The state-of-charge (SOC) estimation is of extreme importance for the reliability and safety of battery operation. Flow to estimate SOC for an assembly of cells in a battery pack remains a subject of great interest. Here a viable method for SOC determination and tracking for multi-cell assemblies is proposed and validated. Using 3S IP (three in series and one in parallel) strings as an example, an inference of SOC is illustrated in a battery assembly based on a correct open pack voltage (OPV) versus SOC (i.e. OPV = f (SOC)) function. The proposed method only requires the measurements of the rest cell voltages of the single cells at two distinct occasions. This accurate SOC estimation approach shall facilitate reliable battery control and management. (C) The Author(s) 2015. Published by ECS. All rights reserved.