Final answer:
The Interface is the device that is always installed with the ELT and connected via a 9-pin connector, powered by a removable rechargeable battery pack.
Step-by-step explanation:
The device always installed with the Emergency Locator Transmitter (ELT) and connected via a 9-pin connector, powered by a removable rechargeable battery pack, is the Interface. An ELT is an emergency beacon that transmits a signal to assist in aircraft location during search and rescue operations. The interface in this context likely refers to the control interface, which allows the system to be tested and monitored, ensuring it is ready for activation in an emergency.