A method for detecting inhomogeneous cell performance of a battery system comprising a plurality of battery cells, comprising: - determining a charging resistance ratio (ResRtoChg), which is the ratio of a maximum cell charging resistance (ResChgMax) of a battery cell among a plurality of battery cells and an average cell charging resistance (ResChgAvg) of the plurality of battery cells, each determined in response to a charging current applied to the battery cells; and determining a discharging resistance ratio (ResRtoDch), which is the ratio of a maximum cell discharging resistance (ResDchMax) of a battery cell among a plurality of battery cells and the average cell discharging resistance (ResDchAvg) of the battery cells, each determined in response to a discharging current applied to the battery cells, and wherein the determining of the charging resistance ratio (ResRtoChg) is based on a determined average cell voltage (uCellAvg), a determined average open circuit voltage (uOCV) and a determined maximum cell voltage (uCellMax) among a plurality of cell voltages (uCell1, uCell2, uCelln) of the battery system, the determining of the discharging resistance ratio (ResRtoDch) is based on a determined average cell voltage (uCellAvg), a determined average open circuit voltage (uOCV) and a determined minimum cell voltage (uCellMin) among a plurality of cell voltages (uCell1, uCell2, uCelln) of the battery system, and wherein determining the charging resistance ratio (ResRtoChg) comprises calculating the fraction (uCellMax - uOCV) / (uCellAvg - uOCV) and wherein determining the discharging resistance ratio (ResRtoDch) comprises calculating the fraction (uOCV - uCellMin) / (uOCV - uCellAvg); and - determining that the battery system is in a degraded ageing state, when at least one of the determined charging resistance ratio (ResRtoChg) and the determined discharging resistance ratio (ResRtoDch) is above a predetermined threshold value; and - determining that the battery system is in a non-degraded ageing state, when none of the determined charging resistance ratio (ResRtoChg) and the determined discharging resistance ratio (ResRtoDch) is above the predetermined threshold value. - determining a charging resistance ratio (ResRtoChg), which is the ratio of a maximum cell charging resistance (ResChgMax) of a battery cell among a plurality of battery cells and an average cell charging resistance (ResChgAvg) of the plurality of battery cells, each determined in response to a charging current applied to the battery cells; and determining a discharging resistance ratio (ResRtoDch), which is the ratio of a maximum cell discharging resistance (ResDchMax) of a battery cell among a plurality of battery cells and the average cell discharging resistance (ResDchAvg) of the battery cells, each determined in response to a discharging current applied to the battery cells, and wherein the determining of the charging resistance ratio (ResRtoChg) is based on a determined average cell voltage (uCellAvg), a determined average open circuit voltage (uOCV) and a determined maximum cell voltage (uCellMax) among a plurality of cell voltages (uCell1, uCell2, uCelln) of the battery system, the determining of the discharging resistance ratio (ResRtoDch) is based on a determined average cell voltage (uCellAvg), a determined average open circuit voltage (uOCV) and a determined minimum cell voltage (uCellMin) among a plurality of cell voltages (uCell1, uCell2, uCelln) of the battery system, and wherein determining the charging resistance ratio (ResRtoChg) comprises calculating the fraction (uCellMax - uOCV) / (uCellAvg - uOCV) and wherein determining the discharging resistance ratio (ResRtoDch) comprises calculating the fraction (uOCV - uCellMin) / (uOCV - uCellAvg); and determining a discharging resistance ratio (ResRtoDch), which is the ratio of a maximum cell discharging resistance (ResDchMax) of a battery cell among a plurality of battery cells and the average cell discharging resistance (ResDchAvg) of the battery cells, each determined in response to a discharging current applied to the battery cells, and wherein the determining of the charging resistance ratio (ResRtoChg) is based on a determined average cell voltage (uCellAvg), a determined average open circuit voltage (uOCV) and a determined maximum cell voltage (uCellMax) among a plurality of cell voltages (uCell1, uCell2, uCelln) of the battery system, the determining of the discharging resistance ratio (ResRtoDch) is based on a determined average cell voltage (uCellAvg), a determined average open circuit voltage (uOCV) and a determined minimum cell voltage (uCellMin) among a plurality of cell voltages (uCell1, uCell2, uCelln) of the battery system, and wherein determining the charging resistance ratio (ResRtoChg) comprises calculating the fraction (uCellMax - uOCV) / (uCellAvg - uOCV) and wherein determining the discharging resistance ratio (ResRtoDch) comprises calculating the fraction (uOCV - uCellMin) / (uOCV - uCellAvg); and - determining that the battery system is in a degraded ageing state, when at least one of the determined charging resistance ratio (ResRtoChg) and the determined discharging resistance ratio (ResRtoDch) is above a predetermined threshold value; and - determining that the battery system is in a non-degraded ageing state, when none of the determined charging resistance ratio (ResRtoChg) and the determined discharging resistance ratio (ResRtoDch) is above the predetermined threshold value.