15 and differentiated error 16 for each state variable x and each parameter p individual state variables x and parameters p are corrected in battery model 3 by correction equations x corr k 1 p corr k 1 until error dy is compensated the behavior of the battery may be represented in general as a nonlinear time discrete differential equation system of the first order for implementation in a digital computer wherein x k 1 f x k p u k 2 y k h x k p u k 3 u k the vector of the input variables of energy accumulator model 3 in time increment k at least one maximum of two measured battery performance quantities i batt meas u batt meas t batt meas x k the vector of state variables of energy accumulator model 3 in time increment k e g open circuit voltage u c0 polarization u d concentration polarization u k using starting value x 0 y k the vector of the output quantities of energy accumulator model 3 in time increment k and includes modeled performance quantities y that are also available as measurement y meas but without the performance quantities used as input variables u k p the vector of the parameters to be estimated of energy storage device model 3 f x k p u k the right side of state differential equations of energy accumulator model 3 and h x k p u k the vector of the output equations of energy accumulator model.