Final answer:
The primary purpose of an aircraft storage battery is to provide electrical power for engine start and emergencies. This is similar to the role of batteries in vehicles, providing the initial startup power and serving as a backup in case the alternator fails.
Step-by-step explanation:
The primary purpose of an aircraft storage battery is Option 2: To provide electrical power for engine start and emergencies. Aircraft batteries, similar to those in cars and marine engines, are essential for providing the initial power required to start the engine. The lead-acid battery, for example, is a common type used in vehicles, consisting of multiple cells connected in series to produce the necessary voltage. In the context of aircraft, once the engines are started, the alternator primarily takes over to supply power for the rest of the aircraft's needs and recharges the battery if needed.
Unlike ground vehicles, aircraft cannot accommodate excessively heavy batteries for energy storage due to weight constraints and the need for efficiency. Even the best lithium-ion batteries, which are less energy-dense compared to conventional fuels, would require impractical amounts of weight to match the energy provided by jet fuel, severely limiting the aircraft's range and operational efficiency. Therefore, batteries in aircraft are used mainly for starting the engine and as a backup power source in emergencies, not as the primary energy source for sustained flight.