state of charge (SOC)

State of charge is a battery's fuel gauge: the fraction of its usable capacity still available, from 100% (full) down to 0% (empty). It sounds simple — just measure how full the tank is — but a battery has no dipstick. You cannot directly see the charge; you must infer it. The cleanest clue is the open-circuit voltage, since a rested Li-ion cell sits near 4.2 V when full and 3.0 V when empty, but under load the voltage sags and lies, so a bare voltmeter makes a terrible fuel gauge.

Modern gauges therefore combine two methods: coulomb counting, which integrates current in and out over time (charge that flowed = ∫I dt), and voltage-based correction that recalibrates when the battery rests. Even so, SOC drifts because capacity itself fades with age and shrinks in the cold — which is why your phone can jump from 30% to dead, and why an EV's range estimate is really an educated guess that the battery management system keeps re-learning.