Definition
The portion of a battery's available voltage that is high enough to operate aircraft electrical equipment correctly. Once battery voltage drops below the minimum level required by the equipment, the remaining voltage is no longer useful, even though the battery is not yet fully discharged.
Plain English
The part of the battery's charge that is still strong enough to actually run the airplane's instruments and radios. Below a certain point, the battery still has some power left, but it is too weak to be useful.
Context Anchor
Seen when an aircraft is operating on the main battery after the normal charging source, such as an alternator or generator, is no longer supplying power.
Derivation
Voltage comes from volt, the unit named for Alessandro Volta, an early scientist who studied electricity. Thinking of voltage as the electrical push available from the battery helps explain why equipment stops working reliably when that push gets too weak.
Why Pilots Care
It indicates how much time remains before critical systems may fail, guiding decisions on when to land.
Grounding Statement
A battery can still have some charge left but no longer have enough voltage to run essential aircraft equipment correctly.
Intuition Check
Do not assume useful voltage means any voltage at all. Here, useful means enough voltage to operate needed equipment reliably.
Example Sentence 1
After the alternator failed, the pilot shed nonessential loads to extend the useful voltage of the main battery.
Example Sentence 2
Once the voltage dropped below useful levels, the pilot knew it was time to declare an emergency and land.