Self-discharge screening method and apparatus for lithium-ion battery

Abstract

A self-discharge screening method and apparatus for a lithium-ion battery. The method comprises: carrying out high-temperature standing, first room-temperature standing, and second room-temperature standing on batteries under test; on the basis of process parameters of the first standing and process parameters of the foregoing two times standing was performed, performing screening on the batteries under test; and on the basis of process parameters of the three times standing was performed, determining physical self-discharge rates of the remaining batteries under test, and carrying out physical self-discharge screening. By using state parameters to determine the physical self-discharge rates of the batteries under test, batteries affected by non-obvious poor physical self-discharge are screened out.

Description

A method and device for screening self-discharge of lithium-ion batteries

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on July 31, 2023, with application number 202310964643.7. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The embodiments of the present application relate to energy storage battery technology, and in particular to a method and device for screening self-discharge of lithium-ion batteries. Background Art

[0003] Self-discharge is an inherent property of lithium batteries, manifesting as a gradual decrease in capacity over time when left unused. Due to factors such as raw material batches, manufacturing processes, manufacturing environment, and management level, the self-discharge rate of lithium batteries produced on a large scale varies. For client applications, it is crucial to ensure consistent self-discharge rates across integrated battery packs.

[0004] In related technologies, the commonly used method for self-discharge screening 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 of the battery before and after a certain standing time and the standing time, and then screen out batteries with excessive self-discharge based on the self-discharge rate.

[0005] However, the accuracy of self-discharge screening using the K value method is limited by the length of the static time. This results in poor battery screening accuracy in self-discharge screening within a limited time. Technical issues

[0006] The present application provides a self-discharge screening method and device for lithium-ion batteries, so as to improve the accuracy of self-discharge screening on the basis of limited standing time. Technical Solutions

[0007] In a first aspect, an embodiment of the present application provides a method for screening self-discharge of a lithium-ion battery. The method for screening self-discharge of a lithium-ion battery comprises:

[0008] Adjust the batteries of the test batch to the preset state of charge respectively;

[0009] subjecting the battery to be tested in the preset state of charge to high-temperature standing;

[0010] performing high-temperature screening on the batteries of the test batch according to the state parameters of the batteries during the high-temperature static state;

[0011] performing a first normal-temperature resting operation on the batteries remaining in the batch to be tested after the high-temperature screening;

[0012] performing normal-temperature screening on the remaining batteries to be tested in the batch to be tested according to the state parameters of the batteries to be tested during the high-temperature resting state and the first normal-temperature resting state;

[0013] performing a second normal temperature resting operation on the batteries remaining in the batch to be tested after the normal temperature screening;

[0014] determining a physical self-discharge rate of the battery under test according to state parameters of the battery under test during the high-temperature resting state, the first normal-temperature resting state, and the second normal-temperature resting state;

[0015] According to the physical self-discharge rate of the battery to be tested, physical self-discharge screening is performed on the battery to be tested remaining in the batch to be tested after the normal temperature screening.

[0016] In a second aspect, an embodiment of the present application further provides a self-discharge screening device for a lithium-ion battery, the self-discharge screening device for a lithium-ion battery comprising a state of charge adjustment module, a high-temperature static module, a high-temperature screening module, a first normal-temperature static module, a normal-temperature screening module, a second normal-temperature static module, a physical self-discharge determination module, and a physical screening module;

[0017] The state of charge adjustment module is configured to adjust the batteries to be tested in the batch to be tested to a preset state of charge;

[0018] The high-temperature static module is configured to perform high-temperature static treatment on the battery to be tested;

[0019] The high-temperature screening module is configured to perform high-temperature screening on the batteries of the test batch according to the state parameters of the batteries during the high-temperature static state;

[0020] The first normal temperature rest module is configured to perform a first normal temperature rest on the batteries remaining in the batch to be tested after the high temperature screening;

[0021] The normal temperature screening module is configured to perform normal temperature screening on the remaining batteries to be tested in the batch to be tested according to the state parameters of the batteries to be tested during the high temperature resting state and the first normal temperature resting state;

[0022] The second normal temperature rest module is configured to perform a second normal temperature rest on the batteries remaining in the batch to be tested after the normal temperature screening;

[0023] The physical self-discharge determination module is configured to determine the physical self-discharge rate of the battery under test according to state parameters of the battery under test during the high-temperature resting state, the first normal-temperature resting state, and the second normal-temperature resting state;

[0024] The physical screening module is configured to perform physical self-discharge screening on the batteries to be tested remaining in the batch to be tested after the normal temperature screening according to the physical self-discharge rate of the batteries to be tested. Beneficial effects

[0025] Beneficial effects of the present application: The self-discharge screening method and device for lithium-ion batteries provided in the embodiments of the present application sequentially perform high-temperature standing, first normal-temperature standing, and second normal-temperature standing on a plurality of batteries to be tested in a test batch. After the first two standings, the batteries to be tested in the test batch are screened according to the process parameters obtained for the first standing and the first two standings, respectively. After the last standing, the physical self-discharge rate of the remaining batteries to be tested is determined according to the process parameters of the three standing processes, and the remaining batteries to be tested are subjected to physical self-discharge screening based on the physical self-discharge rate, thereby realizing the self-discharge screening of lithium-ion batteries. After two standings and screenings, the physical self-discharge rate of the battery to be tested is determined using the state parameters, and then the poor physical self-discharge batteries with inconspicuous types are screened out, thereby improving the accuracy of self-discharge screening on the basis of a limited standing time. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG1 is a graph showing the voltage variation of a lithium iron phosphate battery at a 20% state of charge as a function of standing time;

[0027] FIG2 is a curve showing the change in voltage drop caused by different types of self-discharge in a battery with poor physical self-discharge;

[0028] FIG3 is a schematic flow chart of a method for screening self-discharge of a lithium-ion battery provided in an embodiment of the present application;

[0029] FIG4 is a schematic flow chart of another method for screening self-discharge of lithium-ion batteries provided in an embodiment of the present application;

[0030] FIG5 is a schematic structural diagram of a self-discharge screening device for lithium-ion batteries provided in an embodiment of the present application. Modes for Carrying Out the Invention

[0031] As described in the background, the K-value method is a commonly used method for self-discharge screening in the industry. Its principle is to calculate the battery's self-discharge rate based on the difference in open-circuit voltage before and after a certain rest period and the rest period. Batteries with excessive self-discharge are then screened out based on this self-discharge rate. However, the accuracy of self-discharge screening using the K-value method is limited by the rest period, resulting in poor accuracy in self-discharge screening within a limited timeframe before shipment. Because lithium-ion batteries are widely used in new energy vehicles and are a part of people's lives, users generally have a high level of concern about their safety. To ensure user trust and ensure that safety does not constrain the development of lithium-ion batteries, the outflow rate of lithium-ion batteries that fail due to self-discharge must be controlled at the ppb level (also known as within one part per billion). However, based on current market reports of spontaneous combustion incidents in new energy vehicles, the inventors have estimated that self-discharge screening methods in related technologies only achieve an outflow rate of lithium-ion batteries that fail due to self-discharge at the ppm level (also known as within one part per million). Therefore, innovations in self-discharge screening methods are crucial for improving lithium battery safety and enhancing customer perception of quality.

[0032] The inventors have found that self-discharge failure is divided into two types: chemical self-discharge failure and physical self-discharge failure. Among them, the common failure causes of chemical self-discharge are generally factors that affect chemical reactions, such as excessive moisture in the electrolyte or electrode, while the common failure causes of physical self-discharge are generally physical reasons such as excessive magnetic material and dust short circuit.

[0033] The voltage drop due to chemical self-discharge can be expressed by the chemical self-discharge formula as V 化 =A(-Ea / RT)t z , where 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 of the battery under test and the formation of the SEI film during screening, and its unit is J / mol; R is the ideal gas constant, which can be 8.314 J / (mol / K); T is the resting temperature, in K; t is the resting time, in days; and z is the exponential power of the resting time, which can be a fixed coefficient. After analyzing the causes of chemical self-discharge failure mentioned above and combining it with the chemical self-discharge formula, the inventors believe that the accuracy of chemical self-discharge screening can be improved by increasing the resting temperature.

[0034] 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 standstill time in days. Batteries that experience physical self-discharge failure later in life typically exhibit three types of failure during initial standstill: obvious, general, and subtle. Subtle self-discharge is highly concealed, with a low physical self-discharge rate. It typically requires a long standstill time of more than one month, or even two to three months, to distinguish it from the rest of the batch. However, obvious and general failures require less time. Figure 1 shows a graph of the voltage variation of a 20% state-of-charge lithium iron phosphate battery as a function of standstill time. The inventors' experimental data show that after 30 days of storage, the voltages of two batteries with poor physical self-discharge (physical self-discharge battery 1# and physical self-discharge battery 2#) begin to show a difference from the other batteries. Only after 60 to 90 days of standstill do their voltages become significantly different from those of normal batteries. When using conventional long-term static testing for voltage drop screening, these two physically self-discharging batteries would need to be static for more than 60 days to be screened out. If similar batteries with subtle physical self-discharge defects appear in the batch to be screened, long-term static storage is unrealistic for large-scale production. Batteries with subtle physical self-discharge defects are likely to flow into customers, causing quality and safety issues.

[0035] To address the aforementioned issues, the inventors conducted further research and, based on routine screening data, studied and disassembled a large number of faulty batteries with poor physical self-discharge. Figure 2 shows a curve showing the voltage drop variation caused by different types of self-discharge in a battery with poor physical self-discharge. Combining Figures 1 and 2, the inventors decomposed the voltage drops caused by different types of self-discharge in the example of physical self-discharge battery 1# in Figure 1. The 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 produce during standstill, and this part is actually the voltage drop caused by chemical self-discharge. The physical self-discharge voltage drop is linearly related to the battery's standstill time, meaning the physical self-discharge rate is constant.

[0036] Based on this, the present application proposes a method for self-discharge screening of lithium-ion batteries. This method is applied to the screening of batches of batteries to be tested. FIG3 is a flow chart of a method for self-discharge screening of lithium-ion batteries provided in the present application. Referring to FIG3 , the method for self-discharge screening of lithium-ion batteries includes:

[0037] S301: Adjust the batteries of the test batch to a preset state of charge.

[0038] Specifically, the number of batteries to be tested in the test batch is multiple. For example, the test batch may include more than one hundred batteries to be tested. The battery system of each battery to be tested in the batch is the same, the production time is similar, the materials used are from the same batch and the capacity is the same. The preset state of charge refers to the state of charge setting value to which the batteries to be tested need to be uniformly adjusted before self-discharge screening. The value of the preset state of charge is related to the battery system of the batteries to be tested in the test batch. For example, if the battery system of the battery to be tested is lithium iron phosphate (also called LFP), then the preset state of charge can take any value in the range of 10% to 20%. The state of charge of the batteries to be tested in the test batch is adjusted to the preset state of charge by charging and discharging.

[0039] S302: subjecting the battery to be tested in a preset state of charge to high temperature static condition.

[0040] Specifically, the standing process in the self-discharge screening method of lithium-ion batteries includes high-temperature standing, a first normal-temperature standing, and a second normal-temperature standing. High-temperature standing refers to the operation of setting the battery to be tested to stand at a temperature higher than normal temperature. For example, the temperature condition of high-temperature standing can be any value between 40-50°C, and the standing time can be any value between 2 and 3 days. The battery to be tested in a preset state of charge is placed at the same preset high temperature for standing operation, so that the rate of chemical self-discharge is significantly increased relative to that at normal temperature, which is convenient for subsequent screening.

[0041] S303 , performing high-temperature screening on the batteries of the test batch according to the state parameters of the batteries during high-temperature static storage.

[0042] Specifically, the state parameters refer to parameters that can reflect the self-discharge of the battery to be tested. For example, the state parameters during high-temperature standing can include the first standing time, the first open-circuit voltage of the battery to be tested before high-temperature standing, and the second open-circuit voltage of the battery to be tested after high-temperature standing. Based on the state parameters of the battery to be tested during high-temperature standing, the self-discharge rate of the battery to be tested during high-temperature standing can be determined. Then, based on the self-discharge rates of multiple batteries to be tested during high-temperature standing, all batteries to be tested in the test batch can be subjected to high-temperature screening to screen out batteries to be tested that have exceeded the self-discharge standard during this high-temperature standing. The batteries to be tested screened out by this high-temperature screening step include batteries with poor chemical self-discharge and batteries with obvious poor physical self-discharge.

[0043] For example, based on the voltage difference between the battery under test before and after high-temperature stasis and the duration of the high-temperature stasis, the first unit voltage drop generated by the battery under test per unit stasis time during the high-temperature stasis can be calculated. The first unit voltage drop of the battery under test is then 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 screened out.

[0044] S304 , performing a first normal-temperature resting operation on the remaining batteries in the batch to be tested after the high-temperature screening.

[0045] Specifically, the first normal temperature resting operation refers to placing the test battery at a normal temperature for rest. For example, the temperature of the first normal temperature resting operation can be anywhere between 20-30°C, and the resting time can be anywhere between 2 and 4 days. The test battery remaining after the high-temperature screening in step S303 is placed at the same preset normal temperature for resting operation.

[0046] S305 , performing normal temperature screening on the remaining batteries to be tested in the batch to be tested according to the state parameters of the batteries to be tested during the high-temperature resting state and the first normal-temperature resting state.

[0047] Specifically, the state parameters refer to parameters that can reflect the self-discharge of the battery under test. For example, the state parameters during high-temperature quiescence may include the first quiescence time, the first open-circuit voltage of the battery under test before high-temperature quiescence, and the second open-circuit voltage of the battery under test after high-temperature quiescence; the state parameters during the first normal-temperature quiescence may include the second quiescence time and the third open-circuit voltage of the battery under test after the first normal-temperature quiescence. Based on the state parameters of the battery under test during high-temperature quiescence and the first normal-temperature quiescence, the total self-discharge rate of the battery under test during the high-temperature quiescence and the first normal-temperature quiescence can be determined. Based on the total self-discharge rate of multiple batteries under test during high-temperature quiescence and the first normal-temperature quiescence, a first normal-temperature screening is performed on all remaining batteries under test in the test batch to screen out batteries whose total self-discharge levels during high-temperature quiescence and the first normal-temperature quiescence exceed the standard. This normal-temperature screening step can screen out batteries under test, including those with general poor physical self-discharge.

[0048] For example, based on the voltage difference between the battery under test before and after the first normal-temperature rest period, as well as the total duration of the high-temperature and first normal-temperature rest periods, the second unit voltage drop generated per unit time by the battery under test during the high-temperature and first normal-temperature rest periods can be calculated. The second unit voltage drop of the battery under test is then 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 exhibiting abnormal self-discharge and are screened out.

[0049] S306 , subjecting the remaining batteries in the batch to be tested after the normal temperature screening to a second normal temperature resting state.

[0050] Specifically, the second normal temperature resting operation refers to placing the test battery at room temperature for rest. This differs from the first normal temperature resting operation in terms of the duration and order of rest. For example, the temperature conditions for the second normal temperature resting operation can be the same as those for the first normal temperature resting operation, ranging from 20-30°C, and the resting time can be anywhere from 3 to 5 days. The test battery remaining from the first normal temperature screening in step S305 is placed at the same preset normal temperature for resting operation.

[0051] S307 , determining a physical self-discharge rate of the battery under test according to state parameters of the battery under test during the high-temperature resting state, the first normal-temperature resting state, and the second normal-temperature resting state.

[0052] Specifically, the first two screenings can screen out batteries with chemical self-discharge anomalies, obvious physical self-discharge anomalies, and general physical self-discharge anomalies. However, a few days of resting time is not enough to screen out batteries with subtle physical self-discharge anomalies through traditional methods such as voltage drop or unit voltage drop. In this step, based on the state parameters of the remaining batteries in the test batch during high-temperature rest, first normal-temperature rest, and second normal-temperature rest, the normal value of the self-discharge voltage drop that a normal battery will generate during each rest process, that is, the intrinsic voltage drop, can be determined. For example, the state parameters of the second normal-temperature rest include the fourth open-circuit voltage of the battery to be tested after the third rest time and the second normal-temperature rest. Based on the deviation between multiple batteries to be tested and the intrinsic voltage drop, the physical self-discharge voltage drop of the battery to be tested during multiple rest processes can be determined. By combining the rest time of multiple rest processes with their physical self-discharge voltage drop, the physical self-discharge rate of the battery to be tested can be determined.

[0053] S308 , performing physical self-discharge screening on the batteries to be tested remaining in the batch to be tested after the normal temperature screening according to the physical self-discharge rate of the batteries to be tested.

[0054] Specifically, based on the physical self-discharge rate of the remaining batteries to be tested in the batch to be tested, it can be determined whether the physical self-discharge rate of the batteries to be tested is normal, and then the physical self-discharge of the remaining batteries to be tested in the batch to be tested can be screened. For example, the way to determine whether the physical self-discharge rate of the batteries to be tested is normal can be, on the one hand, to compare the physical self-discharge rate with a preset rate, and on the other hand, to use the physical self-discharge rate of the batteries to be tested to predict the voltage drop caused by physical self-discharge of the batteries to be tested when they are first used, and then compare the voltage drop with a preset voltage drop threshold, which is not limited here. In this step, the physical self-discharge rate of the batteries to be tested can be used to screen out batteries to be tested with subtle physical self-discharge anomalies.

[0055] The self-discharge screening method for lithium-ion batteries provided in this embodiment sequentially subjects multiple batteries to be tested in a test batch to high-temperature standing, a first normal-temperature standing, and a second normal-temperature standing. After the first two standings, the batteries to be tested in the test batch are screened based on the process parameters obtained for the first standing and the first two standings, respectively. After the last standing, the physical self-discharge rate of the remaining batteries to be tested is determined based on the process parameters of the three standing processes, and the remaining batteries to be tested are subjected to physical self-discharge screening based on the physical self-discharge rate, thereby realizing the self-discharge screening of lithium-ion batteries. After two standings and screenings, the physical self-discharge rate of the battery to be tested is determined using the state parameters, and then the poor physical self-discharge batteries are screened out, thereby improving the accuracy of the self-discharge screening on the basis of a limited standing time.

[0056] FIG4 is a flow chart of another method for screening self-discharge of lithium-ion batteries provided in an embodiment of the present application. Referring to FIG4 , based on the above embodiment, the method for screening self-discharge of lithium-ion batteries includes:

[0057] S401 , adjusting the batteries of the test batch to a preset state of charge.

[0058] S402: placing the battery under test in a preset state of charge at high temperature.

[0059] Steps S401 and S402 have the same contents as the aforementioned steps S301 and S302, respectively, and are not repeated here.

[0060] S403 : Calculate a first unit voltage drop of the battery under test during high-temperature standing according to the first standing time, the first open-circuit voltage, and the second open-circuit voltage.

[0061] Specifically, the first unit voltage drop refers to the average voltage drop per unit time generated by the battery under test during high-temperature rest. The first rest time, the first open-circuit voltage, and the second open-circuit voltage are combined with a first calculation formula to calculate the first unit voltage drop of the battery under test during high-temperature rest. The first calculation formula is K1 = (OCV1-OCV2) / △t1, where K1 is the first unit voltage drop (also known as the first self-discharge rate) of the battery under test during high-temperature rest, OCV1 is the first open-circuit voltage of the battery under test before high-temperature rest, OCV2 is the second open-circuit voltage of the battery under test after high-temperature rest, and △t1 is the first rest time of high-temperature rest, that is, the time difference between collecting the second open-circuit voltage and collecting the first open-circuit voltage.

[0062] S404 , performing high-temperature screening on the batteries of the test batch according to the relative relationship between the first unit voltage drop of the battery to be tested and the first preset voltage drop.

[0063] Specifically, the first preset voltage drop is the voltage drop threshold for high-temperature screening and is related to the lithium battery material system of the battery under test, the preset state of charge value, and the first high-temperature standstill time. For example, the first preset voltage drop can be determined based on the long-term storage history of the battery under test and disassembly data of batteries that have failed due to chemical self-discharge. A determination is made as to whether the first unit voltage drop of the multiple batteries 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 standstill exceeds the normal range, and the battery can be determined to be a poor self-discharge battery 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 standstill is within the normal range, and the battery can be determined to be a normal battery and retained. Using this high-temperature screening method, batteries with abnormal chemical self-discharge and obvious physical self-discharge anomalies can be screened out from the batch of batteries under test.

[0064] S405 , performing a first normal-temperature resting operation on the remaining batteries in the batch to be tested after the high-temperature screening.

[0065] Step S405 is identical to the corresponding step S305 and will not be described again here.

[0066] S406 , calculating a second unit voltage drop of the battery under test during the high-temperature rest period and the first normal-temperature rest period according to the first rest period, the second rest period, the first open-circuit voltage, and the third open-circuit voltage.

[0067] Specifically, the second unit voltage drop refers to the average voltage drop per unit time of the battery under test during the combined process of high-temperature rest and the first normal-temperature rest. The first rest time, the second rest time, the first open-circuit voltage, and the third open-circuit voltage are combined with a second calculation formula to calculate the second unit voltage drop of the battery under test during the high-temperature rest and the first normal-temperature rest. The second calculation formula is K2 = (OCV1 - OCV3) / (Δt1 + Δt2), where K2 is the second unit voltage drop (also known as the second self-discharge rate) of the battery under test during the high-temperature rest and the first normal-temperature rest, OCV1 is the first open-circuit voltage of the battery under test before the high-temperature rest, OCV3 is the third open-circuit voltage of the battery under test after the first normal-temperature rest, and Δt2 is the second rest time of the first normal-temperature rest, that is, the time difference between the end and the start of the first normal-temperature rest.

[0068] S407 , performing normal temperature screening on the remaining batteries to be tested in the batch to be tested according to the relative relationship between the second unit voltage drop of the batteries to be tested and the second preset voltage drop.

[0069] Specifically, the second preset voltage drop is the voltage drop threshold for normal temperature screening and is related to the lithium battery material system of the battery under test, the preset state of charge value, the first high-temperature resting time, and the second normal-temperature resting time. For example, the second preset voltage drop can be determined based on the long-term storage history of the battery under test and disassembly data of batteries that have failed due to general physical self-discharge. A determination is made as to whether the second unit voltage drop of the remaining batteries 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 during the combined high-temperature resting and first normal-temperature resting processes exceeds the normal range, and the battery can be determined to be a poor self-discharge battery 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 during the combined high-temperature resting and first normal-temperature resting processes is within the normal range, and the battery can be determined to be a normal battery and retained. This normal temperature screening method can screen out batteries with general physical self-discharge anomalies from the test batch.

[0070] S408 , performing a second normal temperature resting step on the remaining batteries in the test batch after the normal temperature screening.

[0071] Step S408 is identical to the corresponding step S306 and will not be described again here.

[0072] S409 , calculating the static voltage difference generated by the remaining batteries to be tested in the batch to be tested during the static process based on the open circuit voltages before and after the multiple static processes.

[0073] Specifically, the rest process includes a first process, a second process, and a third process. The first process includes high-temperature rest, the second process includes high-temperature rest and a first normal-temperature rest, and the third process includes high-temperature rest, a first normal-temperature rest, and a second normal-temperature rest. 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 static process, the static pressure difference generated by the remaining batteries to be tested in the batch to be tested during the static process can be calculated. For example, the calculation formulas for the precise pressure difference during the static process are △V1=OCV1-OCV2, △V2=OCV1-OCV3 and △V3=OCV1-OCV4, where △V1 is the static pressure difference generated in the first process, recorded as the first pressure difference; △V2 is the static pressure difference generated in the second process, recorded as the second pressure difference; △V3 is the static pressure difference generated in the third process, recorded as the third pressure difference.

[0074] S410 , determining the intrinsic voltage drop of the battery to be tested during the static process according to the average level of static pressure difference generated by the remaining batteries to be tested in the batch to be tested during the static process.

[0075] Specifically, based on the static pressure difference generated by the remaining multiple test batteries in the test batch during the static process, the average level of the static pressure difference during the static process can be determined. For example, the average level can be expressed as an arithmetic mean or median. During a static process, if the average level of the static pressure difference of the remaining multiple test batteries is X, then the intrinsic voltage drop of the test battery during the static process is X. The intrinsic voltage drop can represent the self-discharge voltage drop generated by the test battery with normal self-discharge during the static process due to its own chemical properties. This self-discharge voltage drop conforms to the chemical self-discharge formula and is caused by chemical self-discharge. The difference between this intrinsic voltage drop and the voltage drop caused by poor chemical self-discharge is that the greater the chemical self-discharge activation energy of the test battery, the greater the intrinsic voltage drop, while there is no such relationship between the voltage drop caused by poor chemical self-discharge and the chemical self-discharge activation energy.

[0076] For example, the average level is represented by the median. The median represents the true self-discharge voltage drop of a normal battery under test while being unaffected by outliers, making it ideal for representing the intrinsic voltage drop during the static state. The static voltage differences generated by the remaining batteries under test in the test batch during the static state are sorted in order of magnitude. Based on the order of these static voltage differences, the median of these static voltage differences can be determined as the intrinsic voltage drop of the battery under test during the static state.

[0077] S411 , determining the physical self-discharge voltage drop of the battery to be tested during the static process according to the difference between the static voltage difference of the remaining batteries to be tested in the batch to be tested and the corresponding intrinsic voltage drop.

[0078] Specifically, since the aforementioned high-temperature screening has already screened out batteries with abnormal chemical self-discharge, it can be determined that the remaining batteries to be tested are all batteries with normal chemical self-discharge. Therefore, the voltage drop caused by chemical self-discharge during the static process of these batteries to be tested is the intrinsic voltage drop. The difference between the static voltage difference of the remaining batteries to be tested in the test batch and the corresponding intrinsic voltage drop can represent the voltage difference caused by physical self-discharge of the test batteries, that is, the physical self-discharge voltage drop of the test batteries during the static process. In this way, the physical self-discharge voltage drop of the remaining batteries to be tested in the test batch can be determined during three different static processes.

[0079] S412 : Determine a physical self-discharge rate of the battery under test according to the physical self-discharge voltage drop and the rest time of the battery under test during multiple rest processes.

[0080] Specifically, the resting time refers to the duration of the resting process, which can be determined based on the sum of the resting times of the multiple resting steps included in the resting process. For example, the resting time of the first process is equal to the first resting time, the resting time of the second process is equal to the sum of the first resting time and the second resting time, and the resting time of the third process is equal to the sum of the first resting time, the second resting time and the third resting time. According to the resting time and the physical self-discharge voltage drop corresponding to the resting process, the physical self-discharge voltage drop of the battery to be tested per unit time in this resting process can be determined. According to the physical self-discharge voltage drop per unit time corresponding to different resting processes of the battery to be tested, the final physical self-discharge voltage drop of the battery to be tested can be determined as the physical self-discharge rate of the battery to be tested.

[0081] For example, a relationship curve between the physical self-discharge voltage drop and the rest time of the battery under test during multiple rest processes can be fitted. The relationship curve can be represented by a third equation: y = kx + c, where y is the physical self-discharge voltage drop during the rest process, x is the rest time during the rest process, 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.

[0082] S413 : Determine a predicted maximum value of the physical self-discharge voltage drop of the battery under test before first use based on the longest factory delivery time and the physical self-discharge rate of the battery under test.

[0083] Specifically, the maximum factory expiration time refers to the maximum storage time that the battery under test can reach from the time it leaves the production line until it is put into use by the user. This maximum factory expiration time is preset by the manufacturer based on experimental data or empirical data to ensure that the self-discharge voltage drop caused during storage does not affect normal user use. The product of the maximum factory expiration time of the battery under test and the physical self-discharge rate is the predicted maximum physical self-discharge voltage drop of the battery under test before its first use.

[0084] S414 , performing physical self-discharge screening on the batteries remaining in the batch to be tested after the normal temperature screening based on the relative relationship between the predicted maximum value of the batteries to be tested and the preset self-discharge voltage drop.

[0085] 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, and can be determined based on the standard voltage difference value before the batteries to be tested are assembled into a battery pack. The standard voltage difference value refers to the maximum allowable voltage difference between batteries in the pack when the batteries to be tested are assembled into a battery pack. For example, the standard voltage difference value can be 5mV, and the corresponding preset self-discharge voltage drop can be equal to the standard voltage difference value of 5mV, or slightly less than the standard voltage difference value. For example, the preset self-discharge voltage drop is equal to 4.8mV.

[0086] The self-discharge screening method for lithium-ion batteries provided in this embodiment is designed with three screening steps. In the high-temperature screening, the principle of the influence of temperature on the chemical reaction rate is utilized to screen out batteries with abnormal chemical self-discharge and obvious physical self-discharge by increasing the static temperature in combination with voltage drop judgment. In the normal-temperature screening, batteries with general physical self-discharge abnormalities are screened out by using a longer static period in combination with voltage drop judgment. In the physical self-discharge screening, the intrinsic voltage drop of the battery to be tested during multiple static processes is determined based on the median of the self-discharge voltage drop of the remaining batteries to be tested during multiple static processes. The physical self-discharge voltage drop of the battery to be tested during multiple static processes is determined based on the difference between the total voltage drop of the battery to be tested during the static process and the corresponding intrinsic voltage drop, and then a relationship curve between the physical self-discharge voltage drop and the static time is fitted. The slope of the relationship curve is determined as the physical self-discharge rate of the battery to be tested. The physical self-discharge rate is combined with the longest factory time to calculate the predicted maximum value of the physical self-discharge voltage drop. Based on the relative relationship between the predicted maximum value and the preset self-discharge voltage drop, the batteries to be tested 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 self-discharge and chemical self-discharge, has theoretical support, and further improves the accuracy of self-discharge screening.

[0087] The present application also provides a self-discharge screening device for lithium-ion batteries. Figure 5 is a schematic structural diagram of a self-discharge screening device for lithium-ion batteries according to an embodiment of the present application. Referring to Figure 5 , the self-discharge screening device 500 for lithium-ion batteries includes a state-of-charge adjustment module 501, a high-temperature resting module 502, a high-temperature screening module 503, a first normal-temperature resting module 504, a normal-temperature screening module 505, a second normal-temperature resting module 506, a physical self-discharge determination module 507, and a physical screening module 508. The state-of-charge adjustment module 501 is configured to adjust each of the test batteries in a test batch to a preset state of charge. The high-temperature resting module 502 is configured to perform high-temperature resting on the test batteries at the preset state of charge. The high-temperature screening module 503 is configured to perform high-temperature screening on the test batteries in the test batch based on the state parameters of the test batteries during the high-temperature resting. The first normal-temperature resting module 504 is configured to perform a first normal-temperature resting on the test batteries remaining in the test batch after the high-temperature screening. The normal temperature screening module 505 is configured to perform normal temperature screening on the remaining batteries in the test batch based on the state parameters of the batteries under test during the high-temperature resting state and the first normal-temperature resting state. The second normal-temperature resting state module 506 is configured to perform the second normal-temperature resting state on the remaining batteries in the test batch after the normal temperature screening. The physical self-discharge determination module 507 is configured to determine the physical self-discharge rate of the batteries under test based on the state parameters of the batteries under test during the high-temperature resting state, the first normal-temperature resting state, and the second normal-temperature resting state. The physical screening module 508 is configured to perform physical self-discharge screening on the remaining batteries in the test batch after the normal temperature screening based on the physical self-discharge rate of the batteries under test.

[0088] The above-mentioned product can execute the method provided in any embodiment of the present application, and has the corresponding functional modules and beneficial effects of the execution method.

Claims

1. A self-discharge screening method for a lithium-ion battery, comprising: Adjusting the batteries of the batch to be tested to the preset state of charge respectively; Performing high-temperature standing of the battery to be tested in the preset state of charge; Performing high-temperature screening on the batteries to be tested in the batch to be tested according to the state parameters of the batteries to be tested during the high-temperature static state; Performing a first normal-temperature standing on the batteries remaining in the batch to be tested after the high-temperature screening; Performing normal temperature screening on the remaining batteries to be tested in the batch to be tested according to the state parameters of the batteries to be tested during the high temperature static state and the first normal temperature static state; Performing a second normal temperature standing for the batteries remaining in the batch to be tested after the normal temperature screening; Determining a physical self-discharge rate of the battery to be tested according to state parameters of the battery to be tested in the high-temperature resting state, the first normal-temperature resting state, and the second normal-temperature resting state; According to the physical self-discharge rate of the battery to be tested, physical self-discharge screening is performed on the battery to be tested remaining in the batch to be tested after the normal temperature screening.

2. The self-discharge screening method for lithium-ion batteries according to claim 1, wherein: The state parameters in the high-temperature resting state include a first resting time, a first open-circuit voltage of the battery to be tested before the high-temperature resting state, and a second open-circuit voltage of the battery to be tested after the high-temperature resting state; The step of performing high temperature screening on the batteries to be tested in the batch to be tested according to the state parameters of the batteries to be tested during the high temperature static state comprises: Calculating a first unit voltage drop of the battery under test during the high temperature standing time according to the first standing time, the first open circuit voltage, and the second open circuit voltage; According to the relative relationship between the first unit voltage drop and the first preset voltage drop of the battery to be tested, high temperature screening is performed on the batteries to be tested in the batch to be tested.

3. The self-discharge screening method of a lithium ion battery according to claim 2, wherein: The state parameters in the first normal temperature static state include a second static state time and a third open circuit voltage of the battery to be tested after the first normal temperature static state; The step of performing normal temperature screening on the remaining batteries to be tested in the batch to be tested according to the state parameters of the batteries to be tested during the high temperature standing and the first normal temperature standing, comprises: The second static time of the battery under test during the high temperature static time and the first normal temperature static time is calculated according to the first static time, the second static time, the first open circuit voltage and the third open circuit voltage. Unit voltage drop; According to the relative relationship between the second unit voltage drop and the second preset voltage drop of the battery to be tested, the remaining batteries to be tested in the batch to be tested are screened at normal temperature.

4. The self-discharge screening method of a lithium ion battery according to claim 1, wherein: The determining the physical self-discharge rate of the battery to be tested according to the state parameters of the battery to be tested in the high-temperature resting, the first normal-temperature resting and the second normal-temperature resting includes: Calculating the static pressure difference generated by the remaining batteries to be tested in the batch to be tested during the static process according to the open circuit voltages before and after the multiple static processes, wherein the static process includes a first process, a second process and a third process, the first process includes the high-temperature static process, the second process includes the high-temperature static process and the first normal-temperature static process, and the third process includes the high-temperature static process, the first normal-temperature static process and the second normal-temperature static process; Determine the intrinsic voltage drop of the battery to be tested during the static process according to the average level of the static pressure difference generated by the remaining batteries to be tested in the batch to be tested during the static process; Determining the physical self-discharge voltage drop of the battery to be tested during the plurality of static processes according to the difference between the static pressure difference of the remaining batteries to be tested in the batch to be tested and the corresponding intrinsic voltage drop; The physical self-discharge rate of the battery to be tested is determined according to the physical self-discharge voltage drop and the standing time of the battery to be tested in the plurality of standing processes.

5. The self-discharge screening method of a lithium ion battery according to claim 4, wherein: The step of determining the intrinsic voltage drop of the battery to be tested during the standing process according to the average level of the static pressure difference generated by the remaining batteries to be tested in the batch to be tested during the standing process comprises: Arrange the static pressure differences generated by the plurality of batteries to be tested during the static process in order of magnitude; According to the ranking of the static pressure differences, the median of the static pressure differences is determined as the intrinsic voltage drop of the battery to be tested during the static process.

6. The self-discharge screening method of a lithium ion battery according to claim 4, wherein: The step of determining the physical self-discharge rate of the battery to be tested according to the physical self-discharge voltage drop and the standing time of the battery to be tested during the plurality of standing processes comprises: Fitting a relationship curve between the physical self-discharge voltage drop and the standing time of the battery to be tested according to the physical self-discharge voltage drop and the standing time of the battery to be tested during the multiple standing processes; The physical self-discharge rate of the battery to be tested is determined according to the slope of the relationship curve.

7. The self-discharge screening method of a lithium ion battery according to claim 1, wherein: The step of performing physical self-discharge screening on the batteries to be tested remaining in the batch to be tested after the normal temperature screening according to the physical self-discharge rate of the batteries to be tested comprises: Determining a predicted maximum value of a physical self-discharge voltage drop of the battery to be tested before the first use according to the longest factory time of the battery to be tested and the physical self-discharge rate; According to the relative relationship between the predicted maximum value and the preset self-discharge voltage drop of the battery to be tested, physical self-discharge screening is performed on the batteries to be tested remaining in the batch to be tested after the normal temperature screening.

8. The self-discharge screening method for lithium-ion batteries according to any one of claims 1 to 7, wherein: The high-temperature standing temperature is between 40°C and 50°C; the first normal-temperature standing temperature and the second normal-temperature standing temperature are both between 20°C and 30°C.

9. The self-discharge screening method for lithium-ion batteries according to any one of claims 1 to 7, wherein: The high-temperature standing time is between 2 days and 3 days; the first normal-temperature standing time is between 2 days and 4 days; the second normal-temperature standing time is between 3 days and 5 days.

10. A self-discharge screening device for a lithium-ion battery, comprising: A state of charge adjustment module, configured to adjust the batteries of the batch to be tested to a preset state of charge; A high temperature static module, configured to perform high temperature static treatment on the battery to be tested; A high temperature screening module, configured to perform high temperature screening on the batteries to be tested in the batch to be tested according to the state parameters of the batteries to be tested during the high temperature static state; A first normal temperature static module, configured to perform a first normal temperature static treatment on the batteries remaining in the batch to be tested after the high temperature screening; A normal temperature screening module, configured to perform normal temperature screening on the remaining batteries to be tested in the batch to be tested at the preset state of charge according to the state parameters of the batteries to be tested in the high temperature static state and the first normal temperature static state; A second normal temperature static module, configured to perform a second normal temperature static treatment on the batteries remaining in the batch to be tested after the normal temperature screening; a physical self-discharge determination module, configured to determine a physical self-discharge rate of the battery to be tested according to state parameters of the battery to be tested in the high-temperature resting state, the first normal-temperature resting state, and the second normal-temperature resting state; The physical screening module is configured to perform physical self-discharge screening on the batteries to be tested remaining in the batch to be tested after the normal temperature screening according to the physical self-discharge rate of the batteries to be tested.

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