Definition
A secondary, independent battery installed in some aircraft to supply electrical power to essential instruments and avionics if the main electrical system or primary battery fails. It is kept charged by the aircraft's electrical system in normal operation and is switched in (automatically or manually) when normal power is lost, providing a limited period of backup power to support continued safe flight, especially in IMC.
Plain English
An extra battery in the aircraft that takes over and powers your most important instruments if the main electrical system quits.
Context Anchor
Seen in instrument flying when checking backup power before flight, managing electrical load, or responding to an electrical failure.
Derivation
"Standby" comes from the idea of something kept ready and waiting to take over. The battery sits idle during normal operations and only steps in when needed.
Why Pilots Care
Maintains power to attitude, heading, and navigation instruments during total electrical failure, preserving the ability to maintain controlled flight.
Intuition Check
Do not assume a standby battery keeps the whole airplane powered like normal. It is backup power for selected important equipment, and its time is limited.
Example Sentence 1
After the alternator failed, the pilot switched on the standby battery to keep the primary flight display powered while diverting to the nearest airport.
Example Sentence 2
The preflight checklist requires verifying that the standby battery is fully charged and its switch is in the armed position.