A device is described for ascertaining state variables and/or parameters of a mathematical energy accumulator model, in particular a battery model, that describes the electrical properties of the energy accumulator on the basis of various state variables and parameters, the mathematical energy accumulator model having correction equations by which the state variables and/or the parameters are corrected and adjusted to the actual operating performance of the energy accumulator. A particularly rapid and accurate compensation of the energy accumulator model may be achieved if the error between a measured battery performance quantity and the battery performance quantity calculated by the energy accumulator model, a differentiated component of the error and an integrated component of the error are calculated, the individual components are each weighted using a weighting factor and the weighted error components are taken into account in correction of a state variable and/or a parameter.