Definition
The voltage measured across the terminals of a power source, such as a battery or generator, while it is connected to a load and current is flowing through the circuit.
Plain English
The voltage a battery or power source actually delivers when something is switched on and drawing power from it.
Context Anchor
Seen in aircraft electrical-system checks, battery testing, starter troubleshooting, and maintenance discussions about voltage under actual use.
Derivation
Closed-circuit means the electrical loop is complete and current can flow. The opposite is open-circuit voltage, measured when nothing is connected and no current flows. The closed-circuit reading is almost always lower because the source's own internal resistance drops some voltage once current starts moving.
Why Pilots Care
Reveals whether a battery can deliver usable power under real load; a low reading often means the battery will fail to start an engine even if open-circuit voltage appears normal.
Analogy
It is like checking water pressure while a faucet is open. The useful reading is the one you get while the system is actually being used.
Intuition Check
Do not read “closed” as “shut off.” In electrical use, a closed circuit is one with a complete path, so closed-circuit voltage is measured while electricity can flow.
Example Sentence 1
The battery showed 12.6 volts open-circuit but the closed-circuit voltage dropped to 10.2 volts during the start, suggesting it was nearing the end of its service life.
Example Sentence 2
Before flight the technician checked closed-circuit voltage to confirm the battery could handle starter current draw.