JP7125233B2 - Lithium-ion battery visual inspection method and lithium-ion battery visual inspection apparatus - Google Patents

Description

The present invention relates to a method for optically inspecting the appearance of lithium-ion batteries. More specifically, the present invention relates to an inspection method and an inspection apparatus for inspecting the presence or absence of adhesion of an electrolytic solution to the outer surface of a lithium ion battery.

In the manufacturing process of secondary batteries such as lithium ion batteries, the appearance of the manufactured batteries is inspected. For example, Patent Document 1 discloses a technique for irradiating a battery case with a laser beam to measure the three-dimensional shape of the battery case and displaying a three-dimensional image that facilitates detection of scratches and distortions on the outer surface of the battery case. there is

JP 2015-059817 A

A lithium ion battery contains an electrolytic solution containing a lithium salt in a battery case. If the electrolyte adheres to the outside of the battery case, there is a risk of corrosion, etc. Therefore, an appearance inspection is performed to determine whether or not the electrolyte adheres. Appearance inspection methods include, for example, a method using detection paper and a method using visual inspection, but it is desirable to be able to make a determination by image inspection in order to reduce the number of personnel. However, the lithium salt deposited by drying the electrolytic solution is white, and the difference in color from, for example, the battery case and electrode terminals made of a matte aluminum material is small. Therefore, when simply photographing with a camera or the like, the difference in light intensity between the lithium salt and the battery case in the photographed image is small, making it difficult to make stable determination by image inspection.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the prior art described above. That is, the object is to provide a lithium-ion battery visual inspection method and a lithium-ion battery visual inspection apparatus capable of easily determining the presence or absence of electrolyte adhering to the outside of the lithium-ion battery.

A method for visual inspection of a lithium-ion battery according to one aspect of the present invention, which has been made for the purpose of solving this problem, is a method for visual inspection of a lithium-ion battery having an electrolytic solution containing LiPF ₆ , comprising: A near- infrared imaging step of irradiating a subject, which is a lithium ion battery, with light containing near-infrared rays in the wavelength range of 1381 to 1460 nm, and photographing the subject with a camera capable of imaging near- infrared rays ; In the near- infrared image of the subject captured in the imaging step, when there is a region of a predetermined area or more where the light intensity is a predetermined value or less, the electrolyte is adhered to the subject. and a determination step of determining.

In the method for inspecting the appearance of a lithium-ion battery according to one aspect described above, the subject is irradiated with light containing near-infrared rays and photographed. Since the lithium salt absorbs more near-infrared rays than aluminum, the near-infrared reflectance of a portion with the lithium salt is smaller than the near-infrared reflectance of a battery case or the like without the lithium salt. Therefore, in an image captured by irradiating light containing near-infrared rays, the area where the lithium salt is attached has a lower light intensity than other areas. In this aspect, it is determined that the electrolytic solution is adhered when there is a region having a light intensity equal to or less than a predetermined value and having a predetermined area or more. Therefore, erroneous determination due to noise can be suppressed. As a result, it is possible to easily determine whether or not the electrolyte adheres to the outside of the lithium ion battery.

Further, an appearance inspection apparatus for a lithium ion battery according to one aspect of the present invention is an appearance inspection method for inspecting the appearance of a lithium ion battery having an electrolytic solution containing LiPF ₆ , the inspection object being a lithium ion battery. A near-infrared imaging step of irradiating light containing near-infrared rays in the wavelength range of 1381 to 1460 nm, and photographing the subject with a camera capable of imaging near- infrared rays, and the near-infrared imaging step. a determination step of determining that the electrolytic solution is adhered to the subject when there is a region having a predetermined area or more where the light intensity is less than or equal to a predetermined value in the near- infrared image of the subject; is characterized by including According to the external appearance inspection apparatus for a lithium ion battery in this aspect, it is possible to easily determine whether or not there is an electrolytic solution adhering to the outside of the lithium ion battery.
In addition, there is provided a visual inspection method for inspecting the appearance of a lithium ion battery having an electrolyte containing LiPF ₆ , wherein light containing near-infrared rays in the wavelength range of 1381 to 1460 nm is applied to a test subject, which is a lithium ion battery. and a near-infrared imaging step of imaging the subject with a camera capable of imaging near-infrared rays, and a visible light imaging step of imaging the subject with a camera capable of imaging visible light using visible light. a determination step of determining whether or not there is an area of a predetermined area or more with a light intensity of a predetermined value or less in the near-infrared captured image of the subject captured in the near-infrared imaging step; In the determination step, when it is determined that there is a region having a predetermined area or more, an acquisition step of acquiring a difference between the light intensity in the image captured by visible light and the light intensity in the image captured by near-infrared light; and a determination step of determining that the electrolytic solution adheres to the area of the predetermined area or more when the intensity difference is significantly larger in the area of the predetermined area or more than in the surrounding pixels. It can also be used as a feature.
Furthermore, a lithium-ion battery visual inspection apparatus for inspecting the appearance of a lithium-ion battery having an electrolytic solution containing LiPF ₆ , comprising: Equipped with a camera capable of imaging near-infrared light within the range of 1381 to 1460 nm, a light source that irradiates visible light, and a camera capable of imaging visible light, and a subject that is a lithium ion battery to be inspected is attached to the light source. and determining whether or not there is a region of a predetermined area or more where the light intensity is a predetermined value or less in the near-infrared image of the subject photographed by the camera, and determining whether or not there is a region of the predetermined area or more When it is determined that there is It may be characterized by further comprising a computer that determines that the electrolytic solution is adhered to the area of the predetermined area or more when the area is significantly larger than the pixel.

According to the present invention, there are provided a lithium ion battery visual inspection method and a lithium ion battery visual inspection apparatus capable of easily determining the presence or absence of electrolyte adhering to the outside of the lithium ion battery.

FIG. 3 is an explanatory diagram showing a secondary battery; It is an explanatory view showing an example of a visual inspection device. 4 is a graph showing the relationship between the wavelength of light and reflectance; FIG. 5 is an explanatory diagram showing an example of the result of inspection of the lid member of the battery case; It is a process drawing which shows the manufacturing process of a secondary battery.

A first embodiment of the present invention will be described in detail below with reference to the accompanying drawings. In this embodiment, the present invention is applied to an appearance inspection method for optically inspecting the appearance of a manufactured lithium-ion secondary battery.

The battery to be inspected by the inspection method of the present embodiment is a battery in which an electrode body and a non-aqueous electrolyte are sealed inside a battery case. Specifically, a battery case made of aluminum and a lithium salt A lithium ion secondary battery using a non-aqueous electrolyte containing The inspection of this embodiment is a visual inspection, which is an optical inspection performed on each battery from the outside of the battery case.

First, an example of a battery to be tested will be described with reference to FIG. As shown in FIG. 1, the battery 10 has a flat prismatic battery case 11 in which a wound electrode body 12 and an electrolytic solution 13 containing a lithium salt are enclosed. There is.

The battery case 11 is a metal container made of aluminum, an aluminum alloy, or the like. The lid member 112 is provided with positive and negative electrode terminals 21 and 22, a safety valve 23, and an injection port 24, as shown in FIG.

The electrode body 12 is formed by, for example, stacking a strip-shaped positive electrode plate and a strip-shaped negative electrode plate with a separator interposed therebetween and winding them flat. The positive electrode plate is, for example, an aluminum foil on which a positive electrode active material layer is formed, and the negative electrode plate is, for example, a copper foil on which a negative electrode active material layer is formed. The negative plate and the positive plate are respectively connected to electrode terminals 21 and 22 inside the battery case 11 .

The electrolytic solution 13 is a non-aqueous electrolytic solution containing a predetermined amount of lithium salt in a predetermined solvent. The solvent is, for example, an organic solvent containing ethylene carbonate or the like, and the lithium salt is preferably lithium hexafluorophosphate (LiPF ₆ ), for example.

Adhesion of the electrolyte solution 13 to the outside of the battery case 11 may cause corrosion of the battery case 11 or the like. In order to detect the battery 10 to which the electrolytic solution 13 is attached, for example, as shown in FIG. 2, an image inspection using a visual inspection apparatus 100 is performed. A visual inspection apparatus 100 shown in FIG. 2 includes a computer 30 , a lighting 31 , and a camera 32 . Specifically, the computer 30 controls the illumination 31 to illuminate the battery 10 and controls the camera 32 to photograph the illuminated battery 10 . Then, the computer 30 determines the quality of the battery 10 to be inspected based on the image data captured by the camera 32 .

In the image captured by the camera 32 , light and darkness and color differences appear depending on the location according to the intensity of the light emitted from the illumination 31 and reflected by the outer surface of the battery 10 . If there is a significant difference in the intensity and color of the reflected light, for example, it is possible to automatically analyze the photographed image and detect the battery 10 to which the electrolytic solution 13 is adhered. However, when lithium hexafluorophosphate is dried, it is white, which is close to the color of the battery case 11. Therefore, no significant difference can be obtained by inspection using visible light, making discrimination difficult.

Therefore, by changing the wavelength of the illumination 31, the reflectance of light at a portion of the battery case 11 to which the electrolytic solution 13 is not adhered and a portion to which the electrolytic solution 13 is adhered was examined. , the results shown in FIG. 3 were obtained. FIG. 3A shows changes in reflectance at locations where the electrolytic solution 13 is not adhered, and FIG. 3B shows changes in reflectance at locations where the electrolytic solution 13 is adhered. In FIGS. 3A and 3B, the horizontal axis is the wavelength and the vertical axis is the reflectance.

As shown in FIG. 3A, in the case of the battery case 11 to which the electrolytic solution 13 was not attached, the reflectance did not change significantly even when the wavelength was changed. On the other hand, as shown in FIG. 3B, the reflectance of the portion where the electrolytic solution 13 adhered was significantly lower in the wavelength range of L than in the other wavelengths. The wavelength range of L is the near-infrared region with a wavelength range of 1381 to 1460 nm, and the decrease in reflectance was particularly remarkable in the near-infrared region with a wavelength range of 1426 to 1435 nm. It is speculated that this is because LiPF ₆ absorbs near-infrared rays in this range.

In the inspection method of this embodiment, the battery 10 to be inspected is irradiated with near-infrared rays including a wavelength range of 1381 to 1460 nm, and the amount of light reflected from the battery 10 is detected. Specifically, in the visual inspection apparatus 100 shown in FIG. 2, the illumination 31 is irradiated with near-infrared light with a wavelength in the range of 1381 to 1460 nm, more preferably near-infrared light with a wavelength in the range of 1426 to 1435 nm. The camera 32 shall be capable of detecting near-infrared light within the wavelength range of 1381-1460 nm, more preferably near-infrared light within the wavelength range of 1426-1435 nm. In this way, the image of the portion where the electrolytic solution 13 is adhered in the photographed image has a smaller amount of reflected light and a lower light intensity than the other portions.

By the inspection method of this embodiment, the battery 10 with the electrolyte solution 13 adhering to a part of the lid member 112 is photographed, and one line (for example, line S in FIG. 2) of the image of the lid member 112 in the photographed image An example of light intensity is shown in FIG. In FIG. 4, the horizontal axis is the horizontal position in the line S, and the vertical axis is the light intensity (image gradation). This figure is an example of an image captured by the camera 32 with 8-bit gradation, and the range of light intensity is represented by 0 to 255 gradations.

For example, as shown in FIG. 4, the image of the area W where the electrolytic solution 13 is adhered has a lower light intensity than the other areas. The intensity difference f between the light intensities due to the presence or absence of adhesion of the electrolytic solution 13 is greater than 50, which is the gradation difference used as a criterion for stable detection. Therefore, the minimum gradation of the area where the electrolytic solution 13 is not adhered and the minimum gradation of the area W where the electrolytic solution 13 is adhered are obtained from experiments. , an intermediate gradation T between them is set as a detection threshold.

In this embodiment, the computer 30 analyzes the image data of the image captured by the method described above, and determines whether or not there is a location where pixels with gradations equal to or less than the detection threshold are gathered in a predetermined number or more. . For example, when the computer 30 determines that there is an area in which the number of pixels whose gradations are equal to or less than the detection threshold value is greater than the threshold value, the computer 30 determines that the electrolyte solution 13 is adhered to the battery 10 to be inspected. The detection threshold is an example of a predetermined value, and the threshold number is, for example, the number of pixels corresponding to the adhesion area of one droplet of the electrolytic solution 13, and is an example of a predetermined area.

By using near-infrared rays, the gradation difference between the lithium salt and the aluminum in the photographed image is large, making it suitable for automation. Furthermore, since the presence or absence of adhesion of the electrolytic solution 13 is determined depending on whether or not there is an area with pixels of low gradation equal to or greater than the threshold number, the possibility of erroneous determination due to noise or the like is small. Therefore, according to the inspection method of this embodiment, the presence or absence of adhesion of the electrolytic solution 13 can be automatically detected easily and reliably.

Next, the manufacturing process of the battery 10 including the inspection process by the inspection method of this embodiment will be described with reference to FIG. Before starting the manufacturing process shown in this figure, the positive electrode plate and the negative electrode plate are manufactured, respectively, and each member of the battery case 11 is prepared.

In this manufacturing process, an assembly process is first performed (process 1). In step 1, the electrode body 12 is formed by positive and negative electrode plates and a separator, and the electrode body 12 is connected to the electrode terminals 21 and 22 attached to the lid member 112 . Furthermore, the electrode body 12 is housed in the case body 111 and the cover member 112 is fixed to the case body 111 .

After the assembly process, a liquid injection/permeation process is performed (process 2). In step 2, the electrolytic solution 13 is injected into the case main body 111 through the inlet 24 of the lid member 112 , and the injected electrolytic solution 13 is permeated into each electrode plate of the electrode body 12 . Furthermore, the injection port 24 is sealed, and the assembly of the battery 10 is completed.

After the liquid injection/permeation process, a restraining process is performed (process 3). In step 3, the battery case 11 in which the electrode body 12 and the electrolytic solution 13 are sealed is restrained by being pressed from the outside by a restraining jig, for example. As a result, an appropriate surface pressure is applied to the electrode body 12 inside the battery case 11 .

After the restraining step, an initial charging step is performed (step 4). In step 4, the battery 10 is charged to a predetermined voltage while the surface pressure is applied by restraint. As a result, the battery 10 is fully charged.

After the initial charging step, a high temperature aging step is performed (step 5). In step 5, the fully charged battery 10 is removed from the charging device and allowed to stand in a high-temperature environment for a predetermined period of time. In this embodiment, for example, the aging process is performed in an environment of 60° C. for one day.

After the high temperature aging step, a self-discharge step is performed (step 6). In step 6, the aged battery 10 is moved to a room temperature environment and further left with the electrode terminals 21 and 22 left open. Thereby, the battery 10 self-discharges. In this embodiment, for example, the battery is self-discharged at 25° C. for three days.

After the self-discharge process, a performance confirmation process is performed (process 7). In step 7, the self-discharged battery 10 is charged and discharged according to a predetermined procedure, and the battery capacity and internal resistance are measured. At this stage, the battery 10 whose at least one of the battery capacity and the internal resistance value is out of the allowable range is excluded as a defective product.

After the performance confirmation process, an appearance inspection process is performed (process 8). In step 8, the battery 10 is photographed by the camera 32 while irradiating the outer surface of the battery 10 with near-infrared light from the illumination 31 by the appearance inspection apparatus 100 of the present embodiment. That is, for example, as shown in FIG. 2, near-infrared rays are emitted to the battery case 11 by illumination 31 that emits near-infrared rays with a wavelength in the range of 1381 to 1460 nm, more preferably near-infrared rays in the range of 1426 to 1435 nm. The near-infrared rays reflected by the battery case 11 are photographed by a camera 32. - 特許庁Among the steps 8, the step of photographing with the camera 32 is an example of the imaging step.

Furthermore, the computer 30 of the appearance inspection apparatus 100 of the present embodiment extracts pixels whose gradation is equal to or less than a predetermined detection threshold from the obtained image, and if there is an area in which the number of extracted pixels is equal to or greater than the threshold, , the battery 10 is determined to be an NG product. This step is an example of the determination step. Batteries 10 with no pixel whose gradation is equal to or less than the detection threshold, or batteries 10 whose gradation is not continuous with pixels whose gradation is equal to or less than the detection threshold are judged to be acceptable products and sent to the next step.

Note that the positional relationship among the battery 10, the illumination 31, and the camera 32 is not limited to the example shown in FIG. In other words, the illumination 31 may be positioned so as to irradiate light onto a portion of the battery 10 to be inspected. Moreover, the camera 32 may be located at a position where the reflected light from the portion illuminated by the illumination 31 can be photographed. For example, a dome-shaped illumination that surrounds the battery 10 may be used. Further, for example, a plurality of lights 31 may illuminate from a plurality of directions so as to cover the battery 10 . In this way, shadows are less likely to occur in the irradiated area, and in particular, erroneous determinations due to shadows that tend to occur depending on the shape of the battery case 11 are suppressed.

Further, the imaging gradation of the camera 32 is not limited to 8 bits, and for example, higher gradation may be used. For example, if a 10-bit or 12-bit camera is used, the detection threshold can be controlled in more detail than the above example.

As described in detail above, according to the inspection method of the first embodiment, near-infrared rays with a wavelength in the range of 1381 to 1460 nm, more preferably near-infrared rays with a wavelength in the range of 1426 to 1435 nm are applied to the battery 10. to shoot. If there is a portion where the electrolytic solution 13 is adhered, it can be easily determined by image analysis because the amount of reflected near-infrared light from that portion is small. Furthermore, in the present embodiment, by performing the visual inspection process after the processes such as aging and self-discharge, instead of immediately after the assembly process, it is possible to detect even if electrolyte leakage occurs in the process after the assembly process. In addition, it is possible to reliably detect the battery 10 in which the electrolyte solution 13 adhered after the assembling process has dried up and the lithium salt is deposited. Furthermore, since the appearance inspection can be automated, it is possible to contribute to labor saving.

Next, a second embodiment of the present invention will be described. This embodiment has substantially the same inspection object and inspection method as the first embodiment, but differs from the first embodiment in the determination method. Configurations similar to those of the first embodiment are denoted by the same reference numerals, and descriptions thereof are omitted.

In the present embodiment, as in the first embodiment, before or after photographing the battery 10 using near-infrared light, the same battery 10 to be inspected is photographed using, for example, visible light instead of near-infrared light. Add a process. In other words, a visible light source is arranged in the same manner as the illumination 31, and a camera capable of photographing visible light is arranged in the same manner as the camera 32, and an image of the same battery 10 captured by visible light is acquired. As described above, the lithium salt of the electrolytic solution 13 hardly absorbs visible light, and in an image captured with visible light, the adhesion of the electrolytic solution 13 causes almost no difference in light intensity. It should be noted that a light source having a low absorption rate by the electrolytic solution 13 may be used instead of visible light.

Then, in the determination step of the present embodiment, the computer 30 compares the image captured by visible light and the image captured by near-infrared light, and determines whether or not there is a portion where the light intensity difference appears only in the image captured by near-infrared light. Based on this, it is determined whether or not the electrolytic solution 13 is adhered. For example, as in the first embodiment, the computer 30 detects a location where light intensity is low in an image captured by near-infrared light, specifically, a location where a predetermined threshold number or more of pixels whose gradations are equal to or lower than the detection threshold are gathered. It is determined whether or not there is, and if it is determined that there is, the two images are aligned with reference to the outline of the edge of the battery case 11 or the like. Then, the difference between the light intensity in the image captured by visible light and the light intensity in the image captured by near-infrared light is acquired for the location where the light intensity is determined to be low and the pixels around the location. If the difference in light intensity between these two images is significantly larger than the surrounding pixels at the above location, for example, if it exceeds a predetermined threshold that can be determined to be significantly greater, It is determined that the electrolytic solution 13 is adhered.

Note that the peripheral image is, for example, a pixel adjacent to the location where the light intensity is determined to be low, or a location within several pixels from the location where the light intensity is determined to be low. Moreover, the method of obtaining the difference is not limited to the difference in the light intensity value for each pixel, and may use statistics such as the average value, standard deviation value, median value, etc. of the light intensity values of the surrounding pixels. Moreover, in order to obtain the statistic, preprocessing such as filtering may be appropriately applied to the captured image. Also, in the determination, for example, if the difference is significantly large within a range of a predetermined area or more, the determination may be NG, and if the range where the difference is large is less than the predetermined area, the determination may be OK.

According to the inspection method of this embodiment, the presence or absence of the electrolytic solution 13 adhering to the battery case 11 can be easily determined by image analysis, as in the first embodiment. Furthermore, in this embodiment, based on the image captured with visible light, it is determined whether the difference in light intensity in the image captured with near-infrared light is due to the imaging configuration and the external shape of the battery 10, or due to the adhesion of the electrolyte solution 13. discriminate. As a result, it is possible to suppress the influence of the light intensity difference that occurs due to the external shape of the battery 10 and the imaging configuration. Become.

It should be noted that this embodiment is merely an example and does not limit the present invention in any way. Therefore, the present invention can naturally be improved and modified in various ways without departing from the scope of the invention. For example, the battery 10 to be inspected is not limited to a flat rectangular shape, and may be of any shape. Moreover, the electrode body 12 is not limited to the wound type, and may be a laminated type.

Further, the computer 30 only needs to be able to acquire image data captured by the camera 32 and does not have to be connected to the lighting 31 or the camera 32 .

Also, in the present embodiment, the electrolytic solution 13 containing LiPF ₆ is used, but the electrolytic solution containing other lithium salts may be used. Any solvent may be used in the electrolytic solution.

REFERENCE SIGNS LIST 10 battery 11 battery case 30 computer 31 lighting 32 camera 100 appearance inspection device

Claims (4)

An appearance inspection method for inspecting the appearance of a lithium ion battery having an electrolyte containing LiPF ₆ ,
A near-infrared imaging step of irradiating a subject, which is a lithium-ion battery to be inspected, with light containing near-infrared rays within a wavelength range of 1381 to 1460 nm, and photographing the subject with a camera capable of imaging near- infrared rays ;
In the near- infrared captured image of the subject captured in the near- infrared imaging step, when there is a region having a predetermined area or more where the light intensity is equal to or less than a predetermined value, the electrolyte solution is present in the subject. A determination step of determining that it is attached;
A method for visual inspection of a lithium ion battery, comprising: A lithium-ion battery visual inspection apparatus for inspecting the appearance of a lithium-ion battery having an electrolyte containing LiPF ₆ ,
A light source that irradiates light containing near-infrared rays with a wavelength in the range of 1381 to 1460 nm;
a camera capable of imaging near-infrared rays within a wavelength range of 1381 to 1460 nm;
with
A subject, which is a lithium-ion battery to be inspected, is irradiated with light from the light source, and in a near- infrared image of the subject photographed with the camera, the light intensity is less than or equal to a predetermined value, and an area of a predetermined area or more is obtained. Further comprising a computer that determines that the electrolyte is attached to the subject when there is a region
Appearance inspection equipment for lithium-ion batteries. An appearance inspection method for inspecting the appearance of a lithium ion battery having an electrolyte containing LiPF ₆ ,
A near-infrared imaging step of irradiating a subject, which is a lithium-ion battery to be inspected, with light containing near-infrared rays within a wavelength range of 1381 to 1460 nm, and photographing the subject with a camera capable of imaging near- infrared rays ;
a visible light imaging step of imaging the subject using visible light with a camera capable of imaging visible light;
a determination step of determining whether or not there is a region of a predetermined area or more with a light intensity of a predetermined value or less in the near- infrared captured image of the subject captured in the near- infrared imaging step;
an acquiring step of acquiring the difference between the light intensity in the image captured by visible light and the light intensity in the image captured by near-infrared light when it is determined in the determination step that there is a region having a predetermined area or more;
a determination step of determining that the electrolyte is adhered to the area of the predetermined area or more when the difference in the light intensity is significantly larger than that of the surrounding pixels in the area of the predetermined area or more. Appearance inspection method for lithium-ion batteries. A lithium-ion battery visual inspection apparatus for inspecting the appearance of a lithium-ion battery having an electrolyte containing LiPF ₆ ,
A light source that irradiates light containing near-infrared rays with a wavelength in the range of 1381 to 1460 nm;
a camera capable of imaging near-infrared rays within a wavelength range of 1381 to 1460 nm;
a light source that emits visible light;
a camera capable of imaging visible light;
with
A subject, which is a lithium-ion battery to be inspected, is irradiated with light from the light source, and in a near- infrared image of the subject photographed with the camera, the light intensity is less than or equal to a predetermined value, and an area of a predetermined area or more is obtained. Determine whether there is an area,
When it is determined that there is a region having a predetermined area or more, obtaining the difference between the light intensity in the image captured by visible light and the light intensity in the image captured by near-infrared light,
The computer further comprises a computer that determines that the electrolytic solution is attached to the area of the predetermined area or more when the difference in light intensity is significantly larger than that of the surrounding pixels in the area of the predetermined area or more.
Appearance inspection equipment for lithium-ion batteries.

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