A method of controlling an electric vehicle including an internal combustion engine, a battery having a state of charge (SOC) and an open circuit voltage (OCV), includes establishing a system for estimating battery SOC. The system includes (i) a parameter estimation subsystem including a recursive parameter estimator for identifying battery parameters and (ii) an OCV estimation subsystem including a nonlinear adaptive observer for estimating battery OCV. Estimated battery OCV is related to estimated battery SOC by a mapping. An output is generated based on the estimated battery SOC.