A METHOD FOR ESTIMATING OR PREDICTING AN INTERNAL BATTERY STATE OF A BATTERY UNIT

The invention relates to a method for estimating or predicting an internal battery state of at least one battery unit (203) within an electric energy storage system (201), such as in a vehicle (200), the method comprising: - obtaining operational data of the electric energy storage system relating to operating conditions of the electric energy storage system, - feeding the obtained operational data to a non-linear state observer adapted to estimate and/or predict the internal battery state of the at least one battery unit in a series of time steps, such that an observer error of the non-linear state observer converges towards zero, or towards a value close to zero, - based on at least the obtained operational data, estimating or predicting the internal battery state using the non-linear state observer. [0002] The non-linear state observer is a switched multi-gain observer switching between at least two different static observer gains, wherein the observer gain to be used is selected based on a predicted or estimated value of the internal battery state as determined by the nonlinear state observer.