NOVELTY - The method involves determining electrical performance of cells of an electrochemical storage of an energy source (105). A current impulse or a sinusoidal current curve is generated (110). The capacity of internal resistance and/or internal impedance of each cell is measured (120). The terminal voltage of each cell is raised. The loss of capacity of each cell of the energy source is determined (125) from the increase in internal resistance and/or the impedance of the cell. The cells are classified according to the determined performance (155).    USE - Method for monitoring an energy source e.g. accumulator battery or super-capacitor, to detect failure and aging of the accumulator battery or super-capacitor of a vehicle e.g. car (claimed) such as electric car.    ADVANTAGE - The method enables determining the electrical performance of the cells, and classifying the cells according to the determined performance, so that the state of health of the battery and the state of health of all the cells can be determined so as to decide whether the battery needs to be repaired, and the battery repair can be optimized during a need to repair the battery, thus reducing the defective cells.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a device for monitoring an energy source to detect failure and aging of an accumulator battery or super-capacitor of a vehicle i.e. car.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow diagram of a method for monitoring an energy source to detect failure and aging of an accumulator battery or super-capacitor of a vehicle.'(Drawing includes non-English language text)'    Step for determining electrical performance of cells of an electrochemical storage of an energy source (105)    Step for generating current impulse or sinusoidal current curve (110)    Step for measuring capacity of internal resistance and/or internal impedance of each cell (120)    Step for determining loss of capacity of each cell of an energy source (125)    Step for classifying cells according to determined performance (155)