A method for estimating an operating parameter of a battery unit (202) in an
energy storage system (200) of a vehicle (201) using a set of first estimators, each first estimator being configured to estimate a state residual size of the battery unit based on a given initial value of said operating parameter and using a prediction model, the operating parameter being indicative of one of capacity and impedance of the battery unit, the method comprising: —obtaining measurement data relating to operating conditions of the
energy storage system at a number of points in time tk, —providing a set of initial values of said operating parameter, each initial value being associated with one of said first estimators, —using the set of first estimators, estimating (103) a set of state residual sizes of the battery unit at each point in time tk based on at least the measurement data and on the associated initial values, —determining a first operating parameter estimate of the battery unit based on the estimated set of state residual sizes, —estimating the operating parameter of the battery unit as a function of
time based on at least said first operating parameter estimate, wherein estimating the set of state residual sizes comprises using a recursive
algorithm for estimating the state residual size of each element of a set of state residuals, each estimated state residual size being determined based on the state residual size as estimated at a previous point in time tk−1 and a current magnitude of the state residual.