ERJ-145 Electrical Practice Test 2026 - Free Electrical Practice Questions and Study Guide

Starting the APU uses the second battery because its electrical path is dedicated to the APU start function. Battery 2 provides the high-current DC needed to energize the APU starter motor and to power the APU control electronics during start, keeping this heavy load separate from Battery 1’s essential-bus duties. The GPU can supply external power when connected, but it doesn't substitute the battery path required to initiate the APU start sequence, so it isn’t used to start the APU.