What type of battery is the auxiliary (aux) battery?

The auxiliary battery is a 24V DC lead-acid battery, which is commonly used in many aircraft systems for its reliability and energy storage capabilities. Lead-acid batteries have been in use for a long time due to their robustness, cost-effectiveness, and relatively high power-to-weight ratio, making them suitable for auxiliary power systems that need to supply energy for starting engines or powering various onboard electronics when the main power source is unavailable.

In aircraft, the auxiliary battery provides critical back-up power to essential systems, ensuring functionality during situations where main electrical power is lost or during ground operations. The 24V rating aligns with industry standards for many aviation applications, allowing compatibility with the overall electrical system of the aircraft.

Other types of batteries, such as lithium-ion, nickel-cadmium, or gel batteries, have their specific applications and may be used in different roles but do not typically serve as the auxiliary battery in standard aircraft configurations due to factors like weight considerations, performance characteristics, temperature sensitivity, and relative costs.