The state-of-charge (SOC) estimation is of extreme importance for the reliability and safety of battery operation. How to estimate SOC and, to some degree, the SOC convention itself, is still a subject of great interest. Here a viable SOC convention valid for single cells and multi-cell strings is proposed and validated. Using a 3S1P string as an illustration in this work, the direct inference from a correct open circuit voltage versus SOC (OCV= f(SOC)) correspondence based on the proposed SOC convention is the best method for accurate SOC estimation among several possible approaches for strings. The thermodynamic aspect on this SOC convention is explained. Uncertainties in actual applications are also discussed. The understanding on this accurate SOC estimation approach shall facilitate reliable battery control and management.