Definition
The aircraft's primary onboard electrical storage source, used to start the engine and to supply electrical power to aircraft systems when the alternator or generator is offline or has failed. Once the engine is running, the alternator or generator normally takes over, and the main battery is recharged and held in reserve. In an electrical emergency, the main battery becomes the sole source of power and has a limited duration measured in minutes, not hours.
Plain English
The aircraft's main rechargeable battery — the one that starts the engine and keeps essential systems running if the generator or alternator fails.
Context Anchor
Seen in electrical-system discussions, emergency checklists, and instrument-flying situations where the aircraft must continue on battery power after a charging-system failure.
Derivation
“Main” means primary or most important. “Battery” comes from an older word meaning a group of things used together; in electricity, it came to mean a group of cells that store and deliver electrical power. Together, “main battery” points to the aircraft’s primary stored-power source.
Why Pilots Care
Allows continued use of essential avionics and flight instruments long enough to complete a safe landing after generator loss.
Analogy
It is like running a house from a backup battery during a power outage: the lights may work for a while, but only if you use the stored power carefully.
Intuition Check
Do not assume “main battery” means the whole electrical system is normal. It means the primary battery is supplying power, and that stored power can run out.
Example Sentence 1
After the alternator failed, the pilot was operating on the main battery and immediately turned off non-essential equipment to conserve power.
Example Sentence 2
The preflight checklist includes verifying main battery voltage before engine start.