SOC (State of Charge)
Charge state in %. 100% = full battery; 0% = empty. Estimated by reading open-circuit voltage at rest.
SOC (State of Charge) is the percentage of energy available compared to nominal capacity. A practical way to estimate it in lead-acid at rest (no charge or discharge for at least 1 hour, around 25°C) with a voltmeter: 12.72 V ≈ 100%, 12.60 V ≈ 90%, 12.42 V ≈ 75%, 12.24 V ≈ 50%, 11.99 V ≈ 25%, 11.77 V ≈ 0%. Below 50% sustained the battery suffers sulphation and loses capacity irreversibly — which is why smart chargers detect low SOC and apply an extended absorption phase to reverse it while still possible.
In Start-Stop vehicles the ECU estimates SOC via an IBS sensor on the negative terminal and only activates Start-Stop with SOC above 70%, to guarantee the engine restarts when the brake is released.