Method of selecting used secondary battery and method of manufacturing battery pack

1. A method of selecting a used secondary battery that is reusable from a plurality of collected used secondary batteries, comprising:

an internal resistance measurement step of measuring a specific internal resistance at a specific battery temperature of each of the used secondary batteries; and

a selection step of selecting used secondary batteries having the specific internal resistance smaller than a preset threshold from the plurality of used secondary batteries whose specific internal resistances have been measured;

the threshold being set by a process including a step of measuring the specific internal resistance of each of a plurality of sample secondary batteries that are used secondary batteries prepared for determining the threshold;

continuously discharging each of the sample secondary batteries at a battery temperature equivalent to the specific battery temperature, and measuring a minimum voltage in the each sample secondary battery during a period of the discharge;

plotting the specific internal resistances and the minimum voltages of each of the sample secondary batteries to create a correlation chart;

determining a regression line of sample secondary batteries including at least all of the sample secondary batteries having a specific internal resistance not exceeding a reference value among the plurality of sample secondary batteries in the correlation chart;

determining a lower limit line that is parallel to the regression line and shifted a predetermined value from the regression line to a side where the minimum voltage is lower in the correlation chart;

calculating a value obtained by dividing a ranking number of batteries in region of the plurality of sample secondary batteries in ascending order of the specific internal resistance by a total number of sample secondary batteries, wherein the batteries in region are contained in a region of the correlation chart where the specific internal resistance is larger than the reference value and the minimum voltage does not exceed the lower limit line;

specifying a battery having a smallest specific internal resistance out of the batteries contained in the region and having a calculated value exceeding a preset value; and

setting the specific internal resistance of the specified battery contained in the region as the threshold.

2. The method of selecting a used secondary battery according to claim 1 , wherein the regression line is a regression line of sample secondary batteries having a specific internal resistance not exceeding the reference value, of the plurality of sample secondary batteries.

3. The method of selecting a used secondary battery according to claim 1 , wherein the predetermined value for determining the lower limit line is one half of an allowable voltage difference between the used secondary batteries determined by a control device of a battery pack during use of the battery pack formed by the plurality of used secondary batteries.

4. The method of selecting a used secondary battery according to claim 1 , wherein the specific battery temperature in the internal resistance measurement step is a temperature of 0° C. or less.

5. The method of selecting a used secondary battery according to claim 1 , wherein

the used secondary batteries are used nickel-metal hydride secondary batteries, and

the reference value is a value of 70% of the specific internal resistance a nickel-metal hydride secondary battery initially has.

6. A method of manufacturing a battery pack formed by combining a plurality of used secondary batteries, comprising:

a step of selecting a plurality of used secondary batteries from the plurality of used secondary batteries selected by the method of selecting a used secondary battery according to claim 1 such that a maximum difference between the specific internal resistances is within an allowable limit; and

an assembling step of forming a battery pack by electrically connecting the plurality of selected used secondary batteries.

7. A method of manufacturing a battery pack formed by combining a plurality of used secondary batteries, comprising:

a first internal resistance measurement step of measuring a first internal resistance at a first battery temperature that is a normal temperature of each of the used secondary batteries;

a second internal resistance measurement step of measuring a second internal resistance at a second battery temperature that is lower than the normal temperature of each of the used secondary batteries;

a first selection step of selecting used secondary batteries having the second internal resistance smaller than a preset threshold from the plurality of used secondary batteries whose second internal resistances have been measured;

a second selection step of selecting a plurality of used secondary batteries having a maximum difference between the first internal resistances not exceeding an allowable limit, as well as a maximum difference between the second internal resistances not exceeding the allowable limit, from the plurality of the used secondary batteries selected in the first selection step;

an assembling step of forming the battery pack by electrically connecting the plurality of the used secondary batteries selected in the second selection step,

the threshold being set by a process including a step of measuring the second internal resistance of each of a plurality of sample secondary batteries that are used secondary batteries prepared for determining the threshold;

continuously discharging each of the sample secondary batteries at a battery temperature equivalent to the second battery temperature, and measuring a minimum voltage in the each sample secondary battery during a period of the discharge;

plotting the second internal resistances and the minimum voltages of each of the sample secondary batteries to create a correlation chart;

determining a regression line of sample secondary batteries including at least all of the sample secondary batteries having a specific internal resistance not exceeding a reference value among the plurality of sample secondary batteries in the correlation chart;

determining a lower limit line that is parallel to the regression line and shifted a predetermined value from the regression line to a side where the minimum voltage is lower in the correlation chart;

calculating a value obtained by dividing a ranking number of batteries in region of the plurality of sample secondary batteries in ascending order of the second internal resistance by a total number of the sample secondary batteries, wherein the batteries in region are contained in a region of the correlation chart where the second internal resistance is larger than the reference value and the minimum voltage does not exceed the lower limit line;

specifying a battery having a smallest second internal resistance out of the batteries contained in the region and having a calculated value exceeding a preset value; and

setting the second internal resistance of the specified battery contained in the region as the threshold.

8. The method of manufacturing a battery pack according to claim 7 , wherein the second battery temperature in the second internal resistance measurement step is a temperature of 0° C. or less.

9. The method of manufacturing a battery pack according to claim 7 , wherein

the used secondary batteries are used nickel-metal hydride secondary batteries, and

the reference value is a value of 70% of the second internal resistance a nickel-metal hydride secondary battery initially has.