Computer-implemented method of estimating battery degradation, in particular, battery in a battery energy storage system, BESS, comprising the steps of: a) acquiring (S1) battery parameters of a battery, characteristics of calendar ageing wearing coefficient BWC 1 and/or characteristics of cycle ageing wearing coefficient BWC 2 wherein BWC 1 is a function of State of Charge, SoC, and BWC 2 is a function of charging/dis-charging rate, C-rate, b) acquiring and/or calculating (S2) instantaneous values of SoC and C-rate of a battery in a defined period, c) reading (S3) instantaneous values of calendar ageing wearing coefficient BWC 1 and/or instantaneous values of cycle ageing wearing coefficient BWC 2 , corresponding to instantaneous values of SoC and C-rate of a battery acquired in step (S2), using characteristics of ageing wearing coefficients acquired in step (S1) and d) determining (S4): - value of calendar ageing wearing index BWI 1 by referring integrated instantaneous values of BWC 1 determined in step (S3) to the integrated nominal values of BWC 1 for a period of nominal operation time with maximum allowable value of the SoC, and/or - values of cycle ageing wearing index BWI 2 by referring integrated instantaneous values of BWC 2 determined in step (S3) to the integrated nominal values of BWC 2 for full battery charging or discharging with nominal C-rate thereby indicating degree of battery degradation. a) acquiring (S1) battery parameters of a battery, characteristics of calendar ageing wearing coefficient BWC 1 and/or characteristics of cycle ageing wearing coefficient BWC 2 wherein BWC 1 is a function of State of Charge, SoC, and BWC 2 is a function of charging/dis-charging rate, C-rate, b) acquiring and/or calculating (S2) instantaneous values of SoC and C-rate of a battery in a defined period, c) reading (S3) instantaneous values of calendar ageing wearing coefficient BWC 1 and/or instantaneous values of cycle ageing wearing coefficient BWC 2 , corresponding to instantaneous values of SoC and C-rate of a battery acquired in step (S2), using characteristics of ageing wearing coefficients acquired in step (S1) and d) determining (S4): - value of calendar ageing wearing index BWI 1 by referring integrated instantaneous values of BWC 1 determined in step (S3) to the integrated nominal values of BWC 1 for a period of nominal operation time with maximum allowable value of the SoC, and/or - values of cycle ageing wearing index BWI 2 by referring integrated instantaneous values of BWC 2 determined in step (S3) to the integrated nominal values of BWC 2 for full battery charging or discharging with nominal C-rate thereby indicating degree of battery degradation. - value of calendar ageing wearing index BWI 1 by referring integrated instantaneous values of BWC 1 determined in step (S3) to the integrated nominal values of BWC 1 for a period of nominal operation time with maximum allowable value of the SoC, and/or - values of cycle ageing wearing index BWI 2 by referring integrated instantaneous values of BWC 2 determined in step (S3) to the integrated nominal values of BWC 2 for full battery charging or discharging with nominal C-rate thereby indicating degree of battery degradation. <EOS>