electronics powered and able to command the start sequence.