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Overview

Aircraft batteries serve as the last-resort power source when all generators are offline. Unlike a car battery that merely starts an engine, an aircraft battery must sustain critical avionics, flight instruments, and cockpit lighting long enough for the crew to restore generated power or execute an emergency landing. Modern commercial aircraft carry at least two batteries—typically a main battery and an APU battery—with some types carrying additional hot-battery bus supplies or dedicated lithium-ion packs for specific functions such as the Boeing 787's flight-deck recorder batteries.

Battery chemistry has evolved considerably since the lead-acid cells used on early jet transports. Nickel-cadmium (NiCd) cells dominated commercial aviation for decades, valued for their ability to deliver high current on demand, tolerate deep discharge, and survive extreme temperatures. More recently, lithium-ion (Li-ion) technology has entered service, offering substantially higher energy density and lower weight. The Boeing 787 Dreamliner adopted large Li-ion main and APU batteries, which delivered weight savings but triggered a significant airworthiness crisis in 2013 due to thermal runaway events, ultimately leading to revised battery management and containment designs.

How It Works

During normal operation, aircraft batteries remain connected to the DC bus through a battery charger. The charger converts 115V AC or 28V DC bus power to a regulated charging current, maintaining the battery at full state of charge while drawing minimal energy. A battery charge controller monitors cell temperature and voltage to prevent overcharge, which in NiCd cells causes electrolyte venting and in Li-ion cells can initiate thermal runaway.

When all AC power sources are lost, static inverters or batteries supply the essential DC bus. The main battery powers essential avionics through the hot battery bus, a permanently energised bus that bypasses all contactors and remains live even when the aircraft master switch is off. The APU battery, isolated from the main battery, provides the dedicated starting energy required to motor the APU starter-generator to light-off speed. On the Boeing 787, large 32 V Li-ion batteries replace the traditional architecture, powering essential avionics, brake-by-wire, and the APU start function through a dedicated battery management electronics unit.

Key Components

• Main Battery: Typically 24V NiCd, 36 to 50 Ah, housed in a temperature-monitored battery box ventilated overboard to prevent hydrogen accumulation.
• APU Battery: Similar chemistry to the main battery but optimised for high-rate discharge needed for APU starting, usually 24V 40 to 50 Ah.
• Battery Charger: Solid-state AC-to-DC converter maintaining constant-voltage or constant-current charging profile matched to battery chemistry.
• Battery Temperature Monitor: Thermocouple or thermistor array within the battery case providing cockpit indication and automatic disconnect on overtemperature.
• Hot Battery Bus: Unfused, always-energised DC bus directly connected to the battery output terminals, powering standby flight instruments and smoke detectors even with all switches off.

Aircraft Applications

The Boeing 737-800 carries a 24V 36 Ah NiCd main battery and a separate APU battery, both in the electronics and equipment bay beneath the forward fuselage. The Airbus A320-200 uses a similar dual NiCd architecture with an additional static inverter that converts battery DC to single-phase AC for standby attitude and navigation displays. The Boeing 777-300ER carries NiCd batteries with an enhanced monitoring system derived from lessons learned on earlier types. The Boeing 787-9 uses two 32V 75 Ah Li-ion batteries, each contained in a stainless-steel enclosure with dedicated ventilation ducting following the 2013 design changes mandated by the FAA and EASA.

Advantages and Limitations

NiCd batteries offer proven reliability, tolerance of extreme temperatures, and straightforward maintenance, but suffer from the memory effect if not periodically deep-discharged and contain cadmium, a toxic heavy metal requiring careful disposal. Li-ion batteries deliver roughly twice the energy per kilogram, reducing airframe weight meaningfully on large aircraft, but require sophisticated battery management electronics to prevent thermal runaway and mandate robust containment in the event of a cell failure. Both chemistries provide limited capacity measured in minutes rather than hours; the battery system buys time for crew response, not a substitute for restoring generator power.