A current measuring circuit measures a charging current of a battery comprising a plurality of cell groups while the battery is being charged using one of a plurality of charging systems. A voltage measuring circuit measures voltages of the cell groups. A controller defines a plurality operating regions in a charging current profile of the battery during a charging cycle, filters the charging current and the voltages measured in the operating regions, calculates internal resistances of the cell groups in the operating regions based on the filtered current and voltages, generates at least one of statistical values and distance metrics based on the internal resistances of the cell groups, and determines whether one or more of the cell groups is faulty based on the at least one of the statistical values and the distance metrics irrespective of the plurality of charging systems used to charge the battery.