Method of grouping single cells of power sources to build optimal packs using parameters obtained by analysis of impedance spectrum

An optimization method for selection of unit cells using impedance measurement and analysis to make a battery pack, the method comprising:

• (a) measuring complex impedances at different frequencies in a predetermined frequency range for power sources of a different internal performance characteristic;
• (b) doing approximation fitting of the impedance spectrum curve measured in step (a) to a defined impedance function of the equivalent circuit model of power sources to determine several parameters as separated into resistance and capacitance components;
• (c) determining, for the individual power sources, one or more total resistances (a DC approximation resistance as the real value of impedance data at a frequency of 0 Hz) corresponding to the resistance component determined in step (b);
• and (d) calculating a probability density distribution of the power sources by the total resistances, based on the total resistances determined for the individual power sources in step (c), and sorting the power sources of a similar internal performance characteristic into the same group to make the battery pack.