Battery capacity test method

Abstract

The application relates to a battery capacity test method, which comprises the following steps: S1, discharging a standard battery cell in a low charge condition in six stages, and recording the charge capacity, static voltage and working voltage in the discharging process; S2, fitting two equations according to the charge capacity and static voltage; S3, discharging a battery to be predicted in two stages, taking the above working voltage as a cutoff condition, and obtaining the static voltage of the battery to be predicted after two-stage discharging; and S4, calculating the capacity of the battery to be predicted according to a formula. The battery capacity prediction method can test the battery capacity without full charging, the thickness of the battery is not too large, and the process and time of the battery capacity test are less than those of a conventional capacity grading method.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a method for testing battery capacity. Background Technology

[0002] Capacity is an important performance indicator of lithium-ion batteries. During the later stages of battery assembly, multiple individual cells need to be connected in series and parallel to form modules and battery packs. Therefore, how to determine the capacity of individual cells before assembly is a technical problem that urgently needs to be solved in battery technology. Summary of the Invention

[0003] Therefore, the present invention provides a battery capacity testing method that can obtain the capacity of a single battery cell while ensuring that the thickness of the battery cell is not affected by the capacity test.

[0004] To address the aforementioned technical problems, this invention provides a method for testing battery capacity. Includes the following steps: S1: Discharge a fully charged standard battery cell at a constant current rate to the first state of charge value, control the standard battery cell to enter the resting stage, and obtain the first static voltage of the standard battery cell after the resting stage ends. Continue discharging the standard battery cell at a constant current rate to the second state of charge value, and obtain the working voltage V1 of the standard battery cell at the end of the discharge; control the standard battery cell to enter the resting stage, and obtain the second static voltage of the standard battery cell after the resting stage ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the third state of charge value, control the standard battery cell to enter the resting stage, and obtain the third static voltage of the standard battery cell after the resting period ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the fourth state of charge value, control the standard battery cell to enter the resting stage, and obtain the fourth static voltage of the standard battery cell after the resting period ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the fifth state of charge value, and obtain the working voltage V2 of the standard battery cell at the end of the discharge; control the standard battery cell to enter the resting stage, and obtain the fifth static voltage of the standard battery cell after the resting stage ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the sixth state of charge value, control the standard battery cell to enter the resting stage, and obtain the sixth static voltage of the standard battery cell after the resting period ends. S2: Using static voltage as the abscissa and state of charge (SOC) value as the ordinate, linearly fit the first SOC value, second SOC value, third SOC value, first static voltage, second static voltage, and third static voltage to obtain the first fitting equation y1 = a1x1 + b1; Using static voltage as the abscissa and SOC value as the ordinate, linearly fit the fourth SOC value, fifth SOC value, sixth SOC value, fourth static voltage, fifth static voltage, and sixth static voltage to obtain the second fitting equation y2 = a2x2 + b2. S3: Obtain a cell to be predicted that is the same type as the standard cell, and give the cell to be predicted an initial voltage V0, where V0 > V1; S4: Discharge the cell to be predicted at a constant current rate. When the cell to be predicted is discharged to the working voltage V1, stop discharging and control the cell to be predicted to enter the resting stage. Obtain the voltage of the cell to be predicted after the resting stage is V3. S5: Continue to discharge the cell to be predicted at a constant current rate. When the cell to be predicted discharges to the working voltage V2, stop discharging and obtain the discharge capacity C0 during the discharge period. Control the cell to be predicted to enter the resting stage and obtain the voltage of the cell to be predicted after the resting period ends as V4. S6: Based on steps S2-S5 and the formula C=C0÷(y1-y2), we get C=C0÷[(a1V3+b1)-(a2V4+b2)], where C is the capacity of the battery cell to be predicted after being fully charged and discharged to the discharge cutoff voltage.

[0005] Furthermore, in step S3, the battery cell to be predicted is a battery cell of the same type as the standard battery cell and formed.

[0006] Further, in step S1, the first state of charge value ranges from 11% to 50%; the difference between the second state of charge value and the first state of charge value ranges from 1% to 5%; and the difference between the third state of charge value and the second state of charge value ranges from 1% to 5%.

[0007] Further, in step S1, the value of the fourth state of charge is between 5% and 30%; the difference between the fifth state of charge and the fourth state of charge is between 1% and 5%; and the difference between the sixth state of charge and the fifth state of charge is between 1% and 5%.

[0008] Furthermore, in step S1, the values ​​of the first state of charge and the fourth state of charge are both between 0% and 20%; or, the values ​​of the first state of charge and the fourth state of charge are both between 20% and 50%.

[0009] Furthermore, in step S1, the differences between the second state of charge value and the first state of charge value, the differences between the third state of charge value and the second state of charge value, the differences between the fifth state of charge value and the fourth state of charge value, and the differences between the sixth state of charge value and the fifth state of charge value are all equal.

[0010] Furthermore, in step S1, the fifth state of charge value is 0%, and the operating voltage V2 is the discharge cutoff voltage of the standard battery cell.

[0011] Furthermore, in step S1, the operating voltage of the standard battery cell when it is discharged at constant current to the sixth state of charge value is less than the over-discharge protection voltage of the standard battery cell.

[0012] Further, in step S2, the first fitting equation is y1=1.8634x1-5.8872; the second fitting equation is y2=0.1251x2-0.375.

[0013] Furthermore, in step S3, the difference between the thickness of the cell at the initial voltage V0 and the thickness of the cell before formation is controlled to be within 0.5 mm.

[0014] The technical solution of the present invention has the following advantages over the prior art: 1) The battery capacity testing method described in this invention uses two fitting equations derived from the linear fitting of a standard battery cell, with static voltage as the abscissa and state of charge as the ordinate. In other words, it is not necessary to fully charge the battery cell to be predicted. It is only necessary to measure the discharge capacity C0 of the battery cell to be predicted from the working voltage V1 to the working voltage V2. The capacity C of the battery cell to be predicted after being fully charged and discharged to the discharge cutoff voltage can be predicted through the two fitting equations. Thus, on the one hand, it solves the problem of battery cell thickening caused by fully charging the battery cell in the existing capacity grading process; on the other hand, the capacity test is faster than the existing capacity grading process. Attached Figure Description

[0015] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0016] Figure 1 The graph shows the equations when the first fitting equation is obtained in this invention. Figure 2 The graph shows the equations when the second fitting equation is obtained in this invention. Detailed Implementation

[0017] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0018] The inventors learned that the current related technologies rely on the capacity grading process to determine the capacity of the battery cell. Typically, the battery cell is fully charged with constant current and constant voltage in the capacity grading cabinet, and after being left to stand for a period of time, it is discharged with a certain rate of constant current according to the process or customer requirements. After 2-4 complete full charge and discharge cycles, the capacity of the last full discharge is taken as the capacity grading capacity for classification.

[0019] The inventors also learned that, in order to pursue long battery life, the only way to achieve this at the cell level, besides improving the materials themselves, is to make full use of the overall space. In other words, the group margin of individual battery cells is often relatively large.

[0020] Thus, the inventors discovered that when the group margin of a battery cell is relatively large, such as when the group margin is greater than 92%, the battery cell will bulge during the capacity testing stage, i.e., the full charge and discharge stage, because the electrode sheet expands and there is no space to release the stress. This causes the battery cell to become thicker, which in turn affects the assembly of the battery module or battery pack in the later stage.

[0021] Therefore, in order to solve the above problems, the present invention proposes a battery capacity testing method, comprising the following steps: S1: Discharge a fully charged standard battery cell at a constant current rate to the first state of charge value, control the standard battery cell to enter the resting stage, and obtain the first static voltage of the standard battery cell after the resting stage ends, wherein the discharge temperature is 25℃. Continue discharging the standard battery cell at a constant current rate to the second state of charge value, and obtain the working voltage V1 of the standard battery cell at the end of the discharge; control the standard battery cell to enter the resting stage, and obtain the second static voltage of the standard battery cell after the resting stage ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the third state of charge value, control the standard battery cell to enter the resting stage, and obtain the third static voltage of the standard battery cell after the resting period ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the fourth state of charge value, control the standard battery cell to enter the resting stage, and obtain the fourth static voltage of the standard battery cell after the resting period ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the fifth state of charge value, and obtain the working voltage V2 of the standard battery cell at the end of the discharge; control the standard battery cell to enter the resting stage, and obtain the fifth static voltage of the standard battery cell after the resting stage ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the sixth state of charge value, control the standard battery cell to enter the resting stage, and obtain the sixth static voltage of the standard battery cell after the resting period ends. S2: Using static voltage as the abscissa and state of charge (SOC) value as the ordinate, linearly fit the first SOC value, second SOC value, third SOC value, first static voltage, second static voltage, and third static voltage to obtain the first fitting equation y1 = a1x1 + b1; Using static voltage as the abscissa and SOC value as the ordinate, linearly fit the fourth SOC value, fifth SOC value, sixth SOC value, fourth static voltage, fifth static voltage, and sixth static voltage to obtain the second fitting equation y2 = a2x2 + b2. S3: Obtain a cell to be predicted that is the same type as the standard cell, and give the cell to be predicted an initial voltage V0, where V0 > V1; S4: Discharge the cell to be predicted at a constant current rate. When the cell to be predicted is discharged to the working voltage V1, stop the discharge and control the cell to be predicted to enter the resting stage. Obtain the voltage of the cell to be predicted after the resting stage is V3. The discharge temperature is 25℃. S5: Continue to discharge the cell to be predicted at a constant current rate. When the cell to be predicted discharges to the working voltage V2, stop discharging and obtain the discharge capacity C0 during the discharge period. Control the cell to be predicted to enter the resting stage and obtain the voltage of the cell to be predicted after the resting period ends as V4. S6: Based on steps S2-S5 and the formula C=C0÷(y1-y2), we get C=C0÷[(a1V3+b1)-(a2V4+b2)], where C is the capacity of the battery cell to be predicted after being fully charged and discharged to the discharge cutoff voltage.

[0022] In this embodiment, in order to solve the problem of battery cell thickening caused by fully charging the battery cell in the existing capacity grading process, and to make the capacity testing of the battery cell faster, two fitting equations are obtained by linearly fitting a standard battery cell to obtain the relationship between static voltage and state of charge value. Therefore, it is not necessary to fully charge the battery cell to be predicted. It is only necessary to measure the discharge capacity C0 of the battery cell to be predicted from the working voltage V1 to the working voltage V2. The capacity C of the battery cell to be predicted after being fully charged and discharged to the discharge cutoff voltage can be predicted by the two fitting equations.

[0023] It should be noted that in step S1, the standard battery cells are selected after the formation and capacity testing process. In other words, the SOC-OCV characteristic curve and discharge characteristic curve of the standard battery cells can be obtained by existing methods. The specific methods of obtaining them will not be elaborated here.

[0024] It should also be noted that the state of charge (SOC) refers to the ratio of the battery's current charge to its full capacity. When not charging, the battery's capacity can be considered A0, corresponding to 0% SOC; when the battery is fully charged, it can be considered to have reached its rated capacity A, corresponding to 100% SOC; during charging, when the battery's capacity reaches x% (A-A0), it corresponds to x% SOC, where x% is any value from 0 to 100. For example, if x is 50, the battery is charged to 50% (A-A0), corresponding to 50% SOC.

[0025] In some embodiments, in order to improve the accuracy of the two fitting equations obtained by linear fitting in step S2, multiple standard battery cells are designed, the capacity of the multiple standard battery cells is highly consistent, and each standard battery cell has undergone step S1. After averaging the first to sixth static voltages of the multiple standard battery cells, step S2 is performed to fit the first fitting equation and the second fitting equation respectively.

[0026] In some embodiments, in order to ensure that the thickness of the cell to be predicted at the initial voltage V0 in step S3 does not affect the subsequent assembly of the battery module or battery pack, the thickness of the cell to be predicted at the initial voltage V0 is limited, that is, the difference between the thickness of the cell to be predicted at the initial voltage V0 and the thickness of the cell to be predicted before formation is controlled to be within 0.5 mm. For example, the difference between the thickness of the cell to be predicted at the initial voltage V0 and the thickness of the cell to be predicted before formation can be 0 mm or 0.1 mm or 0.15 mm or 0.2 mm or 0.25 mm or 0.3 mm or 0.35 mm or 0.4 mm or 0.45 mm or 0.5 mm.

[0027] Specifically, the standard battery cell is first fully discharged, then undergoes a constant current charging stage followed by a constant voltage charging stage to reach full charge. To detect the thickness change of the standard battery cell in real time during charging, a thickness testing fixture is designed. This fixture includes two clamping plates, which are respectively set on both sides of the thickness direction of the standard battery cell. Each clamping plate is equipped with a displacement sensor. Since the bulging area of ​​the standard battery cell is mainly in the middle region, the test end of the displacement sensor is directly facing the middle region of the standard battery cell. The displacement sensor transmits the measured size information to the processor. After processing, the processor can obtain the mapping relationship between time and thickness. Since establishing the mapping relationship between time and operating voltage during the charging process of the standard battery cell is a known existing method, the mapping relationship between thickness and operating voltage can be fitted and constructed.

[0028] Therefore, in step S3, to ensure that the difference between the thickness of the predicted battery cell at the initial voltage V0 and its thickness before formation is within 0.5 mm, the initial voltage V0 can be determined through the mapping relationship between the constructed thickness dimension and the operating voltage. For example, the thickness of the predicted battery cell at the initial voltage V0 is designed to be B0 + 0.4 mm, where B0 is the thickness of the predicted battery cell before formation. Then, based on the mapping relationship between the constructed thickness dimension and the operating voltage, and B0 + 0.4 mm, the voltage V can be obtained. Therefore, as long as the initial voltage V0 is less than or equal to the voltage V, it is acceptable. For example, if B0 is 35 mm, then as long as the thickness of the predicted battery cell at the initial voltage V0 is less than or equal to 35.5 mm, it is acceptable.

[0029] Preferably, since the inventors have found that battery cells after general formation are almost or even without bulging, in order to simplify the testing steps and speed up the testing, the battery cell to be predicted can be designed to be the same type as the standard battery cell after formation.

[0030] In some embodiments, to ensure that the difference between the thickness of the cell to be predicted at its initial voltage V0 and the thickness of the cell before formation is within 0.5 mm, the initial voltage V0 of the cell to be predicted should not be too large. That is, the state of charge value corresponding to the initial voltage V0 of the cell to be predicted should not be too large. Therefore, based on the mapping relationship between the thickness dimension and the working voltage constructed above, the inventors found that the value of the first state of charge should be less than or equal to 50%. Furthermore, in order to ensure that two different fitting equations can be fitted, the difference between the first state of charge value and the fourth state of charge value needs to be designed to be much larger than the difference between the first state of charge value and the second state of charge value, and also larger than the difference between the second state of charge value and the third state of charge value. For example, the difference between the first state of charge value and the fourth state of charge value is 10%, while the difference between the first state of charge value and the second state of charge value is 1%. Therefore, the inventors designed the value of the first state of charge value to be greater than or equal to 11%.

[0031] Specifically, the first state of charge (SOC) value can be 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 134%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, or 50%.

[0032] Furthermore, based on the design range of the first state of charge value being between 11% and 50%, and based on ensuring that two different fitting equations can be fitted, the design range of the fourth state of charge value is between 5% and 30%.

[0033] Specifically, the first state of charge value can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, or 30%.

[0034] Furthermore, when fitting the equation, the smaller the data difference, the smaller the data coverage of the fitted linear equation, the smaller the prediction range, the higher the degree of fit, and the higher the prediction accuracy; conversely, the larger the data difference, the larger the data coverage of the fitted linear equation, the larger the prediction range, the lower the degree of fit, and the lower the prediction accuracy. Therefore, the ranges of the differences between the second and first states of charge, the third and second states of charge, the fifth and fourth states of charge, and the sixth and fifth states of charge are designed to be within appropriate ranges, balancing the prediction range and prediction accuracy of the linear equation. Specifically, the difference between the second and first states of charge is between 1% and 5%; the difference between the third and second states of charge is between 1% and 5%; the difference between the fifth and fourth states of charge is between 1% and 5%; and the difference between the sixth and fifth states of charge is between 1% and 5%.

[0035] Specifically, the difference between the second state of charge value and the first state of charge value can be 1%, 2%, 3%, 4%, or 5%; the difference between the third state of charge value and the second state of charge value can be 1%, 2%, 3%, 4%, or 5%; the difference between the fifth state of charge value and the fourth state of charge value can be 1%, 2%, 3%, 4%, or 5%; and the difference between the sixth state of charge value and the fifth state of charge value can be 1%, 2%, 3%, 4%, or 5%.

[0036] Furthermore, the differences between the second and first states of charge (SOC) values, as well as the differences between the third and second SOC values, can be equal or unequal; the differences between the fifth and fourth SOC values, as well as the differences between the sixth and fifth SOC values, can also be equal or unequal; to further improve the fit, the difference between the second and first SOC values ​​is designed to be equal to the difference between the third and second SOC values, and the difference between the fifth and fourth SOC values ​​is designed to be equal to the difference between the sixth and fifth SOC values.

[0037] For example, when the difference between the second state of charge value and the first state of charge value is 1%, the difference between the third state of charge value and the second state of charge value is also 1%; when the difference between the fifth state of charge value and the fourth state of charge value is 2%, the difference between the sixth state of charge value and the fifth state of charge value is also 2%.

[0038] It should be noted that, in order to ensure that the two fitted equations are fitted under the same conditions, the differences between the second and first states of charge, the third and second states of charge, the fifth and fourth states of charge, and the sixth and fifth states of charge are all designed to be equal.

[0039] For example, the differences between the second and first states of charge, the third and second states of charge, the fifth and fourth states of charge, and the sixth and fifth states of charge are all 1%.

[0040] Furthermore, in order to ensure that the two fitted equations y1=a1x1+b1 and y2=a2x2+b2 are fitted within the range where the degradation trajectory of the standard battery cell is linear or sublinear, when the standard battery cell is a lithium iron phosphate battery, referring to the static OCV-SOC curve of the lithium iron phosphate battery, the range of the first state of charge value and the fourth state of charge value are designed to be between 0% and 20%, or the range of the first state of charge value and the fourth state of charge value is between 20% and 50%.

[0041] Preferably, since the battery cell to be predicted is the same type as the standard battery cell and is a formed battery cell, and the inventors have found that the residual capacity of a generally formed battery cell corresponds to a state of charge of about 30%, the values ​​of the first state of charge and the fourth state of charge are designed to be between 0% and 20%.

[0042] Specifically, regarding the design of the first to sixth state of charge (SOC) values, it should be noted that since the standard battery cells were selected after undergoing full charge and discharge cycles, their static OCV-SOC curves and discharge curves with respect to SOC and operating voltage are known. Therefore, before conducting capacity testing, the operating voltage V1 at the first discharge stop and the operating voltage V2 at the second discharge stop can be designed based on the discharge curves of the standard battery cells with respect to SOC and operating voltage. Then, based on the discharge curves of the standard battery cells with respect to SOC and operating voltage, the second SOC value corresponding to the standard battery cell discharging to operating voltage V1 and the fifth SOC value corresponding to the standard battery cell discharging to operating voltage V2 can be determined. Finally, using the second and fifth SOC values ​​as intermediate values, the first, third, fourth, and sixth SOC values ​​can be determined.

[0043] Specifically, in this embodiment, both the standard battery cell and the battery cell to be predicted are 88.3Ah lithium iron phosphate batteries. The operating voltage V1 at the first discharge stop during the discharge process of the battery cell to be predicted is designed to be 3.108V. Based on the discharge curve of the standard battery cell regarding its state of charge and operating voltage, the second state of charge value corresponding to the standard battery cell discharging to the operating voltage V1 is determined to be 10%. Thus, the first state of charge value can be determined to be 11%, and the third state of charge value to be 9%. The operating voltage V2 at the second discharge stop during the discharge process of the battery cell to be predicted is designed to be the discharge cutoff voltage, i.e., V2 is 2.5V. Based on the discharge curve of the standard battery cell regarding its state of charge and operating voltage, the second state of charge value is determined to be 10%. The discharge curves of the electrical state and operating voltage are used to determine the fifth state of charge (SOC) value of a standard battery cell when it discharges to the operating voltage V1, which is 0%. Thus, the fourth SOC value can be determined as 1%, and the sixth SOC value as -1%. The sixth SOC value of -1% can be understood as the battery cell to be predicted being discharged to the discharge cutoff voltage and then, after the resting phase, being discharged at a constant current rate of 1% SOC. In other words, the battery cell to be predicted is over-discharged. It should be noted that when the battery cell to be predicted is over-discharged, the static voltage measured after the over-discharge and resting phase is less than the over-discharge protection voltage of the standard battery cell.

[0044] After completing the design of the first, third, fourth, and sixth state of charge values, the battery capacity test was started, namely: S1: Discharge multiple standard battery cells in a fully charged state at a constant current of 1C to 11% SOC, let them rest for 30 minutes, and record the static voltage; continue to discharge the standard battery cells at a constant current of 1C to 10% SOC, let them rest for 30 minutes, and record the static voltage; continue to discharge the standard battery cells at a constant current of 1C to 9% SOC. At this time, the average working voltage V1 of multiple standard battery cells at the end of discharge is 3.108V. Let them rest for 30 minutes and record the static voltage; continue to discharge the standard battery cells at a constant current of 1C to 1% SOC, let them rest for 30 minutes, and record the static voltage; continue to discharge the standard battery cells at a constant current of 1C to 0% SOC, let them rest for 30 minutes, and record the static voltage; continue to discharge the standard battery cells at a constant current of 1C to 1% SOC, let them rest for 30 minutes, and record the static voltage. The average of the static voltages recorded after discharging the aforementioned standard battery cells to 11% SOC and allowing them to rest for 30 minutes yields the first static voltage, which is 3.219V. The average of the static voltages recorded after discharging the aforementioned standard battery cells to 10% SOC and allowing them to rest for 30 minutes yields the second static voltage, which is 3.212V. The average of the static voltages recorded after discharging the aforementioned standard battery cells to 9% SOC and allowing them to rest for 30 minutes yields the third static voltage, which is 3.208V. The average of the static voltages recorded after discharging the aforementioned standard battery cells to 1% SOC and allowing them to rest for 30 minutes yields the fourth static voltage, which is 3.0778V. The average of the static voltages recorded after discharging the aforementioned standard battery cells to 0% SOC and allowing them to rest for 30 minutes yields the fifth static voltage, which is 3.008V. The aforementioned standard battery cells are over-discharged by 1%. The average of the static voltages recorded after SOC and 30 min of rest is used to obtain the sixth static voltage, which is 2.998V.

[0045] Next, proceed to step S2, see [link / reference] Figure 1 and Figure 2 Using static voltage as the abscissa and state of charge (SOC) value as the ordinate, a linear fit is performed on the first SOC value, second SOC value, third SOC value, first static voltage, second static voltage, and third static voltage to obtain the first fitting equation y = 1.8634x - 5.8872. Using static voltage as the abscissa and SOC value as the ordinate, a linear fit is performed on the fourth SOC value, fifth SOC value, sixth SOC value, fourth static voltage, fifth static voltage, and sixth static voltage to obtain the second fitting equation y = 0.1251x - 0.375.

[0046] Next, proceed to step S3, where a battery cell of the same type as the standard battery cell and after formation is selected as the battery cell to be predicted. At this time, the battery cell to be predicted has electricity.

[0047] Then proceed to step S4, discharge the cell to be predicted at a constant current of 1C. When the cell to be predicted is discharged to the working voltage of 3.108V, stop discharging and control the cell to be predicted to stand for 30 minutes. Record the voltage V3 at this time as 3.2167V.

[0048] Then proceed to step S5, continue discharging the cell to be predicted at a constant current of 1C. When the cell to be predicted is discharged to the discharge cutoff voltage of 2.5V, stop discharging and obtain the discharge capacity C0 during the discharge period as 11.72Ah. After the cell to be predicted is left to stand for 30 minutes, record the voltage V4 at this time as 2.7923V.

[0049] Finally, in step S6, V3 (3.2167V) is substituted into the first fitting equation 1.8634x-5.8872, V4 (2.7923V) is substituted into the second fitting equation y=0.1251x-0.375, and C0 (11.72Ah) is substituted into C=C0÷(y1-y2), thus obtaining C=11.72÷(10.68%+2.57%)=88.45Ah.

[0050] As can be seen from the above, the capacity C of the lithium iron phosphate battery predicted by the above capacity testing method is 88.45Ah, which is not much different from the actual capacity of the lithium iron phosphate battery of 88.3Ah, and the prediction is accurate.

[0051] It should be noted that, in order to further verify the capacity C of the lithium iron phosphate battery, the state of charge values ​​of the first to sixth stages can be adjusted by adjusting V1 and V2, and then steps 1-6 can be performed again to obtain the capacity C of the lithium iron phosphate battery.

[0052] It should also be noted that in step S1, the fixed rate range used for standard battery cell discharge is between 0.5C and 3C. Specifically, the fixed rate used for standard battery cell discharge can be 0.5C, 0.65C, 0.75C, 1C, 1.2C, 1.25C, 1.4C, 1.55C, 1.75C, 2C, 2.3C, 2.5C, 2.7C, 2.8C, or 3C. In steps S4 and S5, the fixed rate range used for the battery cell to be predicted is between 0.5C and 3C. Specifically, the fixed rate used for the battery cell to be predicted is 0.5C, 0.7C, 0.9C, 1.1C, 1.3C, 1.4C, 1.5C, 1.6C, 1.7C, 1.8C, 1.9C, 2C, 2.1C, 2.2C, 2.3C, 2.4C, 2.5C, 2.6C, 2.7C, 2.8C, 2.9C, or 3C.

[0053] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for testing battery capacity, characterized in that, Includes the following steps: S1: Discharge a fully charged standard battery cell at a constant current rate to the first state of charge value, control the standard battery cell to enter the resting stage, and obtain the first static voltage of the standard battery cell after the resting stage ends. Continue discharging the standard battery cell at a constant current rate to the second state of charge value, and obtain the working voltage V1 of the standard battery cell at the end of the discharge; control the standard battery cell to enter the resting stage, and obtain the second static voltage of the standard battery cell after the resting stage ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the third state of charge value, control the standard battery cell to enter the resting stage, and obtain the third static voltage of the standard battery cell after the resting period ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the fourth state of charge value, control the standard battery cell to enter the resting stage, and obtain the fourth static voltage of the standard battery cell after the resting period ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the fifth state of charge value, and obtain the working voltage V2 of the standard battery cell at the end of the discharge; control the standard battery cell to enter the resting stage, and obtain the fifth static voltage of the standard battery cell after the resting stage ends. Continue discharging the standard battery cell at a constant current rate at a fixed rate until the sixth state of charge value, control the standard battery cell to enter the resting stage, and obtain the sixth static voltage of the standard battery cell after the resting period ends. S2: Using static voltage as the abscissa and state of charge (SOC) value as the ordinate, linearly fit the first SOC value, second SOC value, third SOC value, first static voltage, second static voltage, and third static voltage to obtain the first fitting equation y1 = a1x1 + b1; Using static voltage as the abscissa and SOC value as the ordinate, linearly fit the fourth SOC value, fifth SOC value, sixth SOC value, fourth static voltage, fifth static voltage, and sixth static voltage to obtain the second fitting equation y2 = a2x2 + b2. S3: Obtain a cell to be predicted that is the same type as the standard cell, and give the cell to be predicted an initial voltage V0, where V0 > V1; S4: Discharge the cell to be predicted at a constant current rate. When the cell to be predicted is discharged to the working voltage V1, stop discharging and control the cell to be predicted to enter the resting stage. Obtain the voltage of the cell to be predicted after the resting stage is V3. S5: Continue to discharge the cell to be predicted at a constant current rate. When the cell to be predicted discharges to the working voltage V2, stop discharging and obtain the discharge capacity C0 during the discharge period. Control the cell to be predicted to enter the resting stage and obtain the voltage of the cell to be predicted after the resting period ends as V4. S6: Based on steps S2-S5 and the formula C=C0÷(y1-y2), we get C=C0÷[(a1V3+b1)-(a2V4+b2)], where C is the capacity of the battery cell to be predicted after being fully charged and discharged to the discharge cutoff voltage.

2. The battery capacity testing method according to claim 1, characterized in that, In step S3, the battery cell to be predicted is a battery cell of the same type as the standard battery cell and formed.

3. The battery capacity testing method according to claim 1, characterized in that, In step S1, the first state of charge value ranges from 11% to 50%; the difference between the second state of charge value and the first state of charge value ranges from 1% to 5%; and the difference between the third state of charge value and the second state of charge value ranges from 1% to 5%.

4. The battery capacity testing method according to claim 3, characterized in that, In step S1, the value of the fourth state of charge is between 5% and 30%; the difference between the fifth state of charge and the fourth state of charge is between 1% and 5%; and the difference between the sixth state of charge and the fifth state of charge is between 1% and 5%.

5. The battery capacity testing method according to claim 4, characterized in that, In step S1, the values ​​of the first state of charge and the fourth state of charge are both between 0% and 20%; or, the values ​​of the first state of charge and the fourth state of charge are both between 20% and 50%.

6. The battery capacity testing method according to claim 4, characterized in that, In step S1, the differences between the second state of charge value and the first state of charge value, the differences between the third state of charge value and the second state of charge value, the differences between the fifth state of charge value and the fourth state of charge value, and the differences between the sixth state of charge value and the fifth state of charge value are all equal.

7. The battery capacity testing method according to claim 4 or 6, characterized in that, In step S1, the fifth state of charge value is 0%, and the operating voltage V2 is the discharge cutoff voltage of the standard battery cell.

8. The battery capacity testing method according to claim 7, characterized in that, In step S1, the operating voltage of the standard battery cell when it is discharged at constant current to the sixth state of charge value is less than the over-discharge protection voltage of the standard battery cell.

9. The battery capacity testing method according to claim 8, characterized in that, In step S2, the first fitting equation is y1=1.8634x1-5.8872; the second fitting equation is y2=0.1251x2-0.

375.

10. The battery capacity testing method according to claim 1, characterized in that, In step S3, the difference between the thickness of the cell at the initial voltage V0 and the thickness of the cell before formation is controlled to be within 0.5 mm.

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