Definition
The voltage measured across the terminals of a power source, such as a battery or generator, when no load is connected and no current is flowing through an external circuit.
Plain English
It's the voltage a battery or generator shows when nothing is hooked up to it and drawing power. With no current being pulled, the reading shows the source's full voltage before any drop caused by use.
Context Anchor
Seen in aircraft electrical troubleshooting, especially when checking battery condition or comparing a no-load reading with a reading taken while equipment is operating.
Derivation
An 'open circuit' is one where the path is broken — like an open switch — so current cannot flow. The term simply names the voltage present in that broken-path condition.
Why Pilots Care
A low open-circuit voltage reading tells you a battery is weak before it fails during engine start or in flight.
Analogy
Like checking the pressure gauge on a closed water tank: you see the maximum pressure because nothing is flowing out.
Intuition Check
“Open” does not mean the circuit is turned on. Here, “open” means the electrical path is broken or disconnected, so current is not flowing through an outside device.
Example Sentence 1
The mechanic measured the battery's open-circuit voltage at 12.6 volts, indicating a full charge before the aircraft was returned to service.
Example Sentence 2
A 12-volt battery showing 12.6 volts open-circuit voltage is considered fully charged.