Definition
An electrical bus that is connected directly to the airplane battery and supplies power to a small group of essential items, such as engine start circuits, emergency lighting, and certain avionics. It remains powered whenever the battery is connected, even when the master switch is off and the generators are not online.
Plain English
A wire junction inside the airplane that draws power straight from the battery and feeds a few critical items. It stays live as long as the battery is hooked up, regardless of whether anything else is switched on.
Context Anchor
Seen in turboprop electrical system diagrams, abnormal checklists, and discussions of what equipment remains powered after a generator or main electrical failure.
Derivation
In electrical systems, a 'bus' (short for 'busbar') is a common conductor that distributes power to multiple circuits. A 'battery bus' is simply the bus fed directly from the battery rather than through the main electrical system.
Why Pilots Care
It guarantees that critical systems like certain avionics and engine controls receive battery power during startup or after a generator failure.
Intuition Check
Do not read “bus” as a vehicle here. In aircraft electrical systems, a bus is a shared electrical power point; a battery bus is the bus fed by the battery, not the battery itself.
Example Sentence 1
Even with the master switch off, the clock and emergency lighting stayed powered because they were wired to the battery bus.
Example Sentence 2
With the generator offline, the battery bus continued to power the essential flight instruments.