A computer implemented method combines a simplified equivalent circuit model with a model capturing the variation of the circuit parameters. The components of the equivalent circuit model depend on the internal battery state, and the parameters of the model encode this dependence. The invention then uses actual operational data capturing various modes of operation of the battery and different discharge rates to fit the model parameters (rather than controlled laboratory tests used in previous work). Once this analysis is done (offline), the model can be used in an online phase to adjust estimates of the internal battery state as the battery is operating.