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The Boeing 727 uses a battery for emergency electrical power. In the event of a failure of the primary power sources, the aircraft's battery provides a backup to power essential systems required for safe operation.

Batteries serve as a reliable means of generating electrical power during critical situations when the engines are not producing enough electrical output or when there's a complete power failure. They ensure that key instruments and systems remain operational, allowing the crew to manage emergencies effectively and maintain control of the aircraft.

While other options like generators and ground power units are important for normal operations, they do not serve the same purpose as the battery specifically for emergency power. Generators generate power during flight, and ground power units provide external power on the ground; however, in an emergency scenario where both generators fail, the reliance shifts to the onboard battery system. Capacitors, on the other hand, are generally used for smoothing out electrical supply and not as a primary or emergency power source.