Electrics - Battery & Standby Power System 
Amendment: Description amended to include single battery installations.
- Supplementary Procedures - Battery Start Procedure
- Boeing - Loss of all standby power following battery discharge
The Battery/Standby Power System can supply DC and AC power to selected flight instruments, communications and navigation systems, and other critical systems, if there are main AC and DC electrical power system failures.
The Battery/Standby Power System comprises the following Buses:
If all generator power is lost, these are the only buses that can be powered by the batteries.
- See: Battery Schematic
- See: Bus Equipment List
Batteries
Early production aircraft have a single battery only.
In the event of AC power failure, the standby power system remains operative for 30 minutes.
Most production aircraft have a MAIN and APU battery (which is used for starting and operating the APU). There is no direct cockpit control for the APU battery.
Airplanes With 90-minute Standby Power
On most, but not all, aircraft with 2 batteries, the MAIN and APU batteries are paralled if power is lost to the Left DC Bus, thus allowing the standby power system to remain operative for 90 minutes in the event of AC power failure.
Each battery has a charger powered from the Ground Service Bus.
Hot Battery Bus
The hot battery bus provides power to items that must be continuously powered, such as the Clock's time reference.
Prior to establishing electrical power, the Main Battery powers the Hot Battery Bus. After establishing electrical power, the Main Battery charger powers the Hot Battery Bus.
Battery Bus
Prior to establishing electrical power, [and] when the Battery Switch is ON, the Main Battery powers the Battery Bus.
The Main Battery also powers the Battery Bus if the the Standby Power Selector Switch is in the BAT position.
After establishing electrical power, the Left DC system powers the Battery Bus, and the main battery provides a backup source of power.
The Battery DISCH light illuminates when the main battery is discharging. If EICAS is powered, the advisory message MAIN BAT DISCH also displays.
The MAIN BAT DISCH light illuminates when the Main Battery is discharging. If EICAS is powered, the advisory message MAIN BAT DISCH also displays.
The Battery OFF light illuminates and the EICAS advisory message BATTERY OFF displays if the battery switch is OFF after electrical power is established.
Standby DC Bus
The Standby DC Bus can be powered by several sources.
Prior to establishing electrical power, when the battery switch is ON and the standby power selector is in AUTO, the main battery powers the standby DC bus. The Battery DISCH light illuminates when the main battery is discharging.
After establishing electrical power, the left DC system powers the standby DC bus and the main battery provides a backup source of power.
When the standby power selector is in BAT, the main battery powers the standby DC bus.
Prior to establishing electrical power, when the Battery Switch is ON and the Standby Power Selector is in AUTO, the Main Battery and the APU Battery are paralleled to power the Standby DC bus. The MAIN BAT DISCH light and the APU BAT DISCH light illuminate when the main battery and APU battery are discharging.
After establishing electrical power, the Left DC system powers the Standby DC bus and the Main Battery and APU Battery provide a backup source of power.
When the Standby Power Selector is in BAT, the main battery and the APU battery are paralleled to power the Standby DC Bus.
The Standby Bus OFF light illuminates and the EICAS advisory message STANDBY BUS OFF displays if the Standby DC bus is unpowered.
Standby AC Bus
The Standby AC bus can be powered by several sources.
Prior to establishing electrical power, when the battery switch is ON and the standby power selector is in AUTO, the main battery powers the standby inverter which provides AC power to the standby AC bus.
After establishing electrical power, the left AC system powers the standby AC bus and the main battery and standby inverter provide a backup source of power.
When the standby power selector is in BAT, the main battery and standby inverter power the standby AC bus.
Prior to establishing electrical power, when the Battery Switch is ON and the Standby Power Selector is in AUTO, the Main Battery and the APU Battery parallel to power the Standby Inverter which provides AC power to the Standby AC bus.
After establishing electrical power, the Left AC system powers the Standby AC Bus and the Main Battery, APU Battery and Standby Inverter provide a backup source of power.
When the standby power selector is in BAT, the Main Battery, APU Battery and Standby Inverter power the Standby AC Bus.
The Standby Bus OFF light illuminates and the EICAS advisory message STANDBY BUS OFF displays if the Standby AC Bus is unpowered.
Standby Power Selector
OFF
In the OFF position, the selector cuts off all sources of power to the Standby DC and Standby AC Buses.
Boeing In flight, loss of Standby DC power is sensed as a double engine failure and unlocks the RAT. If the Standby Power Selector is placed OFF (in flight) on the 757, full RAT deployment could be expected due to the spring extension mechanism.
AUTO
The Standby Power Selector affects the power going to the Standby DC and Standby AC buses. With the selector in the AUTO position, left main DC and AC power have priority in supplying the two Standby Buses.
When the Battery Switch is ON, the AUTO position also permits the main battery, APU battery and battery chargers to act as backup power sources for the two buses.
BAT
Selecting the BAT position cuts off main AC and DC power to the standby system. The only power remaining for the standby system is battery power. In this case even the battery chargers are disconnected from the Ground Service Bus.With the Battery Switch ON, the battery supplies Standby DC and Standby AC through the Static Inverter.
AFM Flight beyond 30 minutes (90 minutes if 2 batteries installed) will result in complete loss of electrical power.