Method of grouping single cells of power sources to build optimal packs using parameters obtained by analysis of impedance spectrum
A method for selecting unit cells to make an optimal battery pack improves the performance of the battery pack with primary or secondary batteries connected to one another in series, in parallel, or in a combination of them. The method for selecting unit cells to make the optimal battery pack includes measuring the impedance spectrum of the individual unit cells in a predetermined frequency region, fitting the impedance spectrum to an equivalent circuit model composed of parameters including resistance and capacitance components, calculating total resistances from the fitted parameters, and making the battery pack with unit cells of a similar total resistance.
1. An optimization method for selection of unit cells using impedance measurement and analysis to make a battery pack, the method comprising:
(a) calculating at least two parameters by an indirect and non-destructive testing for understanding the internal performance characteristic of electrochemical power sources;
(b) determining a correlation between the sum of the parameters and the internal performance characteristic of the power sources; and
(c) sorting the power sources of a similar internal characteristic into the same group based on said correlation to make the battery pack.
2. The method as claimed in claim 1 , wherein the electrochemical power sources include primary battery, fuel cell, secondary battery including lithium-ion battery, lithium-ion polymer, lithium polymer, NiCd, NiMH or lead acid battery.
3. The method as claimed in claim 1 , wherein the parameters are determined by a fitting method involving linear or non-linear fitting to the equivalent circuit model.
4. 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.
5. The method as claimed in claim 4 , wherein the electrochemical power sources include primary battery, fuel cell, secondary battery including lithium-ion battery, lithium-ion polymer, lithium polymer, NiCd, NiMH or lead acid battery.
6. The method as claimed in claim 4 , wherein the measurement in step (a) includes a multi-sine wave Fourier transformation impedance spectrum measurement.
7. The method as claimed in claim 4 , wherein the measurement in step (a) includes a transient response Laplace transformation impedance spectrum measurement.
8. The method as claimed in claim 4 , wherein the equivalent circuit model includes an nRC component model.
9. The method as claimed in claim 4 , wherein the total resistance is a DC approximation resistance as the real value of impedance data at a frequency of 0 Hz.
10. The method as claimed in claim 4 , wherein the battery pack comprises the power sources of a same group based on the probability density distribution of the total resistances.