Airtightness inspection method and airtightness inspection apparatus for sealed battery

1. An airtightness inspection method for a sealed battery having a battery element sealed into a battery case in a negative pressure environment, the battery case being made of a deformable film, the method comprising:

enclosing the sealed battery in an airtight container;

measuring a surface shape of the sealed battery enclosed in the airtight container;

decompressing the airtight container under a predetermined negative pressure that is smaller in absolute pressure value than a negative pressure of the negative pressure environment applied for sealing the battery element into the battery case;

measuring a surface shape of the sealed battery in the airtight container in a decompressed state where the predetermined negative pressure is maintained; and

determining airtightness of the sealed battery based on a change in the surface shape of the sealed battery between before decompression of the airtight container and in the decompressed state of the airtight container.

2. The airtightness inspection method for the sealed battery as defined in claim 1 , wherein the surface shape of the airtight container before decompression is measured in a state where an inner space of the airtight container is in an atmospheric pressure,

the decompression of the airtight container under the predetermined negative pressure is performed rapidly, and

the change in the surface shape of the sealed battery includes an expansion of the battery case or generation of invisible fine creases on the surface of the battery case in the decompressed state.

3. The airtightness inspection method for the sealed battery as defined in claim 1 , wherein the surface shape of the sealed battery before decompression and the surface shape of the sealed battery in the decompressed state of the airtight container are measured by capturing images of the sealed battery by a digital photographic apparatus with the sealed battery illuminated; and

the change in the surface shape of the sealed battery is determined by comparing, pixel by pixel, an image of the sealed battery captured before decompression and an image of the sealed battery captured in the decompressed state of the airtight container.

4. The airtightness inspection method for the sealed battery as defined in claim 3 , further comprising

determining whether or not a difference between a luminance of a pixel of the image captured before decompression and a luminance of the pixel of the image captured in the decompression state exceeds a set value; and

determining whether or not airtightness of the sealed battery is maintained based on the number of pixels with a luminance difference exceeding the set value.

5. The airtightness inspection method for the sealed battery as defined claim 4 , further comprising:

calculating an area with a difference in the surface shape from the number of the pixels with a difference exceeding the set value and a resolution of the digital photographic apparatus; and

determining whether or not airtightness of the sealed battery is maintained based on the area with the difference in the surface shape.

6. An airtightness inspection apparatus for a sealed battery, having a battery element sealed into a battery case in a negative pressure environment, the battery case being made of a deformable film, the method, comprising:

an airtight container for containing the sealed battery;

measurement means for measuring a surface shape of the sealed battery enclosed in the airtight container;

decompression means for decompressing the airtight container under a predetermined negative pressure that is smaller in absolute pressure value than a negative pressure of the negative pressure environment applied for sealing the battery element into the battery case; and

determination means for determining airtightness of the sealed battery based on a change in a surface shape of the sealed battery between before decompression of the airtight container and a decompressed state of the airtight container where the predetermined negative pressure is maintained,

wherein the measurement means is configured to measure the surface shape of the sealed battery in the airtight container in the decompressed state where the predetermined negative pressure is maintained.

7. The airtightness inspection apparatus for the sealed battery as defined in claim 6 , wherein the measurement means comprises a digital photographic apparatus for capturing an image of the surface shape of the sealed battery and the determination means comprises a programmable controller programmed to:

store an image captured by the digital photographic apparatus;

determine whether or not a difference between a luminance of a pixel of an image captured before decompression and a luminance of the pixel of an image captured in the decompressed state exceeds a set value; and

determine whether or not airtightness of the sealed battery is maintained based on the number of pixels with a luminance difference exceeding the set value.

8. The airtightness inspection apparatus for the sealed battery as defined in claim 6 , wherein the digital photographic apparatus is installed outside the airtight container; and

at least a part of the airtight container is formed of a transparent material to enable image capturing of the sealed battery by the digital photographic apparatus.

9. The airtightness inspection apparatus for the sealed battery as defined in claim 6 , wherein the airtight container has an opening that is open toward outside air; and

the airtightness inspection apparatus further comprises a carrier device that carries the sealed battery and engages with the opening so as to tightly close the airtight container with the sealed battery enclosed.

10. The airtightness inspection apparatus for the sealed battery as defined in claim 6 , further comprising:

a display device for displaying an image captured by the digital photographic apparatus before decompression, an image captured by the digital photographic apparatus in the decompressed state of the airtight container, and pixels with a luminance difference exceeding a set value.