Definition
The amount of electrical energy currently stored in a battery, expressed as a percentage of its full capacity. A fully charged battery is at 100 percent state of charge; a fully depleted battery is at 0 percent.
Plain English
How full the battery is right now, compared to when it is completely charged.
Context Anchor
Seen in electrical system and generator-failure discussions, especially when deciding how long battery-powered equipment may keep working.
Derivation
Charge comes from an old word meaning to load. In this use, the battery is thought of as being loaded with stored electrical energy, so its state of charge is how much of that stored energy remains.
Why Pilots Care
It tells the pilot how long the battery can continue powering essential instruments and radios after a generator failure.
Analogy
Like the fuel gauge on a car, but for stored electricity instead of fuel.
Intuition Check
State of charge does not mean whether the battery is currently being charged. It means how much stored electrical energy the battery has available right now.
Example Sentence 1
After the alternator failed, the pilot reduced electrical load to preserve the battery's state of charge for the approach.
Example Sentence 2
A low state of charge meant the aircraft had only a short time before critical electrical systems would shut down.