Two 14 V batteries, connected in series, supply the necessary 28 V DC power when no other source is available. The batteries supply power for starting the APU. For pressure refueling, when no other power source is available the batteries will via switches on the fueling control panel and the emergency inverter supply the required electrical power. In an emergency electrical power condition, the batteries will, if fully charged, provide power to certain essential equipment for approximately 45 minutes.
The emergency inverter is of a solid state type with 28 V DC input and single phase 115 V, 400 Hz AC output. The inverter supplies power for operation of essential flight instruments, communication equipment and necessary lights in the event of a complete loss of AC power from the generators.
An external power receptacle enables the connection of a 115/200 V, 3-phase, 400 Hz external power source to supply power to the buses when the engine-driven or APU generators are not operating. External power will not be supplied to a load bus which has power from an engine-driven generator or the APU generator available.