Final answer:
The APU battery is a critical component in an aircraft, located in the tail section or rear compartment, providing power for starting engines and essential functions, especially in emergencies.
Step-by-step explanation:
The APU, or Auxiliary Power Unit, battery is a critical, self-contained unit located in the tail section or rear compartment of an aircraft. This battery is responsible for providing electrical power to the aircraft when the main engines are not running, which is essential for starting the engines and for powering onboard systems during ground operations.
The use of an APU battery is particularly important during emergency situations where it can be used to maintain essential functions of the aircraft.