Lithium ion battery grouping method

Abstract

The invention relates to a lithium ion battery grouping method. The method comprises the following steps: performing low-current formation charging on battery cells, and monitoring and removing abnormal battery cells; then carrying out aging treatment and screening abnormal battery cells; based on the relationship between the voltage and the state of charge, adjusting the electric quantity of the battery cell and evaluating the capacity of the single battery cell through a series of tests; finally, the evaluated battery cells form a battery pack, the overall performance of the battery pack is detected by calculating the voltage difference and large-current pulse discharge impedance, grouping is completed, and effective evaluation and guarantee of the battery performance are achieved. Compared with the prior art, the method has the advantages that the electric quantity is directly adjusted to the shipment electric quantity requirement by adopting a simple process step after the battery cell formation is finished, the traditional full-full discharge capacity test process step is replaced, and control means such as voltage difference and DCR are introduced in the process are replaced, and on the basis of ensuring the battery consistency and capacity, the battery cell production and manufacturing time is shortened, and the cost is greatly reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a lithium ion battery assembly method. Background Art

[0002] With the increasing awareness of energy conservation and environmental protection, the application scope of lithium-ion batteries is becoming wider and wider, especially with the popularization of new energy vehicles. The battery performance degradation, battery life reduction and battery safety issues of pure electric vehicles are becoming more and more prominent, and have become a topic of widespread concern in the industry and society. The battery system or battery pack is generally composed of multiple modules in series, where the module is composed of multiple battery modules in series and parallel. Due to factors such as the environment in the single cell manufacturing process and the errors of the equipment itself, lithium-ion batteries will show certain differences in voltage, capacity, internal resistance and self-discharge, and the consistency difference of the battery itself is a factor that causes battery performance degradation;

[0003] At present, battery manufacturers conduct formation, capacity division and other charge and discharge tests and long-term shelving operations before the cells are grouped into PACK to obtain relevant data of the process. By controlling the cell capacity, DCR, self-discharge and other grouping factors, the consistency of the cells in the battery pack is guaranteed to be within a reasonable range. In terms of cell capacity testing, most battery manufacturers currently use full charge and discharge to obtain the calibrated capacity of the cell and then perform capacity division. The process is relatively simple, mainly using a constant current or constant current constant voltage charging mode to charge the power battery pack to the charging cut-off voltage, and then discharge it at a constant current to the discharge cut-off voltage to obtain the capacity of the cell. However, this capacity division method takes a long time, which makes the overall manufacturing process of lithium-ion batteries take a long time, and it needs to occupy a capacity division cabinet for capacity division, which has a high capacity division cost. Summary of the invention

[0004] The purpose of the present invention is to provide a method for evaluating the consistency of lithium-ion battery packs and battery packs in order to overcome the defects of the above-mentioned prior art. After the battery cell is formed, a simple process is used to directly adjust the power to the shipping power requirement, replacing the traditional full charge and discharge capacity test process. Control measures such as pressure difference and DCR are introduced in the process. On the basis of ensuring battery consistency and capacity, the production time of the battery cell is shortened and the cost is greatly reduced.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] The present invention provides a lithium ion battery assembly method, comprising the following steps:

[0007] S1. Charge the battery cells with a small current to a preset voltage, and monitor the voltage of the battery cells at the same time to remove the battery cells with abnormal voltage;

[0008] S2, aging the battery cells treated in step S1 within a preset temperature range, and screening out battery cells with abnormal voltage after the aging process;

[0009] S3. Based on the relationship between the cell voltage and the state of charge, the cell power is adjusted to the state of charge required for shipment, and the adjusted power is recorded. Then, a discharge DC impedance test is performed and abnormal values ​​are eliminated. Then, the preset time is set aside to obtain the static voltage data of the cell. After eliminating the cells with a voltage lower than the lower limit, the state of charge of the cell at this time is obtained, and then the full-range discharge capacity is calculated, so as to complete the accurate assessment of the capacity of a single cell.

[0010] S4: Assemble the battery packs with the cells evaluated in S3, evaluate the electrical connection contact resistance and module consistency, and complete the battery assembly.

[0011] Furthermore, the method for obtaining the relationship between the cell voltage and the state of charge specifically includes the following steps:

[0012] Battery charging:

[0013] Select from normal cells and fully charge the cells using the rated charging current;

[0014] Discharge operation and data recording:

[0015] The battery cell is discharged at a preset rate. During the discharge process, the operation is performed according to the preset state of charge depth at intervals. At each interval point, the open circuit voltage of the battery cell and the corresponding state of charge are recorded to establish a SOC-OCV data relationship.

[0016] Furthermore, S2 specifically includes the following sub-steps:

[0017] S2-1, High temperature aging:

[0018] The cells that have been screened out by S1 are placed in a preset temperature range for aging;

[0019] S2-2, leave at room temperature:

[0020] After the high-temperature aging is completed, the battery cell is placed at room temperature for a preset time so that the state of the battery cell reaches equilibrium in a stable environment. The voltage data of the battery cell after high-temperature aging is obtained and recorded as OCV1. According to the SOC-OCV data relationship, the power at this time is obtained as SOC1;

[0021] S2-3. Screening abnormal cells:

[0022] Read the voltage data of the battery cells after high-temperature aging, and screen out the abnormal voltage batteries by comparing the battery cell voltage with the normal voltage range.

[0023] Furthermore, S3 specifically includes the following sub-steps:

[0024] S3-1, battery cell power adjustment:

[0025] According to the established SOC-OCV data relationship, adjust the battery cell power to the state of charge required for shipment;

[0026] S3-2, Discharge DC impedance test and outlier elimination:

[0027] Perform a discharge DCR test on the battery cells after the power adjustment, and eliminate abnormal DCR values ​​of the battery cells by comparing them with the normal range of DCR of the same batch of batteries;

[0028] S3-3. Pause and obtain static voltage data:

[0029] The battery cells are left alone for a preset time to obtain static voltage data OCV2 of each battery cell;

[0030] S3-4. Eliminate abnormal voltage cells and obtain charge status:

[0031] Eliminate the cells whose OCV2 voltage is lower than the lower limit, and obtain the SOC value of each single cell according to the corresponding relationship between SOC and OCV, which is recorded as SOC2;

[0032] S3-5. Calculate the discharge capacity of the entire interval

[0033] According to the obtained SOC1, SOC2 and adjusted power Q, the discharge capacity Q0 of the battery cell in the whole range is calculated.

[0034] Furthermore, in S3-2, the specific operation is: discharging when the capacity after formation is higher than the shipping capacity, and charging when the capacity after formation is lower than the shipping capacity, using the voltage as the cutoff condition to adjust the battery cell capacity, and record the adjusted capacity Q charged or discharged to the target SOC at this time.

[0035] Further, in S3-5, in the process of calculating the discharge capacity of the entire interval, the calculation formula is Q0=Q / |SOC2-SOC1|*η

[0036] Wherein, η is the capacity conversion between charging capacity and discharging capacity;

[0037] Introducing the capacity correction factor N, the full interval capacity is Q0′=Q / |SOC2-SOC1|N*η

[0038] Among them, N is obtained by selecting several battery cells after high-temperature aging and obtaining the data Q0 in simplified process steps and the corresponding Q actually measured at full charge and discharge capacity.

[0039] Furthermore, S1 specifically includes the following sub-steps:

[0040] S1-1, small current formation charging:

[0041] The assembled battery cells are charged in stages using different small currents in the formation process to charge the battery cells to a preset voltage;

[0042] S1-2, Shelf Monitoring:

[0043] After the battery is charged to the preset voltage, it is placed on hold. During the placement period, the static voltage is read to monitor the battery cell status.

[0044] S1-3. Eliminate abnormal cells:

[0045] By comparing with the normal voltage range, cells with abnormal voltage are eliminated and cells that meet the requirements are screened out.

[0046] Furthermore, in S1-2, the holding time is ≥ 2h;

[0047] In S1-3, the normal voltage range is to make the cell voltage within the range of Average±3σ.

[0048] Further, in S2-1, the aging temperature used is 45°C to 70°C, and the aging time is 6 to 48 hours;

[0049] In S2-2, the battery cell is left at room temperature for 1 to 3 hours;

[0050] In S2-3, the normal voltage range is such that the cell voltage is within the range of Average±3σ.

[0051] Furthermore, S4 specifically includes the following sub-steps:

[0052] S4-1. Pressure difference calculation:

[0053] Read the voltage of all cells in the battery pack, calculate the voltage difference ΔU2, and determine the consistency of the cells in the battery pack based on the fact that ΔU2 is less than the voltage difference ΔU0 required for battery pack shipment;

[0054] S4-2, DC impedance test:

[0055] Use a large current to perform a short-time pulse discharge test and read the corresponding DCR value. Based on the read DCR value, determine the DCR consistency of the modules in the battery pack, eliminate the obvious poor soldering of the busbar in a single module, record the discreteness of the DCR of each module in the battery pack, and screen out battery packs with problems by evaluating the discreteness of DCR.

[0056] By comparing the DCR value of each single cell with the offline DCR value of the cell, it is possible to determine whether the cell has a large self-discharge.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] 1) In terms of self-discharge detection of battery cells, the use of high-temperature aging of battery cells after formation is a key innovation. By closely monitoring the voltage data before and after aging of the battery cells, abnormal self-discharge cells can be accurately and efficiently screened out. Compared with traditional self-discharge test methods, this method greatly shortens the time required for self-discharge testing and reduces the time cost investment in the production process. At the same time, it eliminates problematic batteries at an early stage, effectively improving the quality and stability of subsequent battery packs, and laying a solid foundation for the long-term and reliable operation of the battery system.

[0059] 2) For the capacity test link, the simple capacity test method proposed in the present invention has many advantages. It replaces the traditional cumbersome and time-consuming full charge and discharge capacity test process, and cleverly introduces control means such as pressure difference and DCR in the test process. On the one hand, these control means effectively guarantee the consistency of the battery, making the performance of each battery cell in the battery pack more coordinated and matched, and significantly improving the overall performance of the battery pack; on the other hand, by utilizing the one-to-one correspondence between SOC and OCV, and introducing the capacity correction factor, the discharge capacity of the battery cell in the full range is obtained based on SOC1 and SOC2 and the interval capacity Q charged or discharged when adjusting the power. It can not only accurately and effectively evaluate the capacity of the battery cell in the full range and other working conditions, but also greatly shorten the test cycle, improve resource utilization, and reduce unnecessary losses to the battery cell caused by full charge and discharge tests, thereby extending the service life of the battery cell, reducing production costs, and enhancing the competitiveness of products in the market.

[0060] 3) In terms of applicability, the simple capacity assessment method proposed in the present invention shows excellent versatility. Whether it is a single battery pack or a battery pack capacity assessment scenario applied to a whole vehicle, this method can perform excellently. It can provide accurate and effective technical support for the subsequent SOH (health state) determination of the battery pack, which helps to achieve real-time monitoring and intelligent management of battery performance throughout the battery life cycle. By predicting the performance degradation or failure risks that may occur in the battery pack in advance, the battery usage strategy and maintenance plan can be optimized in a targeted manner, thereby maximizing the battery's performance and reducing the cost of use, providing a solid technical guarantee and innovation driving force for the sustainable development of the new energy industry, and also providing related companies with more efficient, reliable and forward-looking technical solutions in multiple links such as battery research and development, production, and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 The figure is a schematic diagram of the process of the lithium-ion battery assembly method of the present invention. DETAILED DESCRIPTION

[0062] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. Component models, material names, connection structures, control methods, algorithms and other features not clearly described in this technical solution are all considered to be common technical features disclosed in the prior art.

[0063] Example 1

[0064] This embodiment provides a lithium ion battery assembly method, see Figure 1 , including the following steps:

[0065] S1. Charge the battery cells with a small current to a preset voltage, and monitor the voltage of the battery cells at the same time to remove the battery cells with abnormal voltage;

[0066] S1 specifically includes the following sub-steps:

[0067] S1-1, small current formation charging:

[0068] The assembled battery cells are charged in stages using different small currents in the formation process to charge the battery cells to a preset voltage;

[0069] S1-2, Shelf Monitoring:

[0070] After the battery is charged to the preset voltage, it is placed on hold. During the placement period, the static voltage is read to monitor the battery cell status.

[0071] S1-3. Eliminate abnormal cells:

[0072] By comparing with the normal voltage range, cells with abnormal voltage are eliminated and cells that meet the requirements are screened out.

[0073] Furthermore, in S1-2, the holding time is ≥ 2h;

[0074] In S1-3, the normal voltage range is to make the cell voltage within the range of Average±3σ. “Average” refers to the average value. “σ” represents the standard deviation.

[0075] In specific implementation, the main purpose of step S1 is to initially screen out cells with normal voltage during the cell formation stage, providing a relatively stable quality cell foundation for subsequent aging and further performance testing. By combining low-current formation charging with voltage monitoring, cells with abnormal voltage can be discovered and removed in a timely manner to prevent abnormal cells from entering the subsequent process and affecting the overall quality of the battery pack.

[0076] S1-1 Small current formation charging sub-step

[0077] In specific implementation, the necessity of the formation charging stage: After the battery cell is assembled, the electrode material and the electrolyte inside it need to go through the formation process to form a stable solid electrolyte interface (SEI) film. The charging operation during the formation process can cause a series of electrochemical reactions on the surface of the electrode material, promoting the formation of the SEI film. The SEI film is crucial to the performance of the battery cell. It can prevent the electrolyte from further decomposing and improve the cycle life and safety of the battery cell.

[0078] In specific implementation, the reason for using a small current for charging is: using a small current for charging is to control the rate of the electrochemical reaction. If the charging current is too large, the electrochemical reaction on the electrode surface will be too intense, which will cause the SEI film to form unevenly and produce problems such as dendrites. Using different small current charging in stages can better adjust the charging strategy according to the reaction state inside the battery cell. For example, in the early stage of formation, the active substances inside the battery cell require a lower current to activate. As the formation process proceeds, appropriate adjustment of the current can make the SEI film more dense and evenly formed. Charging the battery cell to a preset voltage is to ensure that the formation process reaches an appropriate level. This preset voltage is determined based on factors such as the material system and design specifications of the battery cell. It can ensure that the formation of the SEI film is basically completed, while avoiding damage to the battery cell caused by overcharging.

[0079] S1-2 Shelf monitoring sub-step

[0080] In specific implementation, the purpose of the suspension is to allow the chemical and physical processes inside the battery cell to reach a relatively stable state after the formation charge reaches the preset voltage. During the charging process, complex electrochemical reactions occur inside the battery cell, including the insertion and extraction of ions. Suspension can gradually balance these processes.

[0081] In specific implementation, the significance of monitoring static voltage is: monitoring the state of the battery cell by reading the static voltage. During the shelf period, the charge distribution inside the battery cell will gradually stabilize, and the static voltage at this time can reflect the true state of the battery cell. If the battery cell has an internal short circuit, micro short circuit or other abnormal conditions, the static voltage will be abnormal. For example, the static voltage of a battery cell with an abnormal self-discharge rate will drop faster than that of a normal battery cell during the shelf process. The requirement for a shelf time of ≥2h is to ensure that the battery cell has enough time to reach a relatively stable state, because the internal reaction speed and time to reach equilibrium of different battery cells are different. This time setting can ensure that the status of most battery cells can be accurately monitored.

[0082] S1-3 Steps to remove abnormal cells

[0083] In specific implementation, the basis for determining the normal voltage range is: the normal voltage range is set to make the cell voltage within the range of Average±3σ, which is based on statistical principles. In the normal cell production process, the distribution of cell voltage conforms to certain statistical laws, where Average represents the average value of the voltage and σ represents the standard deviation. Through a large number of experiments and data analysis, it is found that the voltage of most normal cells will fall within this range. Cells outside this range are likely to have internal structural defects, uneven materials or other problems, resulting in abnormal voltage.

[0084] In specific implementation, the importance of removing abnormal cells: Removing cells with abnormal voltage can prevent these problematic cells from entering the subsequent aging and testing process. Because abnormal cells may cause more serious self-discharge, bulging, or even safety problems during the subsequent high-temperature aging process, and will affect the consistency and performance of the entire battery pack, screening them out at this stage can effectively improve the quality and reliability of subsequent battery packs.

[0085] S2, aging the battery cells treated in step S1 within a preset temperature range, and screening out battery cells with abnormal voltage after the aging process;

[0086] S2 specifically includes the following sub-steps:

[0087] S2-1, High temperature aging:

[0088] The cells that have been screened out by S1 are placed in a preset temperature range for aging;

[0089] S2-2, leave at room temperature:

[0090] After high-temperature aging, the battery cells are placed at room temperature for a preset time to allow the state of the battery cells to reach equilibrium in a stable environment;

[0091] S2-3. Screening abnormal cells:

[0092] Read the voltage data of the battery cells after high-temperature aging, and screen out the abnormal voltage batteries by comparing the battery cell voltage with the normal voltage range.

[0093] In S2-1, the aging temperature used is 45°C to 70°C, and the aging time is 6 to 48 hours;

[0094] In S2-2, the battery cell is left at room temperature for 1 to 3 hours;

[0095] In S2-3, the normal voltage range is such that the cell voltage is within the range of Average±3σ.

[0096] In specific implementation, the S2 step mainly involves aging and re-screening the cells that have been initially screened in S1. The aging process can further stabilize the chemical substances and microstructure inside the cells. Through the combined operation of high-temperature aging and room-temperature storage, the state changes of the cells under different environmental conditions are simulated. The cell voltage is checked after each stage, and abnormal cells are removed to ensure that the cells entering the subsequent steps have better consistency and stability, thereby improving the quality of battery packs.

[0097] S2-1 High temperature aging sub-step

[0098] In specific implementation, the purpose of high-temperature aging is to age the battery cell within a preset temperature range (45°C to 70°C) in order to accelerate the chemical reactions and physical changes inside the battery cell. Within this temperature range, a series of changes will occur in the electrode materials, electrolytes, and solid electrolyte interface (SEI) membranes inside the battery cell. For example, high temperature can promote further stabilization of the crystal structure of the electrode material, allowing the electrolyte to better infiltrate the electrode, and can also repair and optimize the SEI membrane to a certain extent. The aging time is set to 6-48h because different battery cell materials and structures require different times to complete these beneficial changes. Too short a time may result in insufficient aging and fail to achieve the desired effect; too long a time may cause a decline in the performance of the battery cell, such as excessive self-discharge or decomposition of materials.

[0099] In specific implementation, the basis for the temperature and time range: The temperature range of 45℃-70℃ is determined through a large number of experiments. This temperature range can accelerate the aging of the battery cell while avoiding excessively high temperatures that lead to thermal decomposition of the internal materials of the battery cell or other irreversible chemical reactions. Different battery cell systems (such as different positive electrode materials, negative electrode materials and electrolyte combinations) have different sensitivities to temperature. This range is a relatively suitable range determined after comprehensive consideration of a variety of battery cell systems, which can benefit most battery cells during the aging process. The range of aging time is also based on similar considerations. According to the reaction kinetics of the battery cell and actual test results, 6-48h can cover the time required for different battery cells to achieve better aging effects.

[0100] S2-2 Room temperature standby sub-step

[0101] In specific implementation, the necessity of leaving the battery at room temperature: After high-temperature aging, the chemical and physical state inside the battery cell is in a relatively unstable state. Leaving the battery cell at room temperature for 1-3 hours is to allow the battery cell to gradually return to a balanced state in a stable environment. During the high-temperature aging process, the ion distribution inside the battery cell, the state of the electrolyte, etc. have changed. Leaving the battery cell at room temperature can adjust these changes under relatively mild conditions. For example, ions can be redistributed in the electrode material, and the components in the electrolyte can be restored to a uniform state.

[0102] In specific implementation, the determination of the shelf time: The shelf time is set to 1-3h because the balance process inside the battery cell is relatively slow at room temperature. This time range can ensure that most batteries can complete the transition from an unstable state after high-temperature aging to a relatively stable state. If the shelf time is too short, the battery cell may not fully restore balance, which will affect the accuracy of subsequent voltage measurements; if the shelf time is too long, it will increase the production cycle and reduce production efficiency.

[0103] S2-3 Mechanism of screening abnormal cells

[0104] In specific implementation, the significance of re-screening: Although the cells have been preliminarily screened in step S1, during the high-temperature aging process, the cells may have new voltage anomalies due to internal chemical reactions, material changes, etc. Therefore, it is necessary to read the voltage data of the cells after high-temperature aging again, and screen out cells with abnormal voltage by comparing the cell voltage with the normal voltage range (making the cell voltage within the range of Average±3σ).

[0105] In specific implementation, the principle of the normal voltage range is similar to the principle in S1-3. The normal voltage range of Average±3σ is determined based on statistical principles. After high-temperature aging and room-temperature storage, the voltage of a normal battery cell should still conform to this statistical distribution law. Battery cells outside this range may have potential quality problems, such as abnormal self-discharge, internal structural damage, or changes in material properties. Eliminating these abnormal batteries can ensure that the batteries entering subsequent steps have better consistency and stability, which is beneficial to the quality and performance of subsequent battery packs.

[0106] The method for obtaining the relationship between the cell voltage and the state of charge specifically includes the following steps:

[0107] Extract normal cells, fully charge them first, and then discharge them at a preset rate and interval with a preset depth of charge. Record the open circuit voltage and state of charge during the discharge process to establish the relationship between the cell voltage and state of charge.

[0108] It includes the following sub-steps:

[0109] Battery charging:

[0110] Select the normal battery cells after screening in step S2, and fully charge the battery cells using the rated charging current. At this time, the initial state of charge of the battery cells is recorded as SOC1;

[0111] Discharge operation and data recording:

[0112] The battery cell is discharged at a preset rate. During the discharge process, the operation is performed according to the preset state of charge depth at intervals. At each interval point, the open circuit voltage of the battery cell and the corresponding state of charge are recorded to establish the SOC-OCV data relationship.

[0113] Cell Charging Sub-Steps

[0114] In specific implementation, the importance of selecting charging cells: Select the normal cells that have been screened in step S2 for charging, because these cells have been guaranteed in terms of performance and stability after the previous formation, aging and screening steps. Selecting such cells can ensure that the SOC-OCV data relationship established later has high accuracy and reliability, because abnormal cells may cause charging and discharging behaviors that do not conform to normal rules due to internal structural defects, abnormal self-discharge and other problems, thus affecting the establishment of data relationships.

[0115] The reason for using the rated charging current in specific implementation is: Using the rated charging current to fully charge the battery cell is to control the stability and consistency of the charging process. The rated charging current is determined based on the design specifications and material properties of the battery cell. It can ensure that during the charging process, the electrochemical reactions inside the battery cell (such as the insertion and extraction of lithium ions between the positive and negative electrodes) proceed at a relatively stable rate. If the charging current is too large, the electrode polarization phenomenon will be aggravated, affecting the performance and life of the battery cell; if the charging current is too small, the charging time will be extended and the production efficiency will be reduced. At the same time, the initial state of charge of the battery cell is recorded as SOC1 at the end of charging. This SOC1 is an important basic data for subsequent operations such as establishing the SOC-OCV data relationship and evaluating the battery cell capacity. It represents the initial charge level of the battery cell when it is fully charged, and can reflect the initial performance state of the battery cell.

[0116] Discharge operation and data recording sub-steps

[0117] In specific implementation, the significance of preset rate discharge is: discharge the battery cell at a preset rate, and this preset rate is determined based on factors such as the application scenario, material properties, and test requirements of the battery cell. Different discharge rates will affect the discharge curve and performance of the battery cell. For example, when discharging at a high rate, the electrochemical reaction rate inside the battery cell is accelerated, which will cause the electrode polarization phenomenon to be more obvious and the open circuit voltage to drop faster; while at a low rate discharge, the electrochemical reaction is relatively stable. By selecting a suitable preset rate, different operating conditions of the battery cell in actual use can be simulated, making the established SOC-OCV data relationship more valuable for practical application.

[0118] In specific implementation, the purpose of the interval preset state of charge depth operation is: during the discharge process, the operation is performed according to the interval preset state of charge depth, which is to obtain the open circuit voltage data of the battery cell at different states of charge. The state of charge (SOC) is an important indicator to measure the remaining power of the battery cell. By recording the open circuit voltage at different SOC points, the detailed correspondence between SOC and open circuit voltage (OCV) can be obtained. This detailed data relationship is very important for accurately evaluating the performance of the battery cell. For example, in the battery management system (BMS), the remaining power of the battery cell can be more accurately estimated based on the SOC-OCV relationship, providing a basis for the rational use and protection of the battery.

[0119] In specific implementation, the importance of establishing the SOC-OCV data relationship: recording the open circuit voltage of the battery cell at each interval point and the corresponding state of charge, and establishing the SOC-OCV data relationship is the core purpose of the entire step. This data relationship can intuitively reflect the connection between the change in the electrochemical state inside the battery cell and the external voltage performance. In the actual use and management of batteries, the SOC-OCV relationship can be used for a variety of purposes, such as estimating the remaining power of the battery cell, predicting the performance degradation of the battery cell, and optimizing the battery charging and discharging strategy. Through this data relationship, we can have a deeper understanding of the performance characteristics of the battery cell, and provide key data support for subsequent operations such as battery cell capacity evaluation, battery grouping, and battery pack performance optimization.

[0120] S3. Based on the relationship between the cell voltage and the state of charge, the cell power is adjusted to the state of charge required for shipment, and the adjusted power is recorded. Then, a discharge DC impedance test is performed and abnormal values ​​are eliminated. Then, the preset time is set aside to obtain the static voltage data of the cell. After eliminating the cells with a voltage lower than the lower limit, the state of charge of the cell at this time is obtained, and then the full-range discharge capacity is calculated, so as to complete the accurate assessment of the capacity of a single cell.

[0121] S3 specifically includes the following sub-steps:

[0122] S3-1, battery cell power adjustment:

[0123] According to the established SOC-OCV data relationship, adjust the battery cell power to the state of charge required for shipment;

[0124] S3-2, Discharge DC impedance test and outlier elimination:

[0125] Perform a discharge DCR test on the battery cells after the power adjustment, and eliminate abnormal DCR values ​​of the battery cells by comparing them with the normal range of DCR of the same batch of batteries;

[0126] S3-3. Pause and obtain static voltage data:

[0127] The battery cells are left alone for a preset time to obtain static voltage data OCV2 of each battery cell;

[0128] S3-4. Eliminate abnormal voltage cells and obtain charge status

[0129] Eliminate the cells whose OCV2 voltage is lower than the lower limit, and obtain the SOC value of each single cell according to the corresponding relationship between SOC and OCV, which is recorded as SOC2;

[0130] S3-5. Calculate the discharge capacity of the entire interval

[0131] According to the obtained SOC1, SOC2 and adjusted power Q, the discharge capacity Q0 of the battery cell in the whole range is calculated.

[0132] In S3-2, the specific operation is: when the capacity after formation is higher than the shipping capacity, discharge is performed, and when the capacity after formation is lower than the shipping capacity, charge is performed. The voltage is used as the cutoff condition to adjust the battery cell capacity, and the adjusted capacity Q charged or discharged to the target SOC at this time is recorded.

[0133] In S3-5, during the calculation of the discharge capacity of the entire interval, the calculation formula is Q0 = Q / |SOC2-SOC1|*η

[0134] Wherein, η is the capacity conversion between charging capacity and discharging capacity;

[0135] Introducing the capacity correction factor N, the full interval capacity is Q0′=Q / |SOC2-SOC1|N*η

[0136] Among them, N is obtained by selecting several battery cells after high-temperature aging and obtaining the data Q0 in simplified process steps and the corresponding Q actually measured at full charge and discharge capacity.

[0137] S3-1 Cell power adjustment sub-step

[0138] In specific implementation, the principle of adjusting the power is based on the SOC-OCV relationship: the power of the battery cell is adjusted to the state of charge required for shipment according to the SOC-OCV data relationship established in the first step. The SOC-OCV relationship is an important link between the internal electrochemical state of the battery cell and the external measurable voltage. Through this relationship, the charging or discharging process can be controlled according to the voltage value corresponding to the target state of charge. For example, if the voltage corresponding to the state of charge required for shipment is V, and the current battery cell voltage is higher than V, a discharge operation is performed; otherwise, a charging operation is performed, thereby accurately adjusting the battery cell power to the required state. Such an adjustment ensures that the battery cell has an initial power that meets the requirements when shipped, and based on the established scientific data relationship, the adjustment process is more accurate and controllable.

[0139] S3-2 Discharge DC impedance test and outlier elimination sub-step mechanism

[0140] In specific implementation, the significance of DCR test is: to perform discharge DCR (DC resistance) test on the battery cell after adjusting the power. DCR is an important parameter reflecting the internal resistance characteristics of the battery cell. During the discharge process of the battery cell, DCR will affect the voltage drop and energy loss of the battery cell. By measuring DCR, the conductivity inside the battery cell and the contact between the electrode and the electrolyte can be evaluated. For example, a higher DCR means problems such as passivation of the electrode surface or drying of the electrolyte, which will affect the charge and discharge efficiency and power performance of the battery cell.

[0141] In specific implementation, the principle of eliminating abnormal values ​​by comparing with the normal range is: eliminate abnormal values ​​by comparing with the normal range of DCR of the same batch of battery cells. The same batch of battery cells should have similar DCR distribution under normal circumstances because they are manufactured under the same production conditions. The normal range is obtained by statistical analysis of the DCR test data of a large number of battery cells from the same batch. For example, the normal range is defined by the range of mean value ± standard deviation. Battery cell DCR values ​​outside this range are caused by internal structural defects of the battery cells, uneven materials, or abnormal conditions in the production process. Eliminating these abnormal values ​​can ensure that the battery cells entering the subsequent steps have more consistent internal resistance characteristics, thereby improving the overall performance and reliability of the battery pack.

[0142] In specific implementation, the principle of the power adjustment operation is: when the capacity after formation is higher than the shipping capacity, discharge is performed, and when the capacity after formation is lower than the shipping capacity, charge is performed. The voltage is used as the cutoff condition to adjust the battery cell power, and the adjusted power Q is recorded. This adjustment method is based on the comparison between the actual power state of the battery cell and the target shipping power. The voltage is used as the cutoff condition because in the SOC-OCV relationship, voltage is a directly measurable physical quantity and is closely related to the charge state of the battery cell. By controlling the voltage to stop charging or discharging, it can be ensured that the battery cell reaches the target charge state. The recorded adjusted power Q is an important parameter for subsequent operations such as calculating the battery cell capacity.

[0143] S3-3 Suspend and obtain static voltage data sub-step

[0144] In specific implementation, the purpose of shelving is to shelve the battery cell for a preset time to allow the internal electrochemical state of the battery cell to reach equilibrium again after the previous operation (such as power adjustment and DCR test). During the previous operation, the ion distribution and electric field inside the battery cell have changed, and shelving can stabilize these changes.

[0145] In specific implementation, the significance of obtaining static voltage data is: obtaining the static voltage data OCV2 of each battery cell. The static voltage can reflect the potential difference of the battery cell in a stable state, which is an external manifestation of the balance of chemical energy and electrical energy inside the battery cell. OCV2 can be used to subsequently determine whether there is voltage abnormality in the battery cell, and combined with the SOC-OCV relationship, it can further evaluate the performance indicators such as the state of charge of the battery cell.

[0146] S3-4 Mechanism of removing abnormal voltage cells and obtaining charge status sub-steps

[0147] In specific implementation, the importance of removing abnormal voltage cells: remove cells with OCV2 voltage lower than the lower limit, because the abnormal voltage of these cells means that there are abnormal self-discharge, material performance degradation or other potential problems inside. In the battery pack, the abnormal voltage of one cell will affect the performance and safety of the entire battery pack, so these abnormal cells need to be removed to ensure the consistency and stability of the battery pack.

[0148] In specific implementation, the principle of obtaining SOC2 is: according to the corresponding relationship of SOC-OCV, the SOC value of each single cell is obtained, which is recorded as SOC2. The SOC-OCV relationship has been established in the previous steps. Through the measured OCV2, the corresponding SOC value can be found in this relationship. SOC2 represents the state of charge of the cell after a series of operations, and is used together with SOC1 (initial state of charge) for subsequent cell capacity calculation and other operations.

[0149] S3-5 Calculation of the full range discharge capacity sub-step mechanism

[0150] In specific implementation, the principle of the basic calculation formula is: according to the obtained SOC1, SOC2, and adjusted power Q, calculate the discharge capacity Q0 of the battery cell in the full range, the formula is Q0 = Q / |SOC2-SOC1|*η. The principle of this formula is based on the conservation of power and the change of the state of charge. Q is the amount of power charged or discharged in the process of adjusting the power of the battery cell, |SOC2-SOC1| represents the change range of the state of charge, and η is the capacity conversion between the charging capacity and the discharging capacity, taking into account factors such as the energy loss in the charging and discharging process. This formula can be used to estimate the equivalent full-range discharge capacity of the battery cell in the range of SOC1 to SOC2.

[0151] The principle of introducing the capacity correction factor N: By introducing the capacity correction factor N, the full range capacity is Q0′=Q / |SOC2-SOC1|N*η, where N is obtained by selecting a number of cells after high temperature aging and performing simplified process steps to obtain the data Q0 and the corresponding measured data Q of the full charge and discharge capacity. In actual situations, there is a certain difference between the discharge capacity estimated by the simplified process steps and the full charge and discharge capacity, and N is introduced to correct this difference. By conducting experiments on cells after high temperature aging, the corresponding relationship between Q0 and the measured Q is obtained, thereby determining the value of N, so that the calculated full range capacity Q0′ more accurately reflects the actual full range discharge capacity of the cell.

[0152] S4: Assemble the battery pack with the cells evaluated in S3. Measure the overall performance of the battery pack by calculating the voltage difference and measuring the DC impedance with a large current short-time pulse discharge. Evaluate the electrical connection contact resistance and module consistency to complete the battery assembly.

[0153] S4 specifically includes the following sub-steps:

[0154] S4-1. Pressure difference calculation:

[0155] Read the voltage of all cells in the battery pack, calculate the voltage difference ΔU2, and determine the consistency of the cells in the battery pack based on the fact that ΔU2 is less than the voltage difference ΔU0 required for battery pack shipment;

[0156] S4-2, DC impedance test:

[0157] Use a large current to perform a short-time pulse discharge test, read the corresponding DCR value, and determine the DCR consistency of the modules in the battery pack based on the read DCR value. This can eliminate obvious poor soldering of the busbar in a single module, record the discreteness of the DCR of each module in the battery pack, and screen out battery packs with problems by evaluating the discreteness of the DCR.

[0158] By comparing the DCR value of each single cell with the offline DCR value of the cell, it is possible to determine whether the cell has a large self-discharge.

[0159] In specific implementation, the main purpose of step S4 is to conduct overall performance testing on the battery pack composed of cells evaluated in the previous steps to ensure that the quality and performance of the battery pack meet the requirements and complete the battery pack assembly. Through the two key operations of calculating the voltage difference and performing the DC impedance test, the consistency of the cells in the battery pack and the connection of the modules are evaluated from different angles to ensure the stability and reliability of the battery pack during subsequent use.

[0160] S4-1 Pressure difference calculation sub-step

[0161] In specific implementation, the significance of reading the cell voltage and calculating the voltage difference is: reading the voltage of all cells in the battery pack and calculating the voltage difference ΔU2, because the consistency of the cell voltage is crucial to the performance of the battery pack. In a normal battery pack, the voltages of each cell should be relatively close, otherwise it will cause unevenness in the battery pack during charging and discharging. For example, when charging cells with large voltage differences, some cells will be overcharged while others will not be fully charged; when discharging, the cells with low voltage will run out of power first, affecting the battery life of the entire battery pack.

[0162] In specific implementation, the basis for judging consistency is: judging the consistency of the battery cells in the battery pack based on ΔU2 < the voltage difference ΔU0 required for battery pack shipment. The voltage difference ΔU0 required for battery pack shipment is a reasonable voltage difference range determined based on the design specifications and application scenarios of the battery pack. If ΔU2 is less than this range, it means that the voltage consistency of the battery cells in the battery pack is good, which can ensure the performance stability of the battery pack during normal use; conversely, if ΔU2 is greater than ΔU0, there is a problem of inconsistent battery cell performance, which requires further inspection or adjustment.

[0163] S4-2 DC impedance test sub-steps

[0164] In specific implementation, the purpose of the high-current short-time pulse discharge test is to use a large current to perform a short-time pulse discharge test and read the corresponding DCR (direct current resistance) value, because under high-current discharge conditions, some potential problems inside the battery cell and battery pack are more easily exposed. For example, when discharging with a large current, the polarization phenomenon of the battery cell will be more obvious, which can more accurately reflect the resistance characteristics inside the battery cell. At the same time, this test method can simulate the working state of the battery pack in a high-power output scenario, such as the instantaneous high-current discharge of the battery when an electric vehicle accelerates.

[0165] In specific implementation, the principle of determining the consistency of module DCR is as follows: the consistency of module DCR in the battery pack is determined based on the read DCR value. In the battery pack, the DCR of each module should be relatively consistent because they are composed of cells that have been screened and evaluated in the previous steps. If the DCR difference between modules is large, it is caused by problems such as poor connection of the cells inside the module, poor soldering of the busbar, or performance differences of the cells themselves. These potential problems can be discovered by evaluating the discreteness of the DCR. For example, a DCR value with a large degree of dispersion indicates that there are poorly connected modules in the battery pack, which need to be inspected and repaired.

[0166] In specific implementation, the principle of eliminating cold solder joints: This test can eliminate obvious cold solder joints in the busbars within a single module. When there is a cold solder joint on the busbar, when a large current passes through, the contact resistance increases, which will cause the DCR value of the module to be significantly abnormal. This cold solder joint problem can be identified by comparing the DCR values ​​of each module with the normal range. By recording the discreteness of the DCR of each module in the battery pack, the quality of the battery pack can be evaluated as a whole. The discreteness can be measured by statistical methods, such as calculating the standard deviation. If the degree of dispersion exceeds a certain threshold, it can be determined that the battery pack has quality problems and needs to be screened and processed.

[0167] In specific implementation, the principle of judging the self-discharge situation is: by comparing the DCR value of each single cell with the offline DCR value of the cell to determine whether the cell has a large self-discharge. Self-discharge will cause the DCR value of the cell to change, because the chemical substances and electrical states inside the cell change during the self-discharge process. If the DCR value of a single cell changes significantly compared to when it was offline, it is caused by self-discharge. This comparison can help identify cells with abnormal self-discharge, thereby ensuring the long-term performance and reliability of the battery pack.

[0168] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A lithium-ion battery assembly method, characterized in that: The following steps are involved: S1. Charge the battery cells with a small current to a preset voltage, and monitor the voltage of the battery cells at the same time to remove the battery cells with abnormal voltage; S2, aging the battery cells treated in step S1 within a preset temperature range, and screening out battery cells with abnormal voltage after the aging process; S3. Based on the relationship between the cell voltage and the state of charge, the cell power is adjusted to the state of charge required for shipment, and the adjusted power is recorded. Then, a discharge DC impedance test is performed and abnormal values ​​are eliminated. Then, the preset time is set aside to obtain the static voltage data of the cell. After eliminating the cells with a voltage lower than the lower limit, the state of charge of the cell at this time is obtained, and then the full-range discharge capacity is calculated, so as to complete the accurate assessment of the capacity of a single cell. S4: Assemble the battery packs with the cells evaluated in S3, evaluate the electrical connection contact resistance and module consistency, and complete the battery assembly.

2. A lithium-ion battery assembly method according to claim 1, characterized in that: The method for obtaining the relationship between the cell voltage and the state of charge specifically includes the following steps: Battery charging: Select from normal cells and fully charge the cells using the rated charging current; Discharge operation and data recording: The battery cell is discharged at a preset rate. During the discharge process, the operation is performed according to the preset state of charge depth at intervals. At each interval point, the open circuit voltage of the battery cell and the corresponding state of charge are recorded to establish a SOC-OCV data relationship.

3. A lithium-ion battery assembly method according to claim 1, characterized in that: S2 specifically includes the following sub-steps: S2-1, High temperature aging: The cells that have been screened out by S1 are placed in a preset temperature range for aging; S2-2, leave at room temperature: After the high-temperature aging is completed, the battery cell is placed at room temperature for a preset time so that the state of the battery cell reaches equilibrium in a stable environment. The voltage data of the battery cell after high-temperature aging is obtained and recorded as OCV1. According to the SOC-OCV data relationship, the power at this time is obtained as SOC1; S2-3. Screening abnormal cells: Read the voltage data of the battery cells after high-temperature aging, and screen out the abnormal voltage batteries by comparing the battery cell voltage with the normal voltage range.

4. A lithium-ion battery assembly method according to claim 3, characterized in that: S3 specifically includes the following sub-steps: S3-1, battery cell power adjustment: According to the established SOC-OCV data relationship, adjust the battery cell power to the state of charge required for shipment; S3-2, Discharge DC impedance test and outlier elimination: Perform a discharge DCR test on the battery cells after the power adjustment, and eliminate abnormal DCR values ​​of the battery cells by comparing them with the normal range of DCR of the same batch of batteries; S3-3. Pause and obtain static voltage data: The battery cells are left alone for a preset time to obtain static voltage data OCV2 of each battery cell; S3-4. Eliminate abnormal voltage cells and obtain charge status: Eliminate the cells whose OCV2 voltage is lower than the lower limit, and obtain the SOC value of each single cell according to the corresponding relationship between SOC and OCV, which is recorded as SOC2; S3-5. Calculate the discharge capacity of the entire interval According to the obtained SOC1, SOC2 and adjusted power Q, the discharge capacity Q0 of the battery cell in the whole range is calculated.

5. A lithium-ion battery assembly method according to claim 4, characterized in that: In S3-2, the specific operation is: when the capacity after formation is higher than the shipping capacity, discharge is performed, and when the capacity after formation is lower than the shipping capacity, charge is performed. The voltage is used as the cutoff condition to adjust the battery cell capacity, and the adjusted capacity Q charged or discharged to the target SOC at this time is recorded.

6. A lithium-ion battery assembly method according to claim 4, characterized in that: In S3-5, during the calculation of the discharge capacity of the entire interval, the calculation formula is Q0 = Q / |SOC2-SOC1|*η Wherein, η is the capacity conversion between charging capacity and discharging capacity; Introducing the capacity correction factor N, the full interval capacity is Q0′=Q / |SOC2-SOC1|N*η Among them, N is obtained by selecting several battery cells after high-temperature aging and obtaining the data Q0 in simplified process steps and the corresponding Q actually measured at full charge and discharge capacity.

7. A lithium-ion battery assembly method according to claim 1, characterized in that: S1 specifically includes the following sub-steps: S1-1, small current formation charging: The assembled battery cells are charged in stages using different small currents in the formation process to charge the battery cells to a preset voltage; S1-2, Shelf Monitoring: After the battery is charged to the preset voltage, it is placed on hold. During the placement period, the static voltage is read to monitor the battery cell status. S1-3. Eliminate abnormal cells: By comparing with the normal voltage range, cells with abnormal voltage are eliminated and cells that meet the requirements are screened out.

8. A lithium-ion battery assembly method according to claim 7, characterized in that: In S1-2, the holding time is ≥ 2h; In S1-3, the normal voltage range is to make the cell voltage within the range of Average±3σ.

9. A lithium-ion battery assembly method according to claim 3, characterized in that: In S2-1, the aging temperature used is 45°C to 70°C, and the aging time is 6 to 48 hours; In S2-2, the battery cell is left at room temperature for 1 to 3 hours; In S2-3, the normal voltage range is such that the cell voltage is within the range of Average±3σ.

10. A lithium-ion battery assembly method according to claim 1, characterized in that: S4 specifically includes the following sub-steps: S4-1. Pressure difference calculation: Read the voltage of all cells in the battery pack, calculate the voltage difference ΔU2, and determine the consistency of the cells in the battery pack based on the fact that ΔU2 is less than the voltage difference ΔU0 required for battery pack shipment; S4-2, DC impedance test: Use a large current to perform a short-time pulse discharge test and read the corresponding DCR value. Based on the read DCR value, determine the DCR consistency of the modules in the battery pack, eliminate the obvious poor soldering of the busbar in a single module, record the discreteness of the DCR of each module in the battery pack, and screen out battery packs with problems by evaluating the discreteness of DCR. By comparing the DCR value of each single cell with the offline DCR value of the cell, it is possible to determine whether the cell has a large self-discharge.

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