Deterioration level calculation method for secondary battery and non-transitory computer-readable medium storing deterioration level calculation program

1 . A deterioration level calculation method for a secondary battery, the method comprising causing a computer comprising a processor and a memory to perform arithmetic processes including:

measuring an open circuit voltage of a specific secondary battery by connecting the secondary battery to a voltage measurement device at a charging rate in a range from 0% to 100% to obtain measurement data;

a measured value acquisition process of generating a measured value charging rate voltage curve for the secondary battery by using a measurement result of the open circuit voltage at a charging rate in a range from 0% to 100%;

a theoretical value generation process of generating a theoretical value charging rate voltage curve computed from a difference between a cathode open circuit potential theoretical value curve computed from a content of at least one component of a cathode composite of the secondary battery by using a fitting function and an anode open circuit potential theoretical value curve computed from a content of at least one component of an anode composite of the secondary battery;

an evaluation value computation process of computing, with use of an evaluation function to compute an evaluation value, the evaluation value, the evaluation value indicating a magnitude of a difference between the theoretical value charging rate voltage curve and the measured value charging rate voltage curve; and

an analysis process of repeating the theoretical value generation process and the evaluation value computation process with changes of a shift amount parameter and a scaling rate parameter and outputting the shift amount parameter at which the evaluation value is minimized as a deterioration level of the secondary battery, in the fitting function, the shift amount parameter for shifting one of the theoretical positive open circuit potential value curve and the theoretical negative open circuit potential value curve in the charging rate direction, the scaling rate parameter for adjusting the length of the other curve in the charging rate direction; and

determining, based on an outputted deterioration level, whether the secondary battery is suitable for reuse in a different application than its original application,

wherein the evaluation function includes a whole evaluation term representing a difference between the theoretical value charging rate voltage curve and the measured value charging rate voltage curve as a whole, a high charging rate region evaluation term representing a difference between the theoretical value charging rate voltage curve and the measured value charging rate voltage curve in a high charging rate region, and a low charging rate region evaluation term representing a difference between the theoretical value charging rate voltage curve and the measured value charging rate voltage curve in a low charging rate region, the high charging rate region evaluation term and the low charging rate region evaluation term being weighted higher than the whole evaluation term.

2 . The deterioration level calculation method for a secondary battery according to claim 1 , further comprising a measured value differential plot generation process of generating a measured value differential plot where a differential value of the measured value charging rate voltage curve is plotted, wherein

in the theoretical value generation process, a theoretical value differential plot that is a differential value of the theoretical value charging rate voltage curve is generated using the fitting function, and

in the evaluation value computation process, evaluation is made by applying the measured value differential plot as the evaluation function in place of the measured value charging rate voltage curve and applying the theoretical value differential plot in place of the theoretical value charging rate voltage curve.

3 . The deterioration level calculation method for a secondary battery according to claim 2 , wherein at least one of the high charging rate region evaluation term and the low charging rate region evaluation term uses a logarithmic value as a value for evaluating the difference.

4 . The deterioration level calculation method for a secondary battery according to claim 1 , wherein the scaling rate parameter determined in the analysis process is outputted as a parameter for determining a cathode/anode capacity ratio of the secondary battery.

5 . The deterioration level calculation method for a secondary battery according to claim 1 , wherein

the cathode open circuit potential theoretical value curve is computed based on a content of lithium iron phosphate contained in the cathode composite, and

the anode open circuit potential theoretical value curve is computed based on a content of carbon contained in the anode composite.

6 . A non-transitory computer-readable medium storing a deterioration level calculation program for a secondary battery, the program being configured to cause an arithmetic unit of a computer to perform:

measuring an open circuit voltage of a specific secondary battery by connecting the secondary battery to a voltage measurement device at a charging rate in a range from 0% to 100% to obtain measurement data;

a measured value acquisition process of generating a measured value charging rate voltage curve for the secondary battery by using a measurement result of the open circuit voltage at a charging rate in a range from 0% to 100%;

a theoretical value generation process of generating a theoretical value charging rate voltage curve computed from a difference between a cathode open circuit potential theoretical value curve computed from a content of at least one component of a cathode composite of the secondary battery by using a fitting function and an anode open circuit potential theoretical value curve computed from a content of at least one component of an anode composite of the secondary battery;

an evaluation value computation process of computing, with use of an evaluation function to compute an evaluation value, the evaluation value, the evaluation value indicating a magnitude of a difference between the theoretical value charging rate voltage curve and the measured value charging rate voltage curve; and

an analysis process of repeating the theoretical value generation process and the evaluation value computation process with changes of a shift amount parameter and a scaling rate parameter and outputting the shift amount parameter at which the evaluation value is minimized as a deterioration level of the secondary battery, in the fitting function, the shift amount parameter for shifting one of the theoretical positive open circuit potential value curve and the theoretical negative open circuit potential value curve in the charging rate direction, the scaling rate parameter for adjusting the length of the other curve in the charging rate direction; and

determining, based on an outputted deterioration level, whether the secondary battery is suitable for reuse in a different application than its original application,

wherein the evaluation function includes a whole evaluation term representing a difference between the theoretical value charging rate voltage curve and the measured value charging rate voltage curve as a whole, a high charging rate region evaluation term representing a difference between the theoretical value charging rate voltage curve and the measured value charging rate voltage curve in a high charging rate region, and a low charging rate region evaluation term representing a difference between the theoretical value charging rate voltage curve and the measured value charging rate voltage curve in a low charging rate region, the high charging rate region evaluation term and the low charging rate region evaluation term being weighted higher than the whole evaluation term.