Abstract: A battery system includes a multi-cell rechargeable energy storage system (RESS) and an electronic controller configured to process RESS cell data. The controller is programmed with a battery cell open circuit voltage (OCV) versus state of charge (SOC) data table used to calculate a peak OCV movement for a representative battery cell at predetermined capacity and maximum discharge/charge rate. The controller is configured to acquire voltage data for one of the cells during active RESS cycling. The controller is also configured to pass the acquired voltage data through a first-order lag filter algorithm to generate an overpredicted OCV for the subject cell. The controller is additionally configured to determine an overpredicted SOC value for the subject cell in the data table using an OCV value from the overpredicted OCV. Furthermore, the controller is configured to regulate the RESS using the determined overpredicted SOC value for the subject cell.