State-of-Charge (SOC) is a key to modeling and managing the battery system. Several algorithms have been developed to calculate the OCV (open-circuit voltage) based on the terminal voltage and terminal current of batteries. Then SOC can be obtained based on the monotonic mapping between SOC and OCV. Using the measured terminal voltage and current from an EV(electric vehicle) fleet, this thesis solves the following two problems: (1) how to develop a robust mapping between OCV and SOC; and (2) experimentally validate the accuracy of a voltage-based SOC algorithm that is universal to all battery types.