CN113921411A - Method for improving single crystal photovoltaic group string false soldering recognition rate - Google Patents
Method for improving single crystal photovoltaic group string false soldering recognition rate Download PDFInfo
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Abstract
The invention discloses a method for improving the false soldering recognition rate of a single-side single crystal photovoltaic group string, which comprises the following steps of S1, camera and shooting environment calibration: the method takes image gray scale quantization as a detection basis, and introduces a standard first piece to check an EL test camera and a test environment before the production of a production line starts, so as to meet the requirements of no obvious light leakage, no defocus, no overexposure and no halation; s2, identifying the insufficient solder joint of the single crystal photovoltaic module: after identifying the component to be inspected, a bias current I is applied1EL test is carried out to obtain an EL test chart phi1Then with a bias current I2EL test is carried out to obtain an EL test chart phi2Is combined withAnd obtaining a value of the result of the two tests to obtain delta phi (i), defining a concept deviation percentage S (i), and judging whether the front surface is in the front surface cold joint or the back surface is in the back surface cold joint according to the difference value between the judgment threshold value X and the judgment threshold value S (i) and the distribution relation. The improvement is based on the technical improvement provided by the traditional EL test method, the EL hardware equipment does not need to be replaced on a large scale, the improvement cost is low, and the popularization and the application are easy.
Description
Technical Field
The invention belongs to the technical field of single-crystal photovoltaic image recognition and detection, and particularly relates to a method for improving the false soldering recognition rate of a single-side single-crystal photovoltaic group string.
Background
Electroluminescence (EL) is widely used in the field of semiconductor illumination, and by using the technology, defects in solar cell production can be detected. Under a proper forward bias voltage, an electric field built in a potential barrier region of the silicon wafer cell is weakened, the recombination probability of non-equilibrium carriers is increased, and the cell emits near infrared light with the wavelength of 800-1300 nm, and can be captured by a CCD camera. By judging the EL image, the defect condition of most silicon wafer batteries can be screened without damage, and the method is a common means for production quality control of photovoltaic modules.
The series welding process is the first step in the manufacturing process of the photovoltaic module and is also the core step. With the improvement of the automation level, the series welding lamination process is gradually realized by domestic production workshops, and the phenomena of insufficient welding and the like can occur on the back of the battery pack string due to the instability of a series welding machine in the production process. The component with the cold joint phenomenon is easy to generate local heating phenomenon in the service process of a power station, so that the efficiency attenuation phenomenon of the component in the service life is serious, and quality control should be carried out in the production link.
In the existing operation, a factory production line is generally provided with three process points of pre-lamination EL test, post-lamination visual inspection and post-lamination EL and IV detection to inspect the quality of the assembly, manual work is required to be arranged in all three links for judgment, and the positions in the production flow of the photovoltaic assembly are shown in figure 1.
Since the output voltage of the single cell is limited, in order to increase the output voltage of the module, the cells need to be connected in series with the welding bars in the front-rear direction. The automatic stringer can sort the battery pieces in the early stage through the PL technology, quantitatively control the temperature gradient and the welding temperature value of the heating bottom plate, and fully play the characteristics of high efficiency and intellectualization of mechanical production. After series welding is finished, the quality of front series welding is judged by a manual visual inspection method or a camera image identification method, and for judging the situation of back series welding, some factories can require workers at a laying station to confirm the situation of back welding before laying is finished, but the monitoring is not carried out due to full time, and no rewarding measures are supplemented, so that the detection rate is unsatisfactory.
To ameliorate the above-mentioned disadvantages, monitoring is typically accomplished by sampling the cell-ribbon pull test and performing full inspection at the pre-lamination EL test station. However, the battery piece-welding strip tension test is destructive, the welding quality can only be monitored periodically, and the cold joint phenomenon caused by the accidental instability of the machine cannot be detected accurately.
For the process defect of back surface cold joint, in the manufacturing process of the single-surface power generation assembly, screening can be performed only by an EL testing method, but due to the interference of the distance and background factors, the cold joint phenomenon of the battery strings at the edge is not obvious and the manual identification rate is low in the testing image obtained by the traditional EL detection method for the large-area assembly. For the component EL test, because the size of a component sample is larger than that of a single battery, and the distance from each battery to a camera lens is different, in the EL imaging process, the EL intensity is in direct proportion to the reciprocal of the square of the distance, so that the final imaging effect is influenced, the central position of the camera is bright, the edge position of the camera is dark, and the manual interpretation of the false soldering phenomenon is influenced.
Disclosure of Invention
The invention aims to solve the technical problems, designs the method aiming at the characteristics of a factory production line, and provides a method for calculating and identifying the back rosin joint by analyzing the gray level difference of the EL image of the cell at two different bias voltages or currents, so that the defect screening rate of an EL test post before lamination is improved, and the stable quality of a final product is ensured. Based on the above object, the technical scheme of the invention is as follows:
the invention provides a method for improving the false soldering recognition rate of a single-side single-crystal photovoltaic group string, which comprises the following main steps:
s1 calibration of camera and shooting environment
The image brightness is used as a detection basis, and a standard first piece is introduced to check the EL test camera and the test environment before the production of the production line starts, so that the requirements of no obvious light leakage, no defocus, no over exposure and no halation are met.
S2, identifying insufficient solder joint of single-side single-crystal photovoltaic module
After identifying the component to be inspected, a bias current I is applied1EL test is carried out to obtain an EL test chart phi1Then with a bias current I2EL test is carried out to obtain an EL test chart phi2And obtaining delta phi (i) by combining the background strength parameter and the two test result values, defining the concept deviation percentage S (i), and judging whether the front surface cold joint or the back surface cold joint is carried out according to the difference value between the judgment threshold value X and the judgment threshold value S (i) and the distribution relation.
In the step 1, the light leakage phenomenon is used for checking the sealing condition of a shooting darkroom, and if a stable and irremovable internal light source exists, the influence of the light leakage phenomenon can be weakened in the setting of an equipment repair coefficient matrix; the defocusing and overexposure phenomena are optimized by adjusting the shooting parameters of the camera; the halo phenomenon can be corrected by an algorithm carried by the camera, and for a conventional industrial camera, a halo repairing matrix needs to be introduced to carry out post-processing on the image.
The purpose of importing the standard first piece is to obtain the result of EL image display which is uniform and consistent everywhere theoretically when the standard first piece is shot by default, if the result of EL image display is not consistent, the gray level of each image number should be multiplied by a restoration coefficient, and finally the image intensity is uniform and consistent everywhere. The array formed by the series of repair coefficients according to the pixel positions is called a device repair coefficient array, and image distortion caused by halation, internal light sources, inclination of a component placing platform and the like can be reduced.
When the focal length, sensitivity, distance and exposure time are determined, the EL intensity of the device is determined without overexposureELAnd camera register memoryAmount of stored charge ecamThe following relationships exist between:
wherein, texposureIs referred to as exposure time, QcamThe quantum efficiency of the camera is related to aperture N, light sensitivity ISO and spectral wavelength E; BG refers to background light sources. The invention mainly discusses the EL intensity near the main grid, the (x, y) coordinate representation method is replaced by the (i) position of the main grid, the z refers to the distance between the shooting point and the camera, and can be modified by changing parameters, phinoiseIs a noise signal.
The relationship between the amount of charge in the internal registers of the camera and the gray scale image that is ultimately rendered is more complex. The gray scale image mainly represents the relative value size between the charge quantities of the camera register, and for different types of cameras, under the condition that no overexposure occurs, the EL brightness value and the image gray scale value phi basically satisfy the positive correlation relationship:
where f (Φ) is the conversion function between photo-electro-graphs, ΦBGThe result is the comprehensive calculation result of the shooting equipment and the image processing program under the condition that no external light source is applied.
Define the gray scale slope Δ Φ (i) as:
defining a percentage deviation function S (i) as:
wherein, I1And I2Are respectively for the first time andthe magnitude of the applied bias current is tested a second time. The calculation of S (i) takes a single cell as a data set, a represents the area near the first main grid of the cell, b represents the area near the last main grid, and when the cold joint occurs at the position i, S (i)<<0，
The value of the threshold value X is judged to be related to the voltage and current combination applied in the test, and can be obtained through the test before production. The upper limit of the value of X cannot be larger than S caused by noise and other factors under normal series weldingmin(i) (ii) a The lower limit of the value of X cannot be less than S (i) at a known cold joint area; in the production and application process, when S (i)<And X, marking and alarming as a basis for judging that the main grid at the position i has the cold joint.
The step of judging whether the front surface cold joint or the back surface cold joint is carried out according to the difference value between the judgment threshold value X and the judgment threshold value S (i) and the distribution relation is as follows:
uniformly dividing the main grid position of each cell in the single crystal photovoltaic group string into a plurality of areas, and setting that when S (i) < X, the area is marked by a first color (such as gray), when S (i) < X, the area is marked by a second color (such as red), when S (i) >0, the area is marked by a third color (such as yellow), and the rest conditions are not marked specially;
when the battery string is normally series-welded, the calculated S (i) does not cause region specific marking.
When the front surface of one battery piece is subjected to false welding, the area of the battery piece presents a second color (such as red) mark; the main grid area corresponding to the adjacent cell which is welded with the front electrode of the cell through the back electrode is basically not influenced or is close to the judgment condition of the first color (such as grey); the adjacent cell pieces welded to the back electrodes of the cell pieces through the front electrodes are not affected by the front cold joint.
When the back surface of the battery piece is subjected to false soldering, the false soldering area of the back surface presents a first color (such as grey) mark, and other normal soldering areas corresponding to the main grid subjected to false soldering present a third color (such as yellow) mark due to the rise of current density; the adjacent cell welded with the front electrode of the cell through the back electrode is not affected, and the main grid area corresponding to the adjacent cell welded with the back electrode of the cell through the front electrode is specially marked with a phenomenon similar to front cold joint; the poorer the back side series welding effect, the smaller s (i) of the main grid region corresponding to the adjacent cell which is welded with the back side electrode of the cell through the front side electrode.
Action and Effect of the invention
1) According to the improved method for improving the false soldering identification rate of the photovoltaic group string, bias currents with different sizes are applied to the group string to be tested twice, EL intensity values generated by two tests or pixel gray values of pictures are analyzed through a computer and prompted, and therefore workers are assisted to improve screening of false soldering defects. The improvement is based on the technical improvement provided by the traditional EL test method, the EL hardware equipment does not need to be replaced on a large scale, and the method is suitable for the production line with higher automation degree and less manual self-checking times, has low improvement cost and is easy to popularize and apply.
2) The invention has no limit to the types and sizes of single-sided monocrystalline silicon battery pieces, only requires the efficiency to be consistent with the current gear, and has wide application range of products.
3) The method is particularly suitable for screening the insufficient welding defect phenomenon caused by unstable temperature of the series welding machine and deviation of the welding rod, and has more obvious improvement effect.
Drawings
FIG. 1 is a schematic view of a photovoltaic module production process;
FIG. 2 is a schematic diagram of the steps of the detection procedure of the present invention;
FIG. 3 is an equivalent circuit diagram of a single electrode of a photovoltaic cell sheet during EL test;
fig. 4 is a schematic structural diagram of a photovoltaic cell, taking 3BB as an example, and the fine grid is omitted;
fig. 5 is a typical alarm diagram of a photovoltaic cell string under different cold joint conditions, taking a 5BB cell as an example.
Detailed Description
The invention is further illustrated with reference to the following specific examples, which do not limit the scope of the invention.
The improved method for improving the false soldering recognition rate of the photovoltaic module string is suitable for production and quality control of modules adopting battery pieces with the same efficiency and the same current level.
The method for improving the false soldering recognition rate of the single crystal photovoltaic string provided by the embodiment comprises two main steps as follows:
s1 calibration of camera and shooting environment
The image brightness is used as a detection basis, and a standard first piece is introduced to check the EL test camera and the test environment before the production of the production line starts, so that the requirements of no obvious light leakage, no defocus, no over exposure and no halation are met.
S2 false welding identification of single crystal photovoltaic module
After identifying the component to be inspected, a bias current I is applied1EL test is carried out to obtain an EI test pattern phi1Then with a bias current I2EL test is carried out to obtain an EI test pattern phi2And combining the two test result values to obtain delta phi (i), defining the concept deviation percentage S (i), and judging whether the front surface is in the front surface cold joint or the back surface is in the back surface cold joint according to the difference value between the judgment threshold value X and the judgment threshold value S (i) and the distribution relation.
The above two processes are described in detail below with reference to fig. 2.
In S1, the specific steps of calibrating the camera and the shooting environment are as follows:
s1.1, importing a standard first part;
s1.2, setting the focal length, the light sensitivity and the exposure time of the current camera;
s1.3, obtaining a background light source intensity map phi without electrifyingBG；
S1.4, judging whether an obvious light leakage phenomenon exists or not, if so, entering S1.5, and if not, entering S1.6;
s1.5, checking the reason of light leakage, entering S1.3, and recovering phiBG；
S1.6, with a bias current I1EL test is carried out to obtain an EL test chart phi1；
S1.7, judging whether overexposure and defocusing phenomena exist or not according to the test result, returning to S1.2 when the judgment result is yes, and entering S1.8 when the judgment result is no;
s1.8, judging whether a halo phenomenon exists or not, entering S1.9 when the judgment result is yes, and judging whether the camera and the shooting environment meet the requirements of no obvious light leakage, no defocus, no over exposure and no halo when the judgment result is no, and entering S1.10;
s1.9, adjusting the equipment repair coefficient matrix, entering S1.6, and obtaining the EL test chart phi again1；
And S1.10, exporting the standard first part, and entering the step 2.
In the step, the sealing condition of the camera darkroom is checked through the light leakage phenomenon, and if a stable and unaddressed internal light source exists, the influence of the light leakage phenomenon can be weakened in the setting of the equipment repair coefficient matrix; the defocusing and overexposure phenomena are optimized by adjusting the shooting parameters of the camera; the halo phenomenon can be corrected by an algorithm carried by the camera, and for a conventional industrial camera, a halo repairing matrix needs to be introduced to carry out post-processing on the image.
When a standard first piece is shot by default, the display result of the EL image is theoretically uniform everywhere, if the display result is not uniform, the gray level of each image number is multiplied by a restoration coefficient, and finally the image intensity is uniform everywhere. The array formed by the series of repair coefficients according to the pixel positions is called a device repair coefficient array, and image distortion caused by halation, internal light sources, inclination of a component placing platform and the like can be reduced.
EL intensity of the device when focus, sensitivity, distance and exposure time are determinedELAmount of charge e stored with the camera registercamThe following relationships exist:
wherein, texposureIs referred to as exposure time, QcamRefers to the quantum efficiency of the camera; BG refers to background light sources; x and y refer to two-dimensional coordinates of a plane where the object is shot; z refers to the distance from the shooting point to the camera; phinoiseIs a noise signal.
In S2, the step of identifying the false soldering of the single crystal photovoltaic module is as follows:
s2.1, leading in components in sequence;
s2.2, reading the bar code of the current assembly to obtain product information;
s2.3, with a bias current I1EI test is carried out to obtain an EI test pattern phi1；
S2.4, with a bias current I2EI test is carried out to obtain an EI test pattern phi2；
S2.5, according to phi1And phi2Whether the assembly has the insufficient soldering defect or not can be judged by combining manual judgment, if the judgment result is yes, S2.6 is carried out, and if the judgment result is no, S2.9 is carried out;
s2.6, leading out the assembly to a repair station;
s2.7, repairing the component according to the detection result;
s2.8, putting the repaired component into a normal production line, and re-entering S2.1;
s2.9, exporting the qualified assembly to a next station;
and S2.10, judging whether the production is finished or not, transferring the assembly to a specified position when the judgment result is yes, and operating according to the requirement when the judgment result is no.
In step 2.5, the specific method for judging the defect of the insufficient solder is as follows:
when only one cell in a string of cell strings is subjected to cold joint, the series resistance of the normal cell relative to the defective cell is considered to be consistent, and is recorded as Relse. Take 3 main grid cells of FIG. 4 as an example, Relse(1)＝Relse(2)＝Relse(3) The current passing through each main grid of the normal cell is consistent, i.e. Ielse＝Ielse(1)＝Ielse(2)＝Ielse(3)。RsFor the contact resistance of the back electrode of the cell and the interconnection bar, if the first back electrode of the 3-grid cell is in cold joint, R iss(1)>>Rs(2)≈Rs(3)。RshRepresenting the leakage current behavior of the battery plate, the leakage current of the battery plate used in the assembly production lineUniform and small value, i.e. denoted as Rsh＝Rsh(1)＝Rsh(2)＝Rsh(3)。RreIs the resistance characteristic of carriers in the diffusion recombination process and is related to the p-n junction characteristic in order to satisfy Rsh>>RreAnd Rs>>RreThe numerical relationship of (1) and (2) that the voltage or current values used in the two tests are large, the reference I is recommendedscThe numerical value of (c). RreThe assumption is consistent in the same voltage current and the same semiconductor material. When R isreWhen the time is sufficiently small, R can be considered to be different between the two tests although the voltage and the current are differentre＝Rre，1≈Rre，2。
In the EL test, the first shot is performed with the bias current I1And a bias voltage V1Corresponding voltage V loaded on the barrier region of the battery pieced1The following relationships exist:
VTis a thermal voltage; phiELIs the EL emission intensity; c (i) is a coefficient relating to the external quantum efficiency, minority carrier lifetime, and black body emissivity of the cell. When the physical properties of the cell are regulated, C can be considered as a constant, resulting in:
Vd1(i)＝V1-Velse-I1(i)Rs(i)＝Ire，1(i)Rre (3)
I1(i)＝Ish，1(i)+Ire，1(i)≈Ire，1(i) (4)
when the second shooting is performed, the bias current used is I2And a bias voltage V2For example, the voltage V applied to the barrier region of the cell at this timed2The following relationship exists:
Vd2(i)＝V2-Velse-I2(i)Rs(i) (6)
I2(i)＝Ish，2(i)+Ire，2(i)≈Ire，2(i) (7)
amount of charge e stored in a camera registercam(i) Or when it is positively correlated with the image gradation value Φ (i), assuming that the photo-electro-graphic conversion function f (Φ) is a constant value a, the following relationship exists:
in combination with the two test procedures, the following relationship can be obtained:
at this time,. DELTA.phiEL(i) Has a correlation with the back electrode contact resistance, and analog defines the gray scale slope Δ Φ (i):
when the back of the battery is in cold joint, the back of the battery is in cold joint
At this time
And
mean values of the values are indicated. Defining a percentage of conceptual deviation S (i):
ordinal numbers a and b depend on a sequencing method of a main grid region of a cell slice, and S (i) is calculated by taking a single cell as a data set, wherein a represents a region near a first main grid of the cell slice, and b represents a region near a last main grid. When a cold joint occurs at i, S (i) < < 0.
The value of Δ Φ (i) may also be calculated by trial and error in the form of division by equation (9), so as to obtain the result of s (i) with higher sensitivity.
And introducing a judgment threshold value X, and when S (i) < X, alarming the main grid at i. According to the difference value and distribution relationship between X and s (i), it is also possible to subdivide whether the front surface cold joint or the back surface cold joint occurs in the cell, as shown in fig. 5.
In this embodiment, the value of the threshold value X is determined to be related to the voltage-current combination applied in the test, and when the voltage-current value used in the two tests is large and the voltage-current difference between the two tests is large, the selectable value range of X is larger. The upper limit of the value of X cannot be larger than S caused by noise and other factors in the normal series welding conditionmin(i) Otherwise, the misjudgment rate of the algorithm is increased. The lower limit of the value of X cannot be less than S (i) of the cold joint, and the value can be set by introducing a known cold joint component.
With reference to fig. 5, three battery pieces are used to simulate the alarm condition of the method when the battery string is in cold joint. It is assumed that the back electrode of cell a is connected to the front electrode of cell B, and the back electrode of cell B is connected to the front electrode of cell C.
The step of judging whether the front surface cold joint or the back surface cold joint is carried out according to the difference value between the judgment threshold value X and the judgment threshold value S (i) and the distribution relation is as follows:
the main grid position of each cell in the monocrystalline photovoltaic group string is uniformly divided into a plurality of areas (such as 10 areas), and the total number of the areas is 30. And setting that when S (i) < X, the region is marked by a first color (such as gray), when S (i) < < X, the region is marked by a second color (such as red), when S (i) > >0, the region is marked by a third color (such as yellow), and the rest is not marked specially. The judgment criteria of yellow and red need to be set by introducing a battery string of a known cold solder joint type.
When the battery string is normally welded in series, the calculated S (i) does not cause the special mark of the area, but does not exclude the phenomenon of darkening or lightening of the area caused by other factors, and at the moment, the distribution position is irregular although the area has the special mark.
When the front surface of the battery piece B is subjected to false welding, the battery piece area has an obvious blackening phenomenon and shows a red mark. Cell a and cell C are not significantly affected.
When the back surface of the battery piece B is subjected to false soldering, the area subjected to the false soldering has a gray mark, and other normal soldering areas corresponding to the main grid are marked in yellow due to the fact that the current density is increased. The cell piece a is not affected, and the cell piece C is marked with a special mark similar to a front cold joint phenomenon. When the series welding effect of the back surface of the cell B is poorer, the S (i) of the cell C corresponding to the main grid area is smaller.
The foregoing is merely exemplary of the invention and is not intended to limit the invention in any manner. Although the present invention has been described with reference to the preferred embodiments, it is not intended to be limited thereto. Those skilled in the art can make many possible variations and modifications to the disclosed embodiments, or equivalent modifications, without departing from the spirit and scope of the invention, using the methods and techniques disclosed above. Therefore, any simple modification, equivalent replacement, equivalent change and modification made to the above embodiments according to the technical essence of the present invention are still within the scope of the protection of the technical solution of the present invention.
Claims (3)
1. A method for improving the false soldering recognition rate of a single-side single-crystal photovoltaic string is characterized by comprising the following steps:
s1 calibration of camera and shooting environment
By using image brightness quantization as detection basis, before the production of the production line is started, a standard first piece is introduced to inspect an EL test camera and a test environment so as to meet the requirements of no obvious light leakage, no defocus, no over exposure and no halation,
s2 false welding identification of single crystal photovoltaic module
After identifying the component to be inspected, a bias current I is applied1EL test is carried out to obtain an EL test chart phi1Then with a bias current I2EL test is carried out to obtain an EL test chart phi2Obtaining delta phi (i) by combining the two test result values, defining a concept deviation percentage S (i), judging whether the front surface is in the front surface cold joint or the back surface is in the back surface cold joint according to the difference value between the judgment threshold value X and the judgment threshold value S (i) and the distribution relation,
Δ Φ (i) is defined as:
when the cold-joint occurs, the solder joint,
wherein i is the serial number of the marked area of the main grid of the battery piece;
the calculation of S (i) takes a single cell as a data set, a represents the area near the first main grid of the cell, b represents the area near the last main grid, when the cold joint occurs at i, S (i) <0,
judging that the value of the threshold value X is related to the voltage and current combination of the test application, and obtaining the value through a test before production, wherein the upper limit of the value of the threshold value X cannot be larger than S caused by noise and other factors under normal series weldingmin(i) (ii) a The lower limit of the value of X cannot be less than S (i) of the known insufficient solder local area, and in the production and application process, when S (i)<And X, marking and alarming as a basis for judging that the main grid at the position i has the cold joint.
2. The method for improving the false solder joint identification rate of the single crystal photovoltaic string as claimed in claim 1, wherein:
in step 1, the specific steps of calibrating the camera and the shooting environment are as follows:
s1.1, after the focal length, the light sensitivity and the exposure time of the current camera are set, the background light source intensity graph phi is obtained without electrifyingBGAnd determining whether there is light leakage, and if yes, checking the cause of light leakage and recovering phiBGIf the judgment result is negative, entering the next step;
s1.2 with a bias current I1EL test is carried out to obtain an EL test chart phi1Judging whether overexposure and defocusing phenomena exist or not according to the test result, returning to S1.1 when the judgment result is yes, resetting the current focal length, the current light sensitivity and the current exposure time of the camera, carrying out a corresponding process, and entering the next step when the judgment result is no;
s1.3, judging whether a halo phenomenon exists or not, if so, adjusting an equipment repair coefficient matrix, and obtaining an EL test chart phi again1And carrying out a corresponding process, and judging whether the camera and the shooting environment meet the requirements of no obvious light leakage, no defocus, no over exposure and no halo when the judgment result is negative.
3. The method for improving the cold solder joint identification rate of the single-sided single-crystal photovoltaic string as claimed in claim 1, wherein:
in step 2, the step of determining whether the front surface or the back surface is the front surface or the back surface according to the difference value between the determination threshold value X and s (i) and the distribution relationship is as follows:
uniformly dividing the main grid position of each cell in the single-sided single-crystal photovoltaic group string into a plurality of regions, and setting the regions to be marked by a first color when S (i) < X, marking the regions by a second color when S (i) < X, marking the regions by a third color when S (i) >0, and not making special marks in other cases;
when the battery string is normally welded in series, the calculated S (i) does not cause a special region mark;
when the front surface of one battery piece is subjected to false welding, the area of the battery piece presents a second color mark, and the main grid area corresponding to the adjacent battery piece welded with the front surface electrode of the battery piece through the back surface electrode is basically not influenced or is close to the judgment condition of the first color; the adjacent battery piece welded with the back electrode of the battery piece through the front electrode is not affected by front cold joint;
when the back surface of the battery piece is subjected to cold joint, the cold joint area of the back surface presents a first color mark, and other normal welding areas corresponding to the main grid subjected to cold joint present a third color mark due to the rise of current density; the adjacent cell welded with the front electrode of the cell through the back electrode is not affected, and the main grid area corresponding to the adjacent cell welded with the back electrode of the cell through the front electrode is specially marked with a phenomenon similar to front cold joint; the poorer the back side series welding effect, the smaller s (i) of the main grid region corresponding to the adjacent cell which is welded with the back side electrode of the cell through the front side electrode.
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