Self-discharge screening method and device for lithium ion battery

Abstract

This invention discloses a method and apparatus for screening the self-discharge of lithium-ion batteries. The method involves sequentially subjecting multiple batteries from a batch to be tested to high-temperature settling, a first room-temperature settling, and a second room-temperature settling. After the first two settling periods, the batteries in the batch are screened based on the process parameters obtained from the first and second settling periods, respectively. After the final settling, the physical self-discharge rate of the remaining batteries is determined based on the process parameters from the three settling periods. Physical self-discharge screening is then performed on the remaining batteries based on their physical self-discharge rates, thus achieving self-discharge screening of lithium-ion batteries. After two settling periods and screening, the physical self-discharge rate of the batteries is determined using state parameters, thereby screening out batteries with insignificant physical self-discharge defects. This improves the accuracy of self-discharge screening within a limited settling time.

Description

Technical Field

[0001] The present invention relates to energy storage battery technology, and more particularly to a self-discharge screening method and apparatus for lithium-ion batteries. Background Technology

[0002] Self-discharge is an inherent property of lithium batteries, manifesting as a gradual decrease in capacity over time when the battery is stored. Due to factors such as raw material batches, manufacturing processes, manufacturing environment, and management levels, the self-discharge rates of mass-produced lithium batteries vary. For end-user applications, it is necessary to ensure the consistency of self-discharge rates among integrated battery packs.

[0003] Currently, the commonly used method for self-discharge screening in the industry is the K-value method (also known as the self-discharge rate screening method). Its principle is to calculate the self-discharge rate of the battery based on the difference in open-circuit voltage before and after a certain resting time and the resting time, and then screen out batteries that exceed the self-discharge standard based on the self-discharge rate.

[0004] However, the accuracy of self-discharge screening using the K-value method is limited by the settling time. This results in poor screening accuracy for batteries within a limited self-discharge time. Summary of the Invention

[0005] This invention provides a method and apparatus for screening the self-discharge of lithium-ion batteries, so as to improve the accuracy of self-discharge screening based on a limited settling time.

[0006] In a first aspect, embodiments of the present invention provide a self-discharge screening method for lithium-ion batteries, the self-discharge screening method for lithium-ion batteries comprising:

[0007] Adjust the batteries in the batch to be tested to the preset state of charge;

[0008] The battery under test in the preset charged state is subjected to high-temperature static treatment;

[0009] Based on the state parameters of the battery under test during the high-temperature static placement, high-temperature screening is performed on the batteries in the batch to be tested.

[0010] The remaining batteries in the batch to be tested after the high-temperature screening are subjected to a first room-temperature standing period.

[0011] Based on the state parameters of the battery under test during the high-temperature resting and the first room-temperature resting, the remaining batteries in the batch under test are screened at room temperature.

[0012] The remaining batteries in the batch to be tested after the initial room temperature screening are subjected to a second room temperature settling process.

[0013] The physical self-discharge rate of the battery under test is determined based on the state parameters of the battery under test during the high temperature resting, the first room temperature resting, and the second room temperature resting.

[0014] Based on the physical self-discharge rate of the battery under test, physical self-discharge screening is performed on the remaining batteries in the batch under test after the room temperature screening.

[0015] Optionally, the state parameters during the high-temperature settling period include a first settling time, a first open-circuit voltage of the battery under test before the high-temperature settling period, and a second open-circuit voltage of the battery under test after the high-temperature settling period.

[0016] The step of performing high-temperature screening on the batch of batteries to be tested based on the state parameters of the batteries under test during the high-temperature static placement includes:

[0017] Based on the first settling time, the first open-circuit voltage, and the second open-circuit voltage, calculate the first unit voltage drop of the battery under test during the high-temperature settling period;

[0018] Based on the relative relationship between the first unit voltage drop and the first preset voltage drop of the battery under test, high-temperature screening is performed on the batteries in the batch under test.

[0019] Optionally, the state parameters during the first room temperature resting period include the second resting time and the third open-circuit voltage of the battery under test after the first room temperature resting period.

[0020] The step of performing room temperature screening on the remaining batteries in the batch to be tested based on the state parameters of the batteries under test during the high-temperature resting and the first room-temperature resting includes:

[0021] Based on the first settling time, the second settling time, the first open-circuit voltage, and the third open-circuit voltage, calculate the second unit voltage drop of the battery under test during the high-temperature settling and the first room-temperature settling.

[0022] Based on the relative relationship between the second unit voltage drop and the second preset voltage drop of the battery under test, the remaining batteries in the batch under test are screened at room temperature.

[0023] Optionally, determining the physical self-discharge rate of the battery under test based on its state parameters during the high-temperature resting, the first room-temperature resting, and the second room-temperature resting includes:

[0024] Based on the open-circuit voltage before and after each settling process, calculate the settling voltage difference generated by the remaining test batteries in the batch under test during the settling process. The settling process includes a first process, a second process, and a third process. The first process includes the high-temperature settling, the second process includes the high-temperature settling and the first room-temperature settling, and the third process includes the high-temperature settling, the first room-temperature settling, and the second room-temperature settling.

[0025] The intrinsic voltage drop of the battery under test during the resting process is determined based on the average level of the resting voltage difference generated by the remaining batteries under test in the batch under test during the resting process.

[0026] The physical self-discharge voltage drop of the battery under test in each of the resting processes is determined based on the difference between the static voltage difference of the remaining batteries under test in the batch under test and the corresponding intrinsic voltage drop.

[0027] The physical self-discharge rate of the battery under test is determined based on the physical self-discharge voltage drop and resting time during each of the resting processes.

[0028] Optionally, the intrinsic voltage drop of the battery under test during the resting process is determined based on the average level of the resting voltage difference generated by the remaining batteries under test in the batch under test during the resting process, including:

[0029] The static pressure differences generated by each of the batteries under test during the static placement process are sorted in order of magnitude.

[0030] Based on the sorting of the static pressure differences, the median of the static pressure differences is determined as the intrinsic voltage drop of the battery under test during the static period.

[0031] Optionally, determining the physical self-discharge rate of the battery under test based on the physical self-discharge voltage drop and resting time during each of the resting processes includes:

[0032] Based on the physical self-discharge voltage drop and resting time of the battery under test in each of the resting processes, a curve relating the physical self-discharge voltage drop and the resting time of the battery under test is fitted.

[0033] The physical self-discharge rate of the battery under test is determined based on the slope of the relationship curve.

[0034] Optionally, the step of performing physical self-discharge screening on the remaining batteries in the test batch after the room temperature screening, based on the physical self-discharge rate of the batteries under test, includes:

[0035] Based on the longest manufacturing time of the battery under test and the physical self-discharge rate, the predicted maximum value of the physical self-discharge voltage drop of the battery under test before its first use is determined.

[0036] Based on the relative relationship between the predicted maximum value of the battery under test and the preset self-discharge voltage drop, physical self-discharge screening is performed on the remaining batteries under test in the batch after the room temperature screening.

[0037] Optionally, the high-temperature settling temperature is between 40°C and 50°C; the settling temperatures for the first and second room-temperature settling are both between 20°C and 30°C.

[0038] Optionally, the high-temperature settling time is between 2 and 3 days; the first room-temperature settling time is between 2 and 4 days; and the second room-temperature settling time is between 3 and 5 days.

[0039] Secondly, embodiments of the present invention also provide a self-discharge screening device for lithium-ion batteries. The self-discharge screening device for lithium-ion batteries includes a state of charge adjustment module, a high-temperature settling module, a high-temperature screening module, a first room-temperature settling module, a room-temperature screening module, a second room-temperature settling module, a physical self-discharge determination module, and a physical screening module.

[0040] The state of charge adjustment module is used to adjust the batteries in the batch to be tested to a preset state of charge.

[0041] The high-temperature settling module is used to perform high-temperature settling on the battery under test.

[0042] The high-temperature screening module is used to perform high-temperature screening on the batch of batteries to be tested based on the state parameters of the batteries to be tested during the high-temperature static placement.

[0043] The first room temperature settling module is used to perform a first room temperature settling on the remaining batteries in the batch to be tested after the high temperature screening.

[0044] The room temperature screening module is used to perform room temperature screening on the remaining batteries in the batch to be tested based on the state parameters of the batteries under test during the high temperature resting and the first room temperature resting.

[0045] The second room temperature settling module is used to perform a second room temperature settling on the remaining batteries in the batch to be tested after the room temperature screening.

[0046] The physical self-discharge determination module is used to determine the physical self-discharge rate of the battery under test based on the state parameters of the battery under test during the high temperature rest, the first room temperature rest, and the second room temperature rest.

[0047] The physical screening module is used to perform physical self-discharge screening on the remaining batteries in the batch after the room temperature screening, based on the physical self-discharge rate of the batteries to be tested.

[0048] The self-discharge screening method and apparatus for lithium-ion batteries provided in this invention sequentially subject multiple batteries in a batch to high-temperature settling, first room-temperature settling, and second room-temperature settling. After the first two settling periods, the batteries in the batch are screened based on the process parameters obtained from the first and second settling periods, respectively. After the final settling period, the physical self-discharge rate of the remaining batteries is determined based on the process parameters from the three settling periods. Physical self-discharge screening is then performed on the remaining batteries based on their physical self-discharge rates, thus achieving self-discharge screening of lithium-ion batteries. After two settling periods and screening, the physical self-discharge rate of the batteries is determined using state parameters, thereby screening out batteries with insignificant physical self-discharge defects. This improves the accuracy of self-discharge screening within a limited settling time. Attached Figure Description

[0049] Figure 1 This is a graph showing the voltage change of a lithium iron phosphate battery with a 20% state of charge as a function of resting time.

[0050] Figure 2 The curves show the voltage drop variations caused by different types of self-discharge in a battery with poor physical self-discharge.

[0051] Figure 3 This is a schematic flowchart of a self-discharge screening method for lithium-ion batteries provided in an embodiment of the present invention.

[0052] Figure 4 A schematic flowchart of another self-discharge screening method for lithium-ion batteries provided in an embodiment of the present invention;

[0053] Figure 5 This is a schematic diagram of the structure of a self-discharge screening device for lithium-ion batteries provided in an embodiment of the present invention. Detailed Implementation

[0054] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, the accompanying drawings show only the parts relevant to the present invention, and not all of the structures.

[0055] As described in the background section, the commonly used method for self-discharge screening in the industry is the K-value method. Its principle is to calculate the battery's self-discharge rate based on the difference in open-circuit voltage before and after a certain resting time, and then filter out batteries with excessive self-discharge based on this rate. However, the accuracy of self-discharge screening using the K-value method is limited by the resting time, resulting in poor accuracy in self-discharge screening within a limited time before shipment. Since lithium-ion batteries are widely used in new energy vehicles and are closely related to people's lives, users generally have a high level of concern about their safety. To ensure user trust and prevent safety from hindering the development of lithium-ion batteries, the outflow rate of self-discharge-failed lithium-ion batteries needs to be controlled at the ppb level (also known as less than one in a billion). However, based on current market reports of spontaneous combustion incidents in new energy vehicles, the inventors estimate that existing self-discharge screening methods only achieve an outflow rate of self-discharge-failed lithium-ion batteries at the ppm level (also known as less than one in a million). Therefore, innovation in self-discharge screening methods is crucial for improving lithium battery safety and enhancing customer perception of quality.

[0056] The inventors discovered that self-discharge failure can be divided into two main types: chemical self-discharge failure and physical self-discharge failure. Among them, the common causes of chemical self-discharge failure are generally factors that affect the chemical reaction, such as excessive moisture in the electrolyte or electrode. The common causes of physical self-discharge failure are generally physical factors such as excessive magnetic materials and dust short circuits.

[0057] The voltage drop caused by chemical self-discharge can be expressed by the chemical self-discharge formula as V. 化 =A(-Ea / RT)t z In this equation, A is the pre-exponential factor, a coefficient term; Ea is the chemical self-discharge activation energy of the battery under test, which is related to the state of charge and SEI film formation of the battery under test during screening, and its unit is J / mol; R is the ideal gas state constant, which can be taken as 8.314 J / (mol / K); T is the settling temperature, in K; t is the settling time, in days; and z is the exponent of the settling time, which can be a fixed coefficient. After analyzing the causes of chemical self-discharge failure based on the chemical self-discharge formula, the inventors believe that increasing the settling temperature can improve the accuracy of chemical self-discharge screening.

[0058] The voltage drop caused by physical self-discharge can be expressed by the physical self-discharge formula as V. 物=kt, where k is the physical self-discharge rate in mV / d; t is the resting time in days. Batteries that fail due to physical self-discharge later generally exhibit three forms during the initial resting period: obvious, moderate, and inconspicuous. Inconspicuous physical self-discharge has a strong ability to hide, with a low physical self-discharge rate, and generally requires a long resting period of more than one month, or even two to three months, to distinguish it from the whole batch of batteries. Obvious and moderate types do not require such a long time. Figure 1 This is a voltage variation curve of a lithium iron phosphate battery with a 20% state of charge as a function of resting time, referring to... Figure 1 The inventors' experimental data shows that after 30 days of storage, the voltage of two batteries with poor physical self-discharge (Physical Self-Discharge Battery 1# and Physical Self-Discharge Battery 2#) began to show differences from other batteries. It wasn't until a resting period of 60 to 90 days that their voltage became significantly different from other normal batteries. Using conventional long-term resting voltage drop testing techniques, these two batteries would need to be rested for more than 60 days to be screened. If similar batteries with inconspicuous physical self-discharge defects appear in the batches to be screened, long-term resting storage is impractical for large-scale production. Batteries with inconspicuous physical self-discharge defects are very likely to reach customers, causing quality and safety problems.

[0059] To address the aforementioned issues, the inventors conducted further research, studying and disassembling a large number of faulty batteries with poor physical self-discharge based on routine screening data. Figure 2 This is a curve showing the voltage drop variation caused by different types of self-discharge in a battery with poor physical self-discharge, combined with... Figure 1 and Figure 2 The inventor will Figure 1 Taking the #1 physical self-discharge battery as an example, the voltage drop generated by different types of self-discharge was decomposed. Experimental data shows that in batteries with poor physical self-discharge, the total self-discharge voltage drop = intrinsic self-discharge voltage drop + physical self-discharge voltage drop. The intrinsic self-discharge voltage drop is the self-discharge voltage drop that a normal lithium-ion battery would generate during rest; this part is actually the voltage drop generated by chemical self-discharge. The physical self-discharge voltage drop has a linear relationship with the battery's resting time, meaning the physical self-discharge rate is constant.

[0060] Based on this, this invention proposes a self-discharge screening method for lithium-ion batteries, which is applied to the screening of batches of batteries to be tested. Figure 3 This is a schematic flowchart of a self-discharge screening method for lithium-ion batteries provided in an embodiment of the present invention, referring to... Figure 3 The self-discharge screening methods for lithium-ion batteries include:

[0061] S301. Adjust the batteries in the batch to be tested to the preset state of charge.

[0062] Specifically, the batch to be tested contains multiple batteries; for example, a batch may include more than one hundred batteries. All batteries within the batch have the same battery system, similar production time, use the same batch of materials, and have the same capacity. The preset state of charge (SOC) refers to the uniformly adjusted SOC value to which the batteries need to be adjusted before self-discharge screening. The preset SOC value is related to the battery system of the batteries in the batch. For example, if the battery system is lithium iron phosphate (LFP), the preset SOC can be any value between 10% and 20%. The SOC of all batteries in the batch is adjusted to the preset SOC using a charge-discharge method.

[0063] S302. The battery under test is subjected to high-temperature static treatment under a preset charged state.

[0064] Specifically, the settling process in the self-discharge screening method for lithium-ion batteries includes high-temperature settling, a first room-temperature settling, and a second room-temperature settling. High-temperature settling refers to placing the battery under test at a temperature higher than room temperature. For example, the high-temperature settling temperature can be any value between 40-50°C, and the settling time can be any value between 2 and 3 days. Placing the battery under test in a preset state of charge at the same preset high temperature significantly increases the rate of chemical self-discharge compared to room temperature, facilitating subsequent screening.

[0065] S303. Based on the state parameters of the battery under test during high-temperature static placement, perform high-temperature screening on the batch of batteries to be tested.

[0066] Specifically, state parameters refer to parameters that reflect the self-discharge status of the battery under test. For example, state parameters during high-temperature settling may include the first settling time, the first open-circuit voltage of the battery under test before high-temperature settling, and the second open-circuit voltage of the battery under test after high-temperature settling. Based on the state parameters of the battery under test during high-temperature settling, the self-discharge rate of the battery under test during this period can be determined. Then, based on the self-discharge rate of each battery under test during high-temperature settling, all batteries in the test batch can be screened at high temperatures, eliminating those whose self-discharge exceeds the standard during this high-temperature settling. The batteries eliminated in this high-temperature screening step include batteries with poor chemical self-discharge and batteries with obvious physical self-discharge problems.

[0067] For example, based on the voltage difference of the battery under test before and after high-temperature settling and the settling time, the first unit voltage drop generated by the battery under test per unit settling time during high-temperature settling can be calculated. Then, the first unit voltage drop of the battery under test is compared with a first preset voltage drop. Batteries under test with a first unit voltage drop greater than the first preset voltage drop are identified as batteries with abnormal self-discharge and are filtered out.

[0068] S304. The remaining test batteries in the batch after high-temperature screening shall be subjected to a first room temperature standing period.

[0069] Specifically, the first room temperature settling refers to the operation of setting the battery under test at room temperature for settling. For example, the temperature condition for the first room temperature settling can be any value between 20-30°C, and the settling time can be any value between 2 and 4 days. The batteries under test remaining after the high temperature screening in step S303 are placed at the same preset room temperature for settling.

[0070] S305. Based on the state parameters of the battery under test during high-temperature and first-temperature static periods, perform room-temperature screening on the remaining batteries in the batch under test.

[0071] Specifically, state parameters refer to parameters that reflect the self-discharge status of the battery under test. For example, state parameters during high-temperature settling may include a first settling time, a first open-circuit voltage of the battery under test before high-temperature settling, and a second open-circuit voltage of the battery under test after high-temperature settling; state parameters during first room-temperature settling may include a second settling time and a third open-circuit voltage of the battery under test after first room-temperature settling. Based on the state parameters of the battery under test during high-temperature settling and first room-temperature settling, the total self-discharge rate of the battery under test during these processes can be determined. Based on the total self-discharge rate of each battery under test during high-temperature settling and first room-temperature settling, a first room-temperature screening is performed on all remaining batteries in the test batch, filtering out batteries whose total self-discharge exceeds the standard during high-temperature settling and first room-temperature settling. This room-temperature screening step can filter out batteries with general physical self-discharge defects.

[0072] For example, based on the voltage difference between the battery under test before high-temperature settling and after the first room-temperature settling, and the total settling time of the high-temperature settling and the first room-temperature settling, the second unit voltage drop generated by the battery under test per unit time during the high-temperature settling and the first room-temperature settling process can be calculated. Then, the second unit voltage drop of the battery under test is compared with a second preset voltage drop. Batteries under test with a second unit voltage drop greater than the first preset voltage drop are identified as batteries with abnormal self-discharge and are filtered out.

[0073] S306. The remaining test batteries in the batch to be tested after the room temperature screening shall be subjected to a second room temperature standing.

[0074] Specifically, the second room temperature settling refers to setting the battery under test at room temperature for settling, which differs from the first room temperature settling in terms of settling time and sequence. For example, the temperature conditions for the second room temperature settling can be the same as those for the first room temperature settling, taking any value between 20-30°C, and the settling time can be any value between 3 and 5 days. The remaining batteries under test from the first room temperature screening in step S305 are placed at the same preset room temperature for settling.

[0075] S307. Determine the physical self-discharge rate of the battery under test based on the state parameters of the battery under test during high-temperature rest, first room-temperature rest, and second room-temperature rest.

[0076] Specifically, the first two screenings can eliminate test batteries with abnormal chemical self-discharge, obvious physical self-discharge, and general physical self-discharge. However, a few days of resting time is insufficient to screen out test batteries with inconspicuous physical self-discharge through traditional methods such as voltage drop or unit voltage drop. In this step, based on the state parameters of the remaining test batteries in the batch during high-temperature resting, first room-temperature resting, and second room-temperature resting, the normal value of the self-discharge voltage drop that a normal battery would produce during each resting process can be determined, which is the intrinsic voltage drop. For example, the state parameters of the second room-temperature resting include the third resting time and the fourth open-circuit voltage of the test battery after the second room-temperature resting. Based on the deviation between each test battery and the intrinsic voltage drop, the physical self-discharge voltage drop of the test battery during each resting process can be determined. By combining the resting time of each resting process with its physical self-discharge voltage drop, the physical self-discharge rate of the test battery can be determined.

[0077] S308. Based on the physical self-discharge rate of the battery under test, perform physical self-discharge screening on the remaining batteries in the batch under test after room temperature screening.

[0078] Specifically, based on the physical self-discharge rate of the remaining batteries in the test batch, it can be determined whether the physical self-discharge rate of the batteries is normal, and then the remaining batteries in the test batch can be screened for physical self-discharge. For example, determining whether the physical self-discharge rate of the batteries is normal can be done by comparing the physical self-discharge rate with a preset rate, or by using the physical self-discharge rate of the batteries to predict the voltage drop caused by physical self-discharge during the first use of the batteries, and then comparing this voltage drop with a preset voltage drop threshold; no limitation is made here. In this step, the physical self-discharge rate of the batteries can be used to screen out batteries with insignificant physical self-discharge abnormalities.

[0079] The self-discharge screening method for lithium-ion batteries provided in this embodiment involves sequentially subjecting multiple batteries in a batch to high-temperature settling, first room-temperature settling, and second room-temperature settling. After the first two settling periods, the batteries in the batch are screened based on the process parameters obtained from the first and second settling periods, respectively. After the final settling period, the physical self-discharge rate of the remaining batteries is determined based on the process parameters from the three settling periods. Physical self-discharge screening is then performed on the remaining batteries based on their physical self-discharge rates, thus achieving self-discharge screening of lithium-ion batteries. After two settling periods and screening, the physical self-discharge rate of the batteries is determined using state parameters, thereby screening out batteries with insignificant physical self-discharge defects. This improves the accuracy of self-discharge screening within a limited settling time.

[0080] Figure 4 This is a schematic flowchart of another self-discharge screening method for lithium-ion batteries provided in an embodiment of the present invention, referred to... Figure 4 Based on the foregoing embodiments, the self-discharge screening method for lithium-ion batteries includes:

[0081] S401. Adjust the batteries in the batch to be tested to the preset state of charge.

[0082] S402. The battery under test is subjected to high-temperature static treatment under a preset charged state.

[0083] Steps S401 and S402 correspond one-to-one with the aforementioned steps S301 and S302, and will not be repeated here.

[0084] S403. Calculate the first unit voltage drop of the battery under test during high-temperature resting based on the first settling time, the first open-circuit voltage, and the second open-circuit voltage.

[0085] Specifically, the first unit voltage drop refers to the average voltage drop per unit time generated by the battery under test during the high-temperature resting process. By combining the first resting time, the first open-circuit voltage, and the second open-circuit voltage with the first calculation formula, the first unit voltage drop of the battery under test during high-temperature resting can be calculated. The first calculation formula is K1 = (OCV1 - OCV2) / Δt1, where K1 is the first unit voltage drop of the battery under test during high-temperature resting (also known as the first self-discharge rate), OCV1 is the first open-circuit voltage of the battery under test before high-temperature resting, OCV2 is the second open-circuit voltage of the battery under test after high-temperature resting, and Δt1 is the first resting time during high-temperature resting, which is the time difference between collecting the second open-circuit voltage and collecting the first open-circuit voltage.

[0086] S404. Based on the relative relationship between the first unit voltage drop and the first preset voltage drop of the battery under test, high-temperature screening is performed on the batch of batteries under test.

[0087] Specifically, the first preset voltage drop is the voltage drop threshold for high-temperature screening, which is related to the lithium battery material system of the battery under test, the preset state of charge value, and the first settling time at high temperature. For example, the first preset voltage drop can be determined based on the long-term storage of the battery under test and disassembly data of batteries that have failed chemical self-discharge. It is determined whether the first unit voltage drop of each battery under test is greater than or equal to the first preset voltage drop. If the first unit voltage drop of the battery under test is greater than or equal to the first preset voltage drop, it indicates that the voltage drop of the battery under test during high-temperature settling exceeds the normal range, and it can be identified as a battery with poor self-discharge and screened out. If the first unit voltage drop of the battery under test is less than the first preset voltage drop, it indicates that the voltage drop of the battery under test during high-temperature settling is within the normal range, and it can be identified as a normal battery and retained. Using this high-temperature screening method, batteries with abnormal chemical self-discharge and obvious physical self-discharge abnormalities in the batch of batteries under test can be screened out.

[0088] S405. The remaining test batteries in the batch after high-temperature screening shall be subjected to a first room temperature standing period.

[0089] Step S405 is the same as the aforementioned step S305, and will not be repeated here.

[0090] S406. Based on the first settling time, the second settling time, the first open-circuit voltage, and the third open-circuit voltage, calculate the second unit voltage drop of the battery under test during high-temperature settling and the first room-temperature settling.

[0091] Specifically, the second unit voltage drop refers to the average voltage drop per unit time generated by the battery under test during the total process of high-temperature and first room-temperature resting. By combining the first resting time, second resting time, first open-circuit voltage, and third open-circuit voltage with the second calculation formula, the second unit voltage drop of the battery under test during the high-temperature and first room-temperature resting periods can be calculated. The second calculation formula is K2 = (OCV1 - OCV3) / (△t1 + △t2), where K2 is the second unit voltage drop of the battery under test during the high-temperature and first room-temperature resting periods (also known as the second self-discharge rate), OCV1 is the first open-circuit voltage of the battery under test before high-temperature resting, OCV3 is the third open-circuit voltage of the battery under test after the first room-temperature resting, and △t2 is the second resting time during the first room-temperature resting, which is the time difference between the end and beginning of the first room-temperature resting period.

[0092] S407. Based on the relative relationship between the second unit voltage drop and the second preset voltage drop of the battery under test, the remaining batteries under test in the batch under test are screened at room temperature.

[0093] Specifically, the second preset voltage drop is a voltage drop threshold for room temperature screening, which is related to the lithium battery material system of the battery under test, the preset state of charge value, the first settling time at high temperature, and the second settling time at room temperature. For example, the second preset voltage drop can be determined based on long-term storage data of the battery under test and disassembly data of batteries with general physical self-discharge failure. It is determined whether the second unit voltage drop of each remaining battery in the test batch is greater than or equal to the second preset voltage drop. If the second unit voltage drop of the battery under test is greater than or equal to the second preset voltage drop, it indicates that the voltage drop of the battery under test exceeds the normal range during the total process of high-temperature and first room-temperature settling, and it can be identified as a battery with self-discharge failure and screened out. If the second unit voltage drop of the battery under test is less than the second preset voltage drop, it indicates that the voltage drop of the battery under test is within the normal range during the total process of high-temperature and first room-temperature settling, and it can be identified as a normal battery and retained. Using this room-temperature screening method, batteries with general physical self-discharge abnormalities in the test batch can be screened out.

[0094] S408. The remaining test batteries in the batch to be tested after the room temperature screening shall be subjected to a second room temperature standing.

[0095] Step S408 is the same as the aforementioned step S306, and will not be repeated here.

[0096] S409. Based on the open-circuit voltage before and after each settling process, calculate the settling voltage difference generated by the remaining test batteries in the batch under test during the settling process.

[0097] Specifically, the settling process includes a first process, a second process, and a third process. The first process includes high-temperature settling; the second process includes high-temperature settling and a first ambient-temperature settling; and the third process includes high-temperature settling, a first ambient-temperature settling, and a second ambient-temperature settling. The open-circuit voltages before and after the first process are the first open-circuit voltage and the second open-circuit voltage, respectively. The open-circuit voltages before and after the second process are the first open-circuit voltage and the third open-circuit voltage, respectively. The open-circuit voltages before and after the third process are the first open-circuit voltage and the fourth open-circuit voltage, respectively. Based on the difference in open-circuit voltage before and after the settling process, the settling voltage difference generated by the remaining test batteries in the batch under test during the settling process can be calculated. For example, the formulas for calculating the precise voltage difference during the settling process are ΔV1 = OCV1 - OCV2, ΔV2 = OCV1 - OCV3, and ΔV3 = OCV1 - OCV4, where ΔV1 is the settling voltage difference generated in the first process, denoted as the first voltage difference; ΔV2 is the settling voltage difference generated in the second process, denoted as the second voltage difference; and ΔV3 is the settling voltage difference generated in the third process, denoted as the third voltage difference.

[0098] S410. Determine the intrinsic voltage drop of the battery under test during the resting process based on the average level of the resting voltage difference generated by the remaining batteries under test in the batch under test during the resting process.

[0099] Specifically, based on the static pressure difference generated by each remaining battery in the test batch during the resting process, the average level of the static pressure difference during that resting process can be determined. For example, the average level can be expressed as the arithmetic mean or median. If the average static pressure difference of each remaining battery during a resting process is X, then the intrinsic voltage drop of the battery during that resting process is X. The intrinsic voltage drop represents the self-discharge voltage drop generated by a battery with normal self-discharge due to its own chemical properties during the resting process. This portion of the self-discharge voltage drop conforms to the chemical self-discharge formula and belongs to chemical self-discharge. The difference between this intrinsic voltage drop and the voltage drop generated by poor chemical self-discharge is that the greater the chemical self-discharge activation energy of the battery, the greater the intrinsic voltage drop, while there is no such relationship between the voltage drop generated by poor chemical self-discharge and the chemical self-discharge activation energy.

[0100] For example, the average level is represented by the median. The median represents the true self-discharge voltage drop level of a normal battery under test and is unaffected by outliers, making it ideal for representing the intrinsic voltage drop during resting. The resting voltage differences generated by the remaining batteries in the test batch during resting are sorted in order of magnitude. Based on this sorting, the median of the resting voltage differences can be determined as the intrinsic voltage drop of the battery under test during resting.

[0101] S411. Determine the physical self-discharge voltage drop of the battery under test during the resting process based on the difference between the static voltage difference of the remaining batteries in the batch under test and the corresponding intrinsic voltage drop.

[0102] Specifically, the aforementioned high-temperature screening has already eliminated batteries with abnormal chemical self-discharge. Therefore, the remaining test batteries are all those with normal chemical self-discharge. The voltage drop generated by chemical self-discharge during the resting process of these test batteries is their intrinsic voltage drop. The difference between the resting voltage difference of the remaining test batteries in the batch and the corresponding intrinsic voltage drop represents the voltage difference generated by physical self-discharge, which is the physical self-discharge voltage drop of the test batteries during the resting process. In this way, the physical self-discharge voltage drop of the remaining test batteries in the batch during three different resting processes can be determined.

[0103] S412. Determine the physical self-discharge rate of the battery under test based on the physical self-discharge voltage drop and resting time during each resting process.

[0104] Specifically, the settling time refers to the duration of the settling process, which can be determined by the sum of the settling times of each settling step included in the process. For example, the settling time of the first process is equal to the first settling time; the settling time of the second process is equal to the sum of the first and second settling times; and the settling time of the third process is equal to the sum of the first, second, and third settling times. Based on the settling time and physical self-discharge voltage drop corresponding to each settling process, the physical self-discharge voltage drop of the battery under test per unit time during this settling process can be determined. Based on the physical self-discharge voltage drop per unit time corresponding to different settling processes of the battery under test, the final physical self-discharge voltage drop of the battery under test can be determined, which serves as the physical self-discharge rate of the battery under test.

[0105] For example, based on the physical self-discharge voltage drop and resting time of the battery under test during each resting process, a relationship curve between the physical self-discharge voltage drop and resting time can be fitted. This relationship curve can be represented by a third equation, which is y = kx + c, where y is the physical self-discharge voltage drop during the resting process, x is the resting time, k is the slope of the relationship curve, and c is a constant. The physical self-discharge rate of the battery under test is determined based on the slope of the relationship curve.

[0106] S413. Based on the longest manufacturing time and physical self-discharge rate of the battery under test, determine the predicted maximum value of the physical self-discharge voltage drop of the battery under test before its first use.

[0107] Specifically, the maximum manufacturing time refers to the longest storage time that a battery under test can achieve from the time it leaves the production line until it is put into use by the user, according to regulations. This maximum manufacturing time is preset by the manufacturer based on experimental or empirical data to ensure that the self-discharge voltage drop caused during storage does not affect the user's normal use. The product of the maximum manufacturing time of the battery under test and the physical self-discharge rate is the predicted maximum value of the physical self-discharge voltage drop before the first use of the battery under test.

[0108] S414. Based on the relative relationship between the predicted maximum value of the battery under test and the preset self-discharge voltage drop, physical self-discharge screening is performed on the remaining batteries under test in the batch after room temperature screening.

[0109] Specifically, the preset self-discharge voltage drop refers to the screening threshold for the physical self-discharge voltage drop in the physical self-discharge screening process. It can be determined based on the standard value of the voltage difference before the batteries under test are grouped together. The standard value of the voltage difference refers to the maximum allowable voltage difference between the batteries in the group when the batteries under test are grouped together. For example, the standard value of the voltage difference can be 5mV, and the corresponding preset self-discharge voltage drop can be equal to the standard value of the voltage difference of 5mV, or slightly less than the standard value of the voltage difference. For example, the preset self-discharge voltage drop is equal to 4.8mV.

[0110] The self-discharge screening method for lithium-ion batteries provided in this embodiment employs a three-stage screening process. In the high-temperature screening, the principle of temperature's influence on chemical reaction rates is utilized. Batteries exhibiting abnormal chemical self-discharge or significant physical self-discharge abnormalities are eliminated by increasing the settling temperature in conjunction with voltage drop assessment. In the room-temperature screening, batteries exhibiting general physical self-discharge abnormalities are eliminated by using a longer settling time combined with voltage drop assessment. In the physical self-discharge screening, the intrinsic voltage drop of the remaining batteries is determined based on the median self-discharge voltage drop during each settling process. The physical self-discharge voltage drop during each settling process is determined by the difference between the total voltage drop and the corresponding intrinsic voltage drop. A curve showing the relationship between the physical self-discharge voltage drop and the settling time is then fitted. The slope of this curve is defined as the physical self-discharge rate of the battery. By combining the physical self-discharge rate with the longest manufacturing time, the predicted maximum value of the physical self-discharge voltage drop is calculated. Based on the relative relationship between the predicted maximum value and the preset self-discharge voltage drop, the test batteries with abnormal physical self-discharge are screened out, realizing batch screening of lithium-ion batteries. The screening is based on the principles and inherent properties of physical and chemical self-discharge, which has theoretical support and further improves the accuracy of self-discharge screening.

[0111] This invention also provides a self-discharge screening device for lithium-ion batteries. Figure 5 This is a schematic diagram of the structure of a self-discharge screening device for lithium-ion batteries provided in an embodiment of the present invention, with reference to... Figure 5The self-discharge screening device 500 for lithium-ion batteries includes a state of charge adjustment module 501, a high-temperature settling module 502, a high-temperature screening module 503, a first room-temperature settling module 504, a room-temperature screening module 505, a second room-temperature settling module 506, a physical self-discharge determination module 507, and a physical screening module 508. The state of charge adjustment module 501 adjusts the batteries in the test batch to a preset state of charge. The high-temperature settling module 502 performs high-temperature settling on the batteries in the preset state of charge. The high-temperature screening module 503 performs high-temperature screening on the batteries in the test batch based on their state parameters during the high-temperature settling. The first room-temperature settling module 504 performs a first room-temperature settling on the remaining batteries in the test batch after the high-temperature screening. The room-temperature screening module 505 performs room-temperature screening on the remaining batteries in the test batch based on their state parameters during the high-temperature settling and the first room-temperature settling. The second room temperature settling module 506 is used to perform a second room temperature settling on the remaining test batteries in the test batch after room temperature screening. The physical self-discharge determination module 507 is used to determine the physical self-discharge rate of the test batteries based on the state parameters of the test batteries during high temperature settling, the first room temperature settling, and the second room temperature settling. The physical screening module 508 is used to perform physical self-discharge screening on the remaining test batteries in the test batch after room temperature screening based on the physical self-discharge rate of the test batteries.

[0112] The above-described products can perform the methods provided in any embodiment of the present invention, and have the corresponding functional modules and beneficial effects for performing the methods.

[0113] Note that the above description is merely a preferred embodiment of the present invention and the technical principles employed. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and various obvious changes, readjustments, combinations, and substitutions can be made without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments, and may include many other equivalent embodiments without departing from the concept of the present invention, the scope of which is determined by the scope of the appended claims.

Claims

1. A method for screening the self-discharge of lithium-ion batteries, characterized in that, include: Adjust the batteries in the batch to be tested to the preset state of charge; The battery under test in the preset charged state is subjected to high-temperature static treatment; Based on the state parameters of the battery under test during the high-temperature static placement, high-temperature screening is performed on the batteries in the batch to be tested. The remaining batteries in the batch to be tested after the high-temperature screening are subjected to a first room-temperature standing period. Based on the state parameters of the battery under test during the high-temperature resting and the first room-temperature resting, the remaining batteries in the batch under test are screened at room temperature. The remaining batteries in the batch to be tested after the initial room temperature screening are subjected to a second room temperature settling process. Based on the state parameters of the battery under test during the high-temperature settling, the first room-temperature settling, and the second room-temperature settling, the physical self-discharge rate of the battery under test is determined. Specifically, this includes: calculating the settling voltage difference generated by the remaining batteries in the batch under test during the settling process based on the open-circuit voltage before and after each settling process. The settling process includes a first process, a second process, and a third process. The first process includes the high-temperature settling, the second process includes the high-temperature settling and the first room-temperature settling, and the third process includes the high-temperature settling, the second process includes the first room-temperature settling, the third process includes the second room-temperature settling, the third ... third room-temperature settling, the third room-temperature settling, the third room-temperature settling, and the third process include the third room-temperature settling, the third room-temperature settling, and the third room-temperature settling. First, a room temperature resting period; second, a room temperature resting period; based on the average level of the resting voltage difference generated by the remaining test batteries in the test batch during the resting period, determine the intrinsic voltage drop of the test battery during the resting period; based on the difference between the resting voltage difference of the remaining test batteries in the test batch and the corresponding intrinsic voltage drop, determine the physical self-discharge voltage drop of the test battery in each of the resting periods; based on the physical self-discharge voltage drop of the test battery in each of the resting periods and the resting time, determine the physical self-discharge rate of the test battery; Based on the physical self-discharge rate of the battery under test, physical self-discharge screening is performed on the remaining batteries in the batch under test after the room temperature screening.

2. The self-discharge screening method for lithium-ion batteries according to claim 1, characterized in that, The state parameters during the high-temperature settling period include the first settling time, the first open-circuit voltage of the battery under test before the high-temperature settling period, and the second open-circuit voltage of the battery under test after the high-temperature settling period. The step of performing high-temperature screening on the batch of batteries to be tested based on the state parameters of the batteries under test during the high-temperature static placement includes: Based on the first settling time, the first open-circuit voltage, and the second open-circuit voltage, calculate the first unit voltage drop of the battery under test during the high-temperature settling period; Based on the relative relationship between the first unit voltage drop and the first preset voltage drop of the battery under test, high-temperature screening is performed on the batteries in the batch under test.

3. The self-discharge screening method for lithium-ion batteries according to claim 2, characterized in that, The state parameters during the first room temperature resting period include the second resting time and the third open-circuit voltage of the battery under test after the first room temperature resting period; The step of performing room temperature screening on the remaining batteries in the batch to be tested based on the state parameters of the batteries under test during the high-temperature resting and the first room-temperature resting includes: Based on the first settling time, the second settling time, the first open-circuit voltage, and the third open-circuit voltage, calculate the second unit voltage drop of the battery under test during the high-temperature settling and the first room-temperature settling. Based on the relative relationship between the second unit voltage drop and the second preset voltage drop of the battery under test, the remaining batteries in the batch under test are screened at room temperature.

4. The self-discharge screening method for lithium-ion batteries according to claim 1, characterized in that, The step of determining the intrinsic voltage drop of the battery under test during the resting process based on the average level of the resting voltage difference generated by the remaining batteries under test in the batch under test during the resting process includes: The static pressure differences generated by each of the batteries under test during the static placement process are sorted in order of magnitude. Based on the sorting of the static pressure differences, the median of the static pressure differences is determined as the intrinsic voltage drop of the battery under test during the static period.

5. The self-discharge screening method for lithium-ion batteries according to claim 1, characterized in that, The step of determining the physical self-discharge rate of the battery under test based on the physical self-discharge voltage drop and resting time during each of the resting processes includes: Based on the physical self-discharge voltage drop and resting time of the battery under test in each of the resting processes, a curve relating the physical self-discharge voltage drop and the resting time of the battery under test is fitted. The physical self-discharge rate of the battery under test is determined based on the slope of the relationship curve.

6. The self-discharge screening method for lithium-ion batteries according to claim 1, characterized in that, The step of performing physical self-discharge screening on the remaining batteries in the batch after the room temperature screening, based on the physical self-discharge rate of the batteries under test, includes: Based on the longest manufacturing time of the battery under test and the physical self-discharge rate, the predicted maximum value of the physical self-discharge voltage drop of the battery under test before its first use is determined. Based on the relative relationship between the predicted maximum value of the battery under test and the preset self-discharge voltage drop, physical self-discharge screening is performed on the remaining batteries under test in the batch after the room temperature screening.

7. The self-discharge screening method for lithium-ion batteries according to any one of claims 1-6, characterized in that, The high-temperature settling temperature is between 40°C and 50°C; the settling temperatures for the first and second room-temperature settling are both between 20°C and 30°C.

8. The self-discharge screening method for lithium-ion batteries according to any one of claims 1-6, characterized in that, The high-temperature settling time is between 2 and 3 days; the first room-temperature settling time is between 2 and 4 days; and the second room-temperature settling time is between 3 and 5 days.

9. A self-discharge screening device for lithium-ion batteries, characterized in that, include: The state of charge adjustment module is used to adjust the batteries in the batch to be tested to a preset state of charge. A high-temperature settling module is used to perform high-temperature settling on the battery under test; A high-temperature screening module is used to perform high-temperature screening on the batch of batteries to be tested based on the state parameters of the batteries to be tested during the high-temperature static placement. The first room temperature settling module is used to perform a first room temperature settling on the remaining batteries in the batch to be tested after the high temperature screening. A room temperature screening module is used to perform room temperature screening on the remaining batteries in the batch under the preset state of charge based on the state parameters of the batteries under test during the high temperature resting and the first room temperature resting. The second room temperature settling module is used to perform a second room temperature settling on the remaining batteries in the batch to be tested after the room temperature screening. The physical self-discharge determination module is used to determine the physical self-discharge rate of the battery under test based on the state parameters of the battery under test during the high-temperature resting, the first room-temperature resting, and the second room-temperature resting. Specifically, it is used to calculate the resting voltage difference generated by the remaining batteries in the batch under test during the resting process based on the open-circuit voltage before and after each resting process. The resting process includes a first process, a second process, and a third process. The first process includes the high-temperature resting, the second process includes the high-temperature resting and the first room-temperature resting, and the third process includes the high-temperature resting... The test involves three stages: initial static temperature setting, first ambient temperature static temperature setting, and second ambient temperature static temperature setting. Based on the average static voltage difference generated by the remaining test batteries in the batch during the static temperature setting process, the intrinsic voltage drop of the test battery during the static temperature setting process is determined. Based on the difference between the static voltage difference of the remaining test batteries in the batch and the corresponding intrinsic voltage drop, the physical self-discharge voltage drop of the test battery during each static temperature setting process is determined. Based on the physical self-discharge voltage drop of the test battery during each static temperature setting process and the static temperature setting time, the physical self-discharge rate of the test battery is determined. The physical screening module is used to perform physical self-discharge screening on the remaining batteries in the batch after the room temperature screening, based on the physical self-discharge rate of the batteries to be tested.

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