13.5 and 13.9 Electrical power (ATA 24) and lights (ATA 33)

Scope

Both are level 3 for B2 and part of the B2L basic block (13.1, 13.2, 13.5, 13.9).

• 13.5: batteries; DC generation; AC generation; emergency generation; voltage regulation; power distribution; inverters, transformers, rectifiers; circuit protection; external/ground power.
• 13.9: external (navigation, anti-collision, landing, taxiing, ice), internal (cabin, cockpit, cargo), emergency.

Batteries

	Lead-acid	Nickel-cadmium	Lithium-ion
Cell voltage	≈ 2.0 V	≈ 1.2 V	≈ 3.6–3.7 V
24 V battery	12 cells	19–20 cells	Typically 7–8 cells
Charge check	Specific gravity (1.275–1.300 fully charged)	Voltage under charge / capacity test; electrolyte level only valid when fully charged	Battery management system
Hazards	Hydrogen, acid	Thermal runaway, cell imbalance (deep-cycle to restore)	Thermal runaway, needs containment/venting and BMS

Battery uses on large aircraft: APU start, emergency/standby power, power-up. A battery charger (often fed from the AC bus) maintains charge and monitors temperature.

DC generation and regulation

• DC generators and starter-generators: output controlled by varying shunt field current through a voltage regulator (carbon-pile, vibrating-contact on old types; transistorised GCU now).
• Reverse-current cut-out prevents the battery motoring the generator.
• Paralleling: an equalising circuit balances load between generators.
• On most large aircraft DC is obtained from AC via TRUs (28 V DC).