Method for improving battery matching consistency

Abstract

The invention discloses a method for improving battery matching consistency, which comprises the following steps of: in a high-temperature formation stage of battery cells in the same batch, charging the battery cells to a set charge quantity; after the state of the battery cell is stable, charging the battery cell when the state of the battery cell is stable so as to calculate a first direct-current internal resistance; when the state of the battery cell is stable, discharging the battery cell so as to calculate a second DC internal resistance; and matching the battery cells through the first direct-current internal resistance and the second direct-current internal resistance. According to the main technical scheme and effects, charging and discharging are carried out in the high-temperature formation stage to obtain voltage and current data, then the internal resistance is calculated, the battery cell can be effectively made to be in the high-temperature and pressed state of 40 DEG C or above all the time, the test DCR is not prone to being affected by temperature changes caused by charging and discharging of the battery cell at the moment, the high-temperature environment can fully activate materials in the battery cell, and the testing efficiency is improved. And the pressure of the clamp of the formation equipment can simulate the pressed state of the battery cell in real work, so that the most real internal resistance information of the battery cell is obtained.

Description

Technical Field

[0001] The present invention relates to the field of battery assembly, and in particular to a method for improving the consistency of battery assembly. Background Art

[0002] In order to achieve long driving range for electric vehicles, the industry usually increases the voltage and capacity of the battery pack by connecting cells in series / parallel to meet the power requirements of electric vehicles during operation. Since the overall performance of the battery pack is limited by the single cell with the worst performance, the more single cells are used, the more obvious the disadvantages of the "barrel effect" will be. After long-term cycling of the battery pack, the voltage difference at the end of discharge will continue to increase.

[0003] At present, most manufacturers in the industry use the AC internal resistance (ACR) of the battery cell as one of the matching conditions to ensure that the attenuation rate of the battery cells in the same group tends to be consistent. However, the AC internal resistance cannot truly reflect the internal resistance state of the battery cell during the charging and discharging process. Therefore, only a small number of leading manufacturers use the DC internal resistance (DCR) that can better reflect the real internal resistance of the battery cell as one of the battery cell matching conditions. At present, most DCR tests add the test plan to the capacity division process, or place the battery cell at room temperature and perform unified testing through fixed equipment. In the former, since the battery cell will dissipate heat during the charging and discharging process of the capacity division stage, it is difficult to achieve a balance between the internal and external temperatures of the capacity division test cabinet, and the outside of the capacity division cabinet is easily affected by the ambient temperature, which will further aggravate the temperature difference between the inside and outside of the capacity division test cabinet. The latter requires the purchase of expensive high-precision testing equipment, and by setting up a special DCR test station, the battery cell is transferred to a fixed station for short-term charging and discharging testing, so as to obtain a DC internal resistance with low cell differentiation. This process will increase additional costs. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to solve the problem in the prior art that when the AC internal resistance (ACR) of the battery cell is used as one of the matching conditions for matching, it cannot truly reflect the internal resistance state of the battery cell during the charging and discharging process, and when the DC internal resistance (DCR) is used, it is easily affected by temperature, resulting in inaccurate test results and high costs. A method for improving the consistency of battery matching is proposed.

[0005] Technical solution:

[0006] In a first aspect, the present application proposes a method for improving battery pack consistency, comprising the steps of:

[0007] During the high-temperature formation stage of the same batch of cells, charge the cells to a set charge;

[0008] Waiting for the battery cell state to be stable, and when the battery cell state is stable, charging the battery cell to calculate a first DC internal resistance;

[0009] Waiting for the battery cell state to be stable, and when the battery cell state is stable, discharging the battery cell to calculate the second DC internal resistance;

[0010] The battery cells are grouped by using the first DC internal resistance and the second DC internal resistance.

[0011] Preferably, waiting for the battery cell state to be stable, and when the battery cell state is stable, charging the battery cell to calculate the first DC internal resistance comprises:

[0012] Charging the battery cell with a first current;

[0013] Charging the battery cell with a second current;

[0014] The voltage before and after the second current charging is tested to calculate the first DC internal resistance.

[0015] Preferably, testing the voltage before and after charging with the second current to calculate the first DC internal resistance includes:

[0016] The first DC internal resistance formula is calculated as follows:

[0017] ;

[0018] Wherein, DCR1 is the first DC internal resistance, U2 is the voltage after charging with the second current, U1 is the voltage before charging with the second current, I1 is the charging current of the first current, and I2 is the charging current of the second current.

[0019] Preferably, waiting for the battery cell to be in a stable state comprises leaving the battery cell to stand for 1-10 minutes.

[0020] Preferably, waiting for the battery cell state to be stable, and when the battery cell state is stable, discharging the battery cell to calculate the second DC internal resistance comprises:

[0021] discharging the battery cell with a third current;

[0022] discharging the battery cell with a fourth current;

[0023] The voltage before and after the fourth current is discharged is tested to calculate the second DC internal resistance.

[0024] Preferably, testing the voltage before and after charging with the fourth current to calculate the second DC internal resistance includes:

[0025] The second DC internal resistance formula is calculated as follows:

[0026] ;

[0027] Wherein, DCR2 is the second DC internal resistance, U4 is the voltage after the fourth current is discharged, U3 is the voltage before the fourth current is discharged, I4 is the discharge current of the fourth current, and I3 is the discharge current of the third current.

[0028] Preferably, grouping the battery cells by using the first DC internal resistance and the second DC internal resistance includes selecting abnormal battery cells by using the first DC internal resistance, and selecting battery cells whose internal resistance difference is within a set range by using the second DC internal resistance.

[0029] Preferably, the first current charging includes:

[0030] The first current charging time is constant current charging for 10s to 50s, and the current is 0.05C to 0.3C.

[0031] Preferably, the second current charging includes:

[0032] The second current charging time is constant current charging for 1s~15s, and the current is 0.5C~1C.

[0033] Preferably, the third current discharge includes:

[0034] The third current discharge time is constant current discharge for 10s to 50s, and the current is 0.05C to 0.3C.

[0035] Preferably, the fourth current discharge includes:

[0036] The fourth current discharge time is constant current discharge of 1s to 15s, and the current is 0.5C to 1C.

[0037] Beneficial effects: charging and discharging in the high-temperature formation stage to obtain voltage and current data, and then calculating the internal resistance, can effectively keep the battery cell in a high temperature and pressure state above 40°C. At this time, the DCR test is not easily affected by the temperature change caused by the charging and discharging of the battery cell, and the high temperature environment can fully activate the internal materials of the battery cell to obtain the most realistic internal resistance information of the battery cell. The clamp pressure of the formation equipment can also simulate the pressure state of the battery cell when it is actually working; and no additional equipment is required, saving labor and equipment costs, and does not require a lot of time, thereby improving production efficiency; compared with the traditional AC internal resistance ACR solution, the DC internal resistance DCR of the present application can better show the dynamic changes of the battery and has better consistency when grouping. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Provide a schematic diagram of the method framework for the present invention;

[0039] Figure 2 Provide a method flow chart for the present invention;

[0040] Figure 3 It is a schematic diagram for comparing the present invention with the existing parameter matching. DETAILED DESCRIPTION

[0041] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the specific embodiments of the accompanying drawings.

[0042] Example 1

[0043] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be understood by people with general skills in the field to which the present invention belongs. "Including" and similar words used in this article mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0044] In view of the problems existing in the existing technology, such as Figure 1 As shown, a method for improving the consistency of battery packing includes the steps of:

[0045] S101, during the high temperature formation stage of the same batch of battery cells, charging the battery cells to a set charge;

[0046] Specifically, the battery cell is charged with a constant current until it reaches a set charge level (e.g. 30% SOC). The purpose of this process is to provide a stable initial state for subsequent tests while avoiding interference of full charge on the test results.

[0047] S102, waiting for the battery cell state to be stable, and when the battery cell state is stable, charging the battery cell to calculate a first DC internal resistance;

[0048] In some preferred embodiments, waiting for the battery cell state to be stable, and when the battery cell state is stable, charging the battery cell to calculate the first DC internal resistance includes:

[0049] Charging the battery cell with a first current;

[0050] Charging the battery cell with a second current;

[0051] The voltage before and after the second current charging is tested to calculate the first DC internal resistance.

[0052] In some preferred embodiments, testing the voltage before and after charging with the second current to calculate the first DC internal resistance includes:

[0053] The first DC internal resistance formula is calculated as follows:

[0054] ;

[0055] Wherein, DCR1 is the first DC internal resistance, U2 is the voltage after charging with the second current, U1 is the voltage before charging with the second current, I1 is the charging current of the first current, and I2 is the charging current of the second current.

[0056] Specifically, wait for the battery cell state to be stable, let the internal chemical reaction of the battery cell be stable, and avoid the test results being affected by polarization or charging waste heat. At the beginning of charging, use a small current to charge the battery cell to eliminate the polarization phenomenon on the surface of the battery cell, so that the chemical reaction inside the battery cell reaches a preliminary equilibrium state. After the small current depolarization is completed, a large current is applied for secondary charging to simulate the high-power charging state of the battery cell in actual use. (U2-U1): represents the voltage change when the battery cell is changed from a small current charging state to a large current charging state. (I2-I1): represents the change in charging current from a small current to a large current. DC internal resistance (DCR1): The dynamic internal resistance of the battery cell under charging conditions is calculated by the ratio of voltage change to current change.

[0057] Through small current depolarization, the error caused by the polarization of the battery cell is eliminated, making the test results more stable. The large current charging test is closer to the actual use scenario and can accurately reflect the internal resistance performance of the battery cell under high power conditions. Combined with the formation test process, no additional equipment or complex operations are required, the test time is short, and the entire process is controlled within a few minutes, with almost no impact on production efficiency.

[0058] In actual battery applications, BMS charge balancing technology is usually used to balance the voltage of each cell in the battery pack. In order to improve the battery matching rate, only the discharge DCR data can be used as one of the matching conditions, and the charge DCR data is only for reference.

[0059] S103, waiting for the battery cell state to be stable, and when the battery cell state is stable, discharging the battery cell to calculate a second DC internal resistance;

[0060] In some preferred embodiments, waiting for the battery cell to be in a stable state, and when the battery cell is in a stable state, discharging the battery cell to calculate the second DC internal resistance includes:

[0061] discharging the battery cell with a third current;

[0062] discharging the battery cell with a fourth current;

[0063] The voltage before and after the fourth current is discharged is tested to calculate the second DC internal resistance.

[0064] In some preferred embodiments, testing the voltage before and after the fourth current is discharged to calculate the second DC internal resistance includes:

[0065] The second DC internal resistance formula is calculated as follows:

[0066] ;

[0067] Wherein, DCR2 is the second DC internal resistance, U4 is the voltage after the fourth current is discharged, U3 is the voltage before the fourth current is discharged, I4 is the discharge current of the fourth current, and I3 is the discharge current of the third current.

[0068] Specifically, the internal chemical reaction of the battery cell is allowed to reach equilibrium, and the polarization phenomenon caused by the previous discharge or test operation is eliminated. The third current discharge (small current discharge, depolarization) and the fourth current discharge (large current discharge, obtaining dynamic internal resistance information) are as follows: voltage change (U4−U3), the voltage difference between the large current discharge and the small current discharge during the discharge process, reflects the reaction characteristics of the battery cell during the dynamic discharge process, current change (I4−I3), the current change from small current to large current discharge, reflects the response characteristics of the battery cell to the load change, DC internal resistance (DCR2): calculated by the ratio of the change in voltage to current, representing the dynamic internal resistance characteristics of the battery cell under discharge conditions;

[0069] Small current discharge is used for depolarization to ensure that the test value is not disturbed by polarization phenomenon;

[0070] High-current discharge is closer to the actual discharge scenario and can truly reflect the dynamic performance of the battery cell under high-load conditions. In addition, the difference in internal resistance during the discharge process will directly lead to inconsistent heat loss power of the battery cell (series connection) or battery cell bias (parallel connection). Using DCR2 data as one of the parameters for battery cell matching can improve the discharge consistency and stability of the battery pack and reduce performance degradation in long-term cycles.

[0071] S104, grouping the battery cells using the first DC internal resistance and the second DC internal resistance.

[0072] In some preferred embodiments, grouping the battery cells by using the first DC internal resistance and the second DC internal resistance includes screening out abnormal battery cells by using the first DC internal resistance, and selecting battery cells whose internal resistance difference is within a set range by using the second DC internal resistance.

[0073] Specifically, the data of charging DC internal resistance (DCR1) is used to screen out cells that may have abnormalities (such as cells with significantly higher or lower internal resistance). If the DCR1 data deviates significantly from the normal range (the upper and lower thresholds can be set according to the production batch, such as ±3 times the standard deviation of the batch average), the cell is marked as an abnormal cell and does not participate in the subsequent grouping, or the cell formation process is repeated. The data of discharge DC internal resistance (DCR2) is used to accurately group the screened cells to ensure the consistency of the cells in the group. The DCR2 data has a high correlation with actual applications, so it can more stably reflect the actual dynamic internal resistance of the cell under discharge conditions. Compared with the traditional AC internal resistance (ACR), DCR can more realistically reflect the charge and discharge characteristics of the cell and has a higher practical significance.

[0074] For example, the DCR2 difference requirement is: the intra-group difference is less than 0.15 for grouping. When grouping, the cells with similar internal resistance are allocated to the same group in the order of DCR2 from small to large. For the cells with large DCR2 differences within the group, the grouping is readjusted to ensure that the DCR2 difference of each group meets the requirement.

[0075] In some preferred embodiments, waiting for the battery cell to be in a stable state includes leaving the battery cell to stand for 1-10 minutes, preferably 5 minutes, to eliminate polarization of the battery cell that may be caused by previous charging or discharging operations, allowing the chemical reaction inside the battery cell to reach a state of equilibrium, and avoiding temperature fluctuations or chemical instability from interfering with the test results. The standing time is generally 1 to 10 minutes, which is adjusted according to the battery cell capacity and the formation environment conditions. The preferred standing time is 5 minutes, which is the recommended time for most common lithium-ion batteries. It can ensure stability without affecting the production rhythm.

[0076] In some preferred embodiments, the first current charging includes:

[0077] The first current charging time is constant current charging for 10s to 50s, and the current is 0.05C to 0.3C.

[0078] In some preferred embodiments, the second current charging includes:

[0079] The second current charging time is constant current charging for 1s~15s, and the current is 0.5C~1C.

[0080] In some preferred embodiments, the third current discharge comprises:

[0081] The third current discharge time is constant current discharge for 10s to 50s, and the current is 0.05C to 0.3C.

[0082] In some preferred embodiments, the fourth current discharge comprises:

[0083] The fourth current discharge time is constant current discharge of 1s to 15s, and the current is 0.5C to 1C.

[0084] In some embodiments, in combination Figure 2 After measuring DCR2, let the battery cell stand still to make the battery cell state stable, then carry out subsequent formation or transfer the battery cell to aging, secondary filling and capacity division test process, and then carry out grouping.

[0085] Specifically, the same batch of cells refers to:

[0086] Consistent time range: production time is concentrated within a certain range, usually on the same day or in the same production cycle;

[0087] Consistent production lines: All cells are produced on the same production line to ensure consistent production conditions;

[0088] Consistent raw materials: Use the same raw materials for the battery cells (positive and negative electrode materials, electrolyte, diaphragm, etc.);

[0089] Consistent process: All steps and parameters in the production process are exactly the same;

[0090] Consistent specifications: All cells have the same capacity, size, chemistry and model;

[0091] Ensure the consistency of materials, processes and performance of the same batch of cells, providing a reliable basis for subsequent performance evaluation and matching;

[0092] When pairing, other pairing parameters are also included:

[0093] Different types of batteries have different technical document standards. Taking the 20Q battery in this experiment as an example, the required capacity is ≥21Ah, the difference within the capacity group is ≤1.0%*20Ah (control the difference in the capacity of the group to ensure that the battery pack has a consistent capacity during the charge and discharge process), and the voltage group difference is ≤10mV (the difference in open circuit voltage directly reflects whether the state of charge (SOC) of the battery is consistent. The smaller the voltage difference, the better the balance of the battery pack); the aging K value refers to the voltage drop per unit time, which is used to measure the size of the self-discharge of the battery. Generally, the K value is <0.08mV / h (batteries with small self-discharge can maintain voltage stability for a longer time, indicating that there are fewer side reactions inside the battery and higher quality). Data such as capacity, capacity group difference, voltage group difference, etc. can be directly collected through the capacity test cabinet. By strictly controlling the capacity, voltage difference and self-discharge difference, the performance of the batteries in the group is guaranteed to be consistent and the barrel effect is reduced.

[0094] In some embodiments, in combination Figure 3 ,

[0095] Battery model: 20Q soft-pack lithium iron phosphate battery cell;

[0096] Module configuration: 1P20S (single parallel, 20 series monomers form a module);

[0097] Comparative test: The experiment compared the voltage difference changes of battery packs matched with DCR parameters and those matched without DCR parameters;

[0098] Horizontal axis: cycle number, indicating the charge and discharge cycle process of the battery pack;

[0099] Vertical axis: battery pack discharge end voltage difference (mV), which indicates the difference between the maximum single cell voltage and the minimum single cell voltage inside the battery pack at the end of discharge;

[0100] Use DCR matched battery pack:

[0101] The pressure difference always remains small (less than 100mV). Even after more than 300 cycles, the pressure difference only increases slightly. The overall stability is good. As the intervention time of BMS passive equalization increases, the pressure difference changes tend to be stable.

[0102] Battery pack without DCR matching:

[0103] Before 300 cycles, the pressure difference gradually increased, and the growth rate was relatively fast. After 300 cycles, the pressure difference increased sharply, reaching the system threshold, triggering the BMS single-cell protection. Later, due to the increase in the BMS passive balancing intervention time, the pressure difference decreased slightly, but the pressure difference was still much higher than that of the battery pack using DCR.

[0104] It can be seen that the battery pack matched using DCR parameters (charging DCR1 and discharging DCR2) significantly improves the internal consistency of the battery pack, with a small and stable voltage difference and a longer cycle life. During long-term cycles, the performance of the single cells is more balanced, reducing the impact of the "barrel effect" on module performance. For battery packs that do not use DCR matching, the voltage difference of the battery pack increases significantly with the number of cycles because the difference in DC internal resistance is not taken into account, reducing the availability of the battery pack. After about 300 cycles, the battery pack triggers the BMS single cell protection, resulting in charging and discharging interruptions, and the reliability is significantly reduced.

[0105] The above is only a specific implementation of the embodiment of the present invention, but the protection scope of the embodiment of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed in the embodiment of the present invention should be included in the protection scope of the embodiment of the present invention. Therefore, the protection scope of the embodiment of the present invention should be based on the protection scope of the claims.

Claims

1. A method for improving battery pack consistency, characterized in that: Includes steps: During the high-temperature formation stage of the same batch of cells, charge the cells to a set charge; Waiting for the battery cell state to be stable, and when the battery cell state is stable, charging the battery cell to calculate a first DC internal resistance; Waiting for the battery cell state to be stable, and when the battery cell state is stable, discharging the battery cell to calculate the second DC internal resistance; The battery cells are grouped by using the first DC internal resistance and the second DC internal resistance.

2. A method for improving battery pack consistency according to claim 1, characterized in that: Waiting for the battery cell state to be stable, and when the battery cell state is stable, charging the battery cell to calculate the first DC internal resistance includes: Charging the battery cell with a first current; Charging the battery cell with a second current; The voltage before and after the second current charging is tested to calculate the first DC internal resistance.

3. A method for improving battery pack consistency according to claim 2, characterized in that: The voltage before and after the second current charging is tested to calculate the first DC internal resistance, including: The first DC internal resistance formula is calculated as follows: ； Wherein, DCR1 is the first DC internal resistance, U2 is the voltage after charging with the second current, U1 is the voltage before charging with the second current, I1 is the charging current of the first current, and I2 is the charging current of the second current.

4. A method for improving battery pack consistency according to claim 1, characterized in that: Waiting for the battery cell to stabilize includes letting the battery cell sit for 1-10 minutes.

5. A method for improving battery pack consistency according to claim 1, characterized in that: Waiting for the battery cell state to be stable, and when the battery cell state is stable, discharging the battery cell to calculate the second DC internal resistance includes: discharging the battery cell with a third current; discharging the battery cell with a fourth current; The voltage before and after the fourth current is discharged is tested to calculate the second DC internal resistance.

6. A method for improving battery pack consistency according to claim 5, characterized in that: Testing the voltage before and after the fourth current discharge to calculate the second DC internal resistance includes: The second DC internal resistance formula is calculated as follows: ； Wherein, DCR2 is the second DC internal resistance, U4 is the voltage after the fourth current is discharged, U3 is the voltage before the fourth current is discharged, I4 is the discharge current of the fourth current, and I3 is the discharge current of the third current.

7. A method for improving battery pack consistency according to claim 1, characterized in that: Grouping the battery cells by using the first DC internal resistance and the second DC internal resistance includes selecting abnormal battery cells by using the first DC internal resistance and selecting battery cells whose internal resistance difference is within a set range by using the second DC internal resistance.

8. A method for improving battery pack consistency according to claim 2, characterized in that: The first current charging includes: The first current charging time is constant current charging for 10s to 50s, and the current is 0.05C to 0.3C.

9. A method for improving battery pack consistency according to claim 2, characterized in that: The second current charging includes: The second current charging time is constant current charging for 1s~15s, and the current is 0.5C~1C.

10. A method for improving battery pack consistency according to claim 5, characterized in that: The third current discharge includes: The third current discharge time is constant current discharge for 10s to 50s, and the current is 0.05C to 0.3C.

11. A method for improving battery pack consistency according to claim 5, characterized in that: The fourth current discharge includes: The fourth current discharge time is constant current discharge of 1s to 15s, and the current is 0.5C to 1C.

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