Battery and essential power

Set battery sources

With the aircraft secured, inspect the DC electrical panel and ELEC synoptic before starting the APU. ^[94]

Compare the battery-selector positions with the battery and essential-bus indications.

BATT 1 and BATT 2

BAT 2 AUTO supports start bus

FSS says BATT 1 ON connects battery 1 to DC ESS BUS 1. BATT 2 AUTO connects battery 2 to DC ESS BUS 2 and allows it to power the APU START BUS during an APU start. The DC GPU supplies the APU START BUS only; it does not charge the batteries.

Check which essential bus each battery can supply.

Battery and GPU

Check the selector positions, battery indication and any external power. Follow the FSS power-up and shutdown flow.