A method for monitoring battery ageing of a battery unit (202) in an energy storage system (200) of a vehicle (201) or vessel, the method comprising: - predicting (S1) an expected service life of the battery unit based on at least information relating to a planned usage of the battery unit, - during usage of the battery unit, obtaining (S2) operational data relating to an actual usage of the battery unit, - predicting (S3) an actual service life of the battery unit based on at least the obtained operational data, - comparing (S4) the predicted actual service life with the predicted expected service life, thus obtaining a comparison result; - determining (S5) whether a predetermined notification criterion is fulfilled based on the comparison result, - generating (S6) a notification to a user of the battery unit at least if the predetermined notification criterion is fulfilled.