A method for detecting self-discharge of lithium batteries

Abstract

This invention provides a method for detecting the self-discharge of lithium batteries, belonging to the field of battery material recycling and testing technology. The method includes: fully charging and allowing the lithium battery to stand after formation and capacity grading; discharging the lithium battery; allowing the lithium battery to stand and collecting voltage data in real time to plot a voltage recovery curve; obtaining the inflection point time of the voltage recovery curve; and evaluating the self-discharge level of the lithium battery based on the inflection point time. This embodiment achieves the evaluation of the self-discharge consistency of lithium batteries by measuring the time it takes for the inflection point of the voltage recovery curve to appear when the lithium battery is under a low charge state. The method has low overall testing requirements, does not require high-temperature environments or long-term storage of lithium batteries, has a short testing cycle, and does not require the batteries to be removed from the testing equipment after capacity grading; instead, the setting and testing are performed directly on the capacity grading equipment. It is simple, fast, saves energy, and significantly reduces testing costs.

Description

Technical Field

[0001] This invention belongs to the field of battery material recycling and testing technology, and particularly relates to a method for detecting the self-discharge of lithium batteries. Background Technology

[0002] Power batteries include lead-acid batteries, nickel-metal hydride batteries, and lithium-ion batteries. Lithium-ion batteries, due to their high energy density, long cycle life, high safety performance, and environmental friendliness, have become the preferred energy storage device for electric vehicles. However, due to current technological and equipment limitations, the capacity of a single lithium-ion battery typically ranges from 2 to 200 Ah, with a single-cell voltage between 3.2 and 3.7 V. Electric vehicles, on the other hand, require voltages ranging from tens to hundreds of volts. Therefore, multiple individual batteries need to be connected in series or parallel to obtain high-voltage, high-energy battery packs. Due to the "weakest link effect," the consistency of individual battery performance becomes a decisive factor affecting the performance of the battery pack.

[0003] Therefore, before lithium-ion batteries are assembled into a pack, individual cells need to be screened for performance consistency. The main screening parameters include internal resistance, voltage, capacity, and self-discharge rate. Among these, internal resistance, voltage, and capacity parameters are easy to obtain through equipment and instruments, but the self-discharge rate of a single lithium battery is difficult to obtain quickly and easily.

[0004] Self-discharge is the loss of battery capacity caused by spontaneous reactions within a single lithium battery when it is in an open-circuit state (i.e., not connected to an external circuit), resulting in a drop in battery voltage. It is expressed as a percentage of capacity loss or voltage drop per year or month.

[0005] Although the self-discharge rate of rechargeable lithium-ion batteries is much lower than that of nickel-cadmium and nickel-metal hydride batteries, it still maintains a certain value and is dependent on temperature and state of charge. Self-discharge is unavoidable in rechargeable lithium-ion batteries of various cathode materials. The magnitude of self-discharge also depends on the type and modification of materials used in the battery, the battery manufacturing process, the ratio of positive and negative electrodes, the control of battery manufacturing process, the composition and purity of the electrolyte, and the storage time of the battery. The self-discharge of various lithium-ion batteries is very low, generally about 1-5% per month.

[0006] Common methods for detecting the self-discharge of lithium-ion batteries include storing the batteries at a certain temperature for a certain period of time and measuring the capacity or voltage drop before and after storage to measure their self-discharge rate. Examples include patents 201110031739.5 and 201110319504.6. The disadvantages of these methods are that the testing cycle is long (several days), increasing production steps; entire batches of batteries need to be moved into a high-temperature room, and then moved out again after 3-5 days, requiring manual re-listing for capacity testing or manual voltage measurement, significantly increasing labor costs. Summary of the Invention

[0007] To address the above problems, this invention provides a method for detecting the self-discharge of a lithium battery, comprising:

[0008] After the lithium battery is formed and discharged under different capacities, it is fully charged and then left to stand.

[0009] Discharge the lithium battery after it has been left to stand.

[0010] After the lithium battery is discharged, it is left to stand, and the voltage of the lithium battery is collected in real time. A voltage recovery curve is plotted based on the change of voltage over time.

[0011] The time of the inflection point in the voltage recovery curve is obtained; wherein, the inflection point is the turning point in the voltage recovery curve where the voltage drops after reaching its maximum value;

[0012] The self-discharge level of the lithium battery is evaluated based on the time of the inflection point of the curve.

[0013] Preferably, the method for fully charging and allowing the lithium battery to stand includes:

[0014] The device is charged to 3.6V using a constant current and voltage of 0.01C-1.00C, with a cutoff current of 0.01-0.50C and a resting time of 0-30 minutes.

[0015] Preferably, a constant current voltage of 0.5C is used;

[0016] Preferably, the cutoff current is 0.02C.

[0017] Preferably, discharging the lithium battery after it has been left to stand includes:

[0018] Take the lithium battery and discharge it step by step from a high rate to a low rate, and allow it to stand after each discharge step.

[0019] Preferably, the decrease in discharge rate during discharge is 5.0C-0.01C;

[0020] The settling time after each discharge step is 5 minutes to 300 minutes.

[0021] Preferably, the decrease in discharge rate during discharge is 0.1C-0.01C;

[0022] Preferably, the settling time is 30-60 minutes.

[0023] Preferably, the step of gradually discharging the lithium battery from a high rate to a low rate, and then allowing it to stand after each discharge, includes:

[0024] Take the lithium battery and perform the following step-by-step discharge process from high rate to low rate:

[0025] Discharge at 1.0C to 2.0V, then let stand for 5 minutes to 300 minutes;

[0026] Discharge to 2.0V at 0.5C and let stand for 5 minutes to 300 minutes;

[0027] Discharge to 2.0V at 0.2C and let stand for 5 minutes to 300 minutes;

[0028] Discharge to 2.0V at 0.1C and let stand for 5 minutes to 300 minutes;

[0029] Discharge to 2.0V at 0.05C and let stand for 5 minutes to 300 minutes;

[0030] Discharge to 2.0V at 0.02C and let stand for 5 minutes to 300 minutes;

[0031] Discharge to 2.0V at 0.01C.

[0032] Preferably, in the step of allowing the discharged lithium battery to stand still, collecting the voltage of the lithium battery in real time, and plotting a voltage recovery curve based on the voltage change over time, the standing time of the lithium battery is 24-48 hours.

[0033] Preferably, the lithium battery is left to stand for 36 hours.

[0034] Preferably, the lithium battery includes lithium iron phosphate battery, lithium manganese oxide battery, lithium cobalt oxide battery, ternary lithium battery, and lithium titanate battery.

[0035] Preferably, evaluating the self-discharge level of the lithium battery based on the time of the inflection point of the curve includes:

[0036] Calculate the average time of the inflection point of the curve for all lithium batteries in the same batch, and use it as the time average;

[0037] The self-discharge level of the lithium battery is evaluated based on the time of the inflection point of the curve corresponding to the lithium battery and the average time of the same batch.

[0038] Preferably, the evaluation of the self-discharge level of the lithium battery includes:

[0039] The time of the inflection point of the curve corresponding to the lithium battery is compared with the difference between the average time and the preset empirical constant for defective products to obtain a comparison result. The self-discharge level of the lithium battery is evaluated based on the comparison result.

[0040] Preferably, the evaluation of the self-discharge level of the lithium battery includes:

[0041] The difference between the time average and the preset empirical constant for nonconforming products is used as the evaluation value; the evaluation value is calculated using the following formula:

[0042] R Δ =KM;

[0043] Where K≥M≥0; R Δ The evaluation value is K; K is the average time of the curve inflection point of all lithium batteries in the same batch; M is a preset empirical constant for non-conforming products.

[0044] Using the time of the inflection point of the curve of the lithium battery as T, compare T with R Δ The comparisons are performed to obtain the comparison results.

[0045] If the comparison result is T > R Δ If so, the lithium battery is determined to be a low self-discharge product;

[0046] If the comparison result is T≤R Δ If so, the lithium battery is determined to be a high self-discharge product;

[0047] If, during the step of allowing the discharged lithium battery to stand still, collecting the voltage of the lithium battery in real time, and plotting a voltage recovery curve based on the voltage change over time, the lithium battery does not exhibit the inflection point of the curve, then the time during which the lithium battery is allowed to stand still after discharge is taken as the value of T corresponding to the lithium battery.

[0048] Preferably, the value of M ranges from 0 to K;

[0049] Preferably, the value of M is any natural number between 5 and 10.

[0050] This invention provides a method for detecting the self-discharge of a lithium battery, comprising: fully charging and allowing the lithium battery to stand after formation and capacity-controlled discharge; discharging the lithium battery after it has stood; allowing the discharged lithium battery to stand, and collecting the voltage of the lithium battery in real time, and plotting a voltage recovery curve based on the voltage change over time; obtaining the time of the inflection point of the voltage recovery curve; wherein the inflection point is the turning point where the voltage drops after reaching its maximum value in the voltage recovery curve; and evaluating the self-discharge level of the lithium battery based on the time of the inflection point. This invention provides a method for detecting the self-discharge of a lithium battery, overcoming the shortcomings of existing technologies that use voltage or capacity differences to screen for self-discharge in lithium-ion batteries. This embodiment evaluates the self-discharge consistency of lithium batteries by measuring the time it takes for the voltage recovery curve of a lithium battery to reach its inflection point under a low charge state. The method has low overall testing requirements, does not require high-temperature environments or long-term storage of lithium batteries, has a short testing cycle, and does not require removing the batteries after capacity grading. Instead, the setting and testing are performed directly on the capacity grading equipment, which is simple and fast. It also eliminates the need for frequent manual operations such as removing, mounting, moving, and unmounting lithium batteries, saving energy and greatly reducing testing costs. Attached Figure Description

[0051] Figure 1 This is a schematic diagram of the overall process in an embodiment of the lithium battery self-discharge detection method of the present invention;

[0052] Figure 2 This is a schematic diagram of the discharge process AG in step S211 of the lithium battery self-discharge detection method of the present invention.

[0053] Figure 3 This is a detailed flowchart of step S521 in an embodiment of the lithium battery self-discharge detection method of the present invention.

[0054] Figure 4 This is a voltage recovery curve of batch A01 lithium batteries after multiple discharges and 24 hours of rest in Example 1 of the lithium battery self-discharge detection method of the present invention.

[0055] Figure 5 This is a partially enlarged view of the voltage recovery curve of batch A01 lithium batteries after multiple discharges and 24 hours of rest in Example 1 of the lithium battery self-discharge detection method of the present invention.

[0056] Figure 6 This is a voltage recovery curve of lithium batteries in batch A02 after multiple discharges and 48 hours of rest in Example 1 of the lithium battery self-discharge detection method of the present invention.

[0057] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0058] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0059] Unless otherwise defined below, all technical and scientific terms used in the specific embodiments of this invention are intended to have the same meaning as commonly understood by those skilled in the art. While it is believed that the following terms will be well understood by those skilled in the art, the following definitions are set forth to better explain the invention.

[0060] As used in this invention, the terms “comprising,” “including,” “having,” “containing,” or “involving” are inclusive or open-ended and do not exclude other unlisted elements or method steps. The term “consisting of” is considered a preferred embodiment of the term “comprising.” If a group is defined below as comprising at least a certain number of embodiments, this should also be understood to disclose a group that preferably consists only of those embodiments.

[0061] When referring to a singular noun, the indefinite or definite article used, such as "a" or "a kind of," "the," includes the plural form of the noun.

[0062] The term "approximately" in this invention refers to an accuracy range that, as would be understood by those skilled in the art, still guarantees the technical effects of the features in question. This term typically indicates a deviation from the indicated value of ±10%, preferably ±5%.

[0063] Furthermore, the terms first, second, third, (a), (b), (c), and similar terms used in the specification and claims are for distinguishing similar elements and are not necessary for the order of description or chronological sequence. It should be understood that such terms are interchangeable in appropriate contexts, and the embodiments described in this invention can be implemented in a different order than that described or illustrated in this invention.

[0064] Unless otherwise defined or clearly indicated by the context, all technical and scientific terms used in this disclosure have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains.

[0065] The technical solution of the present invention will be further described in detail below with reference to specific embodiments, but this does not constitute any limitation on the present invention. Any limited modifications made by any person within the scope of the claims of the present invention are still within the scope of the claims of the present invention.

[0066] refer to Figure 1 This embodiment provides a method for detecting the self-discharge of a lithium battery, including:

[0067] Step S100: After the lithium battery is formed and discharged under capacity, it is fully charged and left to stand.

[0068] The above-mentioned capacity testing refers to determining the battery capacity by charging and discharging the battery and detecting the discharge capacity when fully charged.

[0069] The above-mentioned capacity testing and discharge can be performed using a battery-based capacity testing system, which can be a capacity testing cabinet.

[0070] Step S200: Discharge the lithium battery after it has been left to stand.

[0071] As mentioned above, after the capacity discharge, the discharge is carried out again by letting it stand still. At this time, the discharge process can be batch discharge, staged discharge, step-by-step discharge, linear discharge, etc.

[0072] Step S300: After discharging, the lithium battery is left to stand, and the voltage of the lithium battery is collected in real time. A voltage recovery curve is plotted based on the change of voltage over time.

[0073] As described above, the lithium battery is placed under static conditions after the discharge process is completed. During this static condition, the battery voltage changes are monitored by a battery formation and capacity detection system to obtain the real-time voltage of the lithium battery. Based on the voltage change over time, a voltage recovery curve is plotted.

[0074] Step S400: Obtain the time of the inflection point of the voltage recovery curve; wherein, the inflection point is the turning point where the voltage drops after reaching the maximum value in the voltage recovery curve.

[0075] Step S500: Evaluate the self-discharge level of the lithium battery based on the time of the inflection point of the curve.

[0076] As mentioned above, when lithium batteries recover their voltage, it will first rise and then fall. When the voltage reaches its highest point, there will be an inflection point on the curve. The self-discharge of each battery is different, and the time corresponding to the inflection point of the curve is different.

[0077] The inflection point of the curve, as described above, is the point where the voltage reaches its maximum value in the voltage recovery curve plotted after the voltage is collected. The point at which the voltage drops is the inflection point of the curve, and the corresponding time is the time of the inflection point.

[0078] The self-discharge level of the tested lithium battery can be directly evaluated by measuring the time of the inflection point of the curve.

[0079] This embodiment provides a method for detecting the self-discharge of lithium batteries, overcoming the shortcomings of existing technologies that use voltage or capacity differences to screen for self-discharge in lithium-ion batteries. This embodiment evaluates the consistency of self-discharge by measuring the time it takes for the inflection point of the voltage recovery curve of a lithium battery under a low state of charge. The method has low overall testing requirements, does not require high-temperature environments or long-term storage of lithium batteries, has a short testing cycle, and does not require removing the batteries after capacity grading; instead, the setting and testing are performed directly on the capacity grading equipment. It is simple and fast, and eliminates the need for frequent manual operations such as removing, mounting, moving, and unmounting lithium batteries, saving energy and significantly reducing testing costs.

[0080] Furthermore, in step S100, the method for fully charging and allowing the lithium battery to stand includes:

[0081] Step S110: Charge the battery to 3.6V using a constant current and voltage of 0.01C-1.00C, cut off the current of 0.01-0.50C, and let it stand for 0-30 minutes.

[0082] Preferably, a constant current voltage of 0.5C is used, and the cutoff current is preferably 0.02C.

[0083] Furthermore, step S200, discharging the lithium battery after it has been left to stand, includes:

[0084] Step S210: Take the lithium battery and discharge it step by step from high rate to low rate, and allow it to stand after each discharge step.

[0085] Furthermore, in step S210, the reduction in discharge rate during discharge is 5.0C-0.01C; and the settling time after each discharge step is 5 minutes-300 minutes.

[0086] Furthermore, the discharge rate decreases by 0.1C-0.01C during discharge;

[0087] Furthermore, the resting time is 30-60 minutes.

[0088] Further, step S210 involves gradually discharging the lithium battery from a high rate to a low rate, and then allowing it to stand after each discharge, including:

[0089] Step S211, refer to Figure 2 The lithium battery is then subjected to the following gradual discharge process (AG) from high rate to low rate:

[0090] A. Discharge at 1.0C to 2.0V, and let stand for 5 minutes to 300 minutes;

[0091] B. Discharge to 2.0V at 0.5C and let stand for 5 minutes to 300 minutes;

[0092] C. Discharge at 0.2C to 2.0V, and let stand for 5 minutes to 300 minutes;

[0093] D. Discharge to 2.0V at 0.1C and let stand for 5 minutes to 300 minutes;

[0094] E. Discharge to 2.0V at 0.05C and let stand for 5 minutes to 300 minutes;

[0095] F. Discharge to 2.0V at 0.02C and let stand for 5 minutes to 300 minutes;

[0096] G. Discharge to 2.0V at 0.01C.

[0097] As mentioned above, in step S211, a gradual discharge process from high rate to low rate is performed. When performing the gradual discharge process from high rate to low rate, the above steps (1) to (6) need to be performed sequentially. Each discharge rate needs to meet the corresponding value. The resting time can be within the range of 5-300 minutes. The resting time in each step can be the same or different, as long as multiple batteries in the same batch are under the same conditions and the same detection level, and the same resting time is set.

[0098] Furthermore, in step S300, the discharged lithium battery is left to stand, and the voltage of the lithium battery is collected in real time. A voltage recovery curve is plotted based on the change of voltage over time. The time for the lithium battery to stand is 24 hours to 48 hours.

[0099] In a preferred embodiment, the lithium battery is left to stand for 36 hours.

[0100] As mentioned above, in the initial stage of resting, as the remaining capacity of the lithium battery recovers, the battery voltage V1 rises accordingly. After rising to a certain level, the capacity recovery process is completed, and the corresponding voltage recovery curve tends to flatten out. The increased V1 slowly approaches 0, reaching a plateau period.

[0101] Because lithium batteries reduce capacity due to self-discharge, the battery voltage V2 continuously decreases. After a certain period, an inflection point appears on the battery voltage drop curve. The magnitude of the battery's self-discharge can be determined based on the timing of this inflection point.

[0102] Furthermore, the lithium battery described in this embodiment may include lithium iron phosphate batteries, lithium manganese oxide batteries, lithium cobalt oxide batteries, ternary lithium batteries, and lithium titanate batteries.

[0103] The lithium iron phosphate battery described above can be a lithium iron phosphate battery with a negative electrode composed of carbon-containing materials and a negative electrode composed of lithium titanate materials.

[0104] The above may also include secondary lithium-ion batteries made of other materials.

[0105] Furthermore, in step S500, the self-discharge level of the lithium battery is evaluated based on the time of the inflection point of the curve, including:

[0106] Step S510: Calculate the average time of the curve inflection point for all lithium batteries in the same batch, and use it as the time average.

[0107] As described above, for all lithium batteries in the same batch, a corresponding voltage recovery curve can be obtained using the method provided in this embodiment. The inflection point and corresponding time of the curve can then be determined from the voltage recovery curve. The average time of all curves in the batch is then calculated to obtain the average time.

[0108] Step S520: Evaluate the self-discharge level of the lithium battery based on the time of the inflection point of the curve corresponding to the lithium battery and the average time of the same batch.

[0109] Furthermore, step S520, evaluating the self-discharge level of the lithium battery, includes:

[0110] Step S521: Compare the time of the inflection point of the curve corresponding to the lithium battery with the difference between the average time and the preset empirical constant for defective products to obtain a comparison result, and evaluate the self-discharge level of the lithium battery based on the comparison result.

[0111] The aforementioned preset experience constant for non-conforming products is the average time of the curve inflection point corresponding to all non-conforming lithium batteries in different batches tested previously. This value is the experience constant and can be dynamically adjusted based on the data from different batches.

[0112] Further reference Figure 3 Step S521, evaluating the self-discharge level of the lithium battery, includes:

[0113] Step S5211: The difference between the average time value and the preset empirical constant for non-conforming products is used as the evaluation value; the evaluation value is calculated using the following formula:

[0114] R Δ =KM;

[0115] Where K≥M≥0; R ΔThe evaluation value is K; K is the average time of the inflection point of the curve of all lithium batteries in the same batch; M is a preset empirical constant for non-conforming products.

[0116] Step S5212: Using the time of the inflection point of the curve of the lithium battery as T, compare T with R. Δ The comparisons are performed to obtain the comparison results.

[0117] Step S5213, if the comparison result is T > R Δ If so, the lithium battery is determined to be a low self-discharge product;

[0118] Step S5214, if the comparison result is T≤R Δ If the lithium battery has a high self-discharge level, it is determined to be a high-self-discharge product; therefore, it is speculated that the lithium battery may be a substandard product.

[0119] In step S300, if the lithium battery does not show an inflection point in the curve during the step of setting the discharged lithium battery to stand still and collecting the voltage of the lithium battery in real time, and plotting the voltage recovery curve based on the change of voltage over time, then the time during which the discharged lithium battery is set to stand still is taken as the value of T corresponding to the lithium battery.

[0120] Furthermore, the numerical range of M is 0-K;

[0121] Furthermore, the value of M is any natural number between 5 and 10.

[0122] As described above, if no inflection point appears on the curve during the testing process, i.e. in step S300, the time the lithium battery is left to stand after discharge is taken as the T value of the lithium battery. For example, if the standing time of this batch of lithium batteries is 48 hours, then the T value of the lithium battery is 48h.

[0123] In this embodiment, multiple deep discharges of different rates are performed directly on the capacity testing cabinet for the lithium battery. During the resting phase, the time of the inflection point of the voltage recovery curve is recorded, and the magnitude of self-discharge is determined based on the time of the inflection point.

[0124] Batteries that do not show an inflection point within the observation period have very low self-discharge; batteries that show an early inflection point have high self-discharge. For batteries in the same batch, the inflection point occurrence time range can be reasonably determined based on the average and standard deviation of the inflection point occurrence time and the customer's performance requirements, thus quickly identifying batteries with high self-discharge.

[0125] The present invention will be further illustrated below with specific embodiments. However, it should be understood that these embodiments are merely for the purpose of more detailed illustration and should not be construed as limiting the present invention in any way.

[0126] Example 1:

[0127] Take 20 50Ah lithium iron phosphate cells, designated as batch A01, and proceed with the following steps:

[0128] (1) Charging: Under production conditions, charge to 3.6V using a constant current and constant voltage of 0.5C, cut off current of 0.02C, and let stand for 30 minutes;

[0129] (2) Discharge: The discharge process for this batch of lithium batteries is as follows (AG):

[0130] A. Discharge at 1.0C to 2.0V and let stand for 30 minutes;

[0131] B. Discharge to 2.0V at 0.5C and let stand for 30 minutes;

[0132] C. Discharge to 2.0V at 0.2C and let stand for 30 minutes;

[0133] D. Discharge to 2.0V at 0.1C and let stand for 30 minutes;

[0134] E. Discharge to 2.0V at 0.05C and let stand for 30 minutes;

[0135] F. Discharge to 2.0V at 0.02C and let stand for 30 minutes;

[0136] G. Discharge to 2.0V at 0.01C.

[0137] (3) After the battery has been left to stand for 24 hours, the voltage curve of the battery over time is automatically recorded using the capacity testing cabinet.

[0138] (4) Obtain the time corresponding to the occurrence of the inflection point of the curve for the 20 lithium batteries in this batch. For batteries that do not show an inflection point, the corresponding time is considered to be 24 hours. Calculate their average value K. The data is as follows:

[0139] Table 1. Timeline of voltage recovery curve inflection point for A01 batch lithium batteries after 24 hours of rest.

[0140]

[0141]

[0142] In the table above, the times when the curves of the 50Ah2D19111 and 50Ah2D19133 lithium batteries reach their inflection points are T and T, respectively. 50Ah2D19111 =12 hours and T 50Ah2D19133 =38 minutes. The remaining 18 batteries did not show an inflection point, so their time is considered to be T=24 hours.

[0143] Based on the process system and on-site control level, the pre-set nonconforming product empirical constant M = 5 for this batch A01 is calculated. According to R... Δ =KM, calculate the evaluation value R Δ =22.5 hours - 5 = 17.2.

[0144] The evaluation value R was calculated. Δ Then, the T value of each lithium battery is compared with R. Δ The values ​​were compared, and lithium batteries with a curve inflection point time of no more than 17.2 hours were identified as high self-discharge products. The self-discharge level of these lithium batteries was relatively high, and it was speculated that they might be unqualified products. Specifically, the two lithium batteries, 50Ah2D19111 and 50Ah2D19133, were batteries with relatively high self-discharge.

[0145] To illustrate the issue more clearly, please refer to the appendix. Figure 4 The voltage recovery curves of four batteries (50Ah2D19074, 50Ah2D19111, 50Ah2D19113, and 50Ah2D19133) after being left to rest for 24 hours in step (3) are plotted on the same voltage recovery curve graph. The voltage recovery curve for the first 1.5 hours is partially magnified and shown in the attached graph. Figure 5 .

[0146] Example 2:

[0147] Take 20 100Ah lithium iron phosphate cells, designated as batch A02, and proceed with the following steps:

[0148] (1) Charging: Under production conditions, charge to 3.6V using a constant current and constant voltage of 0.5C, cut off current of 0.02C, and let stand for 30 minutes;

[0149] (2) Discharge: The discharge process for this batch of lithium batteries is as follows (AG):

[0150] A. Discharge at 1.0C to 2.0V and let stand for 60 minutes;

[0151] B. Discharge to 2.0V at 0.5C and let stand for 60 minutes;

[0152] C. Discharge to 2.0V at 0.2C and let stand for 60 minutes;

[0153] D. Discharge to 2.0V at 0.1C and let stand for 60 minutes;

[0154] E. Discharge to 2.0V at 0.05C and let stand for 60 minutes;

[0155] F. Discharge to 2.0V at 0.02C and let stand for 60 minutes;

[0156] G. Discharge to 2.0V at 0.01C;

[0157] (3) After the battery has been left to stand for 48 hours, the voltage curve of the battery over time is automatically recorded using the capacity testing cabinet.

[0158] (4) Obtain the time corresponding to the inflection point of the curve for the 20 lithium batteries in this batch. For batteries that do not show an inflection point, the corresponding time is considered to be 48 hours. Calculate their average value K. The data is as follows.

[0159] Table 2. Timeline of voltage recovery curve inflection point for A02 batch lithium batteries after 48 hours of rest.

[0160]

[0161] In the table above, the time when the curve of the 100Ah2B22051 lithium battery reaches its inflection point is... T100Ah2B22051 =12 hours. The remaining 19 lithium batteries did not show an inflection point, and the time when they showed an inflection point is considered to be T = 48 hours.

[0162] Based on the process system and on-site control level, the pre-set non-conforming empirical constant M = 10 for this batch of batteries A02 is calculated. According to R... Δ =KM, calculate the evaluation value R Δ =46.2 hours - 10 = 36.2.

[0163] The evaluation value R was calculated. Δ Then, the T value of each lithium battery is compared with R. Δ The values ​​were compared, and lithium batteries with a curve inflection point time of no more than 17.2 hours were considered high self-discharge products. This lithium battery has a high self-discharge level and is suspected to be a defective product. That is, lithium battery with part number 100Ah2B22051 is a battery with a large self-discharge.

[0164] To illustrate the issue more clearly, the voltage recovery curves of four batteries (100Ah2B22040, 100Ah2B22044, 100Ah2B22048, and 100Ah2B22051) after being left to rest for 48 hours in step (3) are plotted on the same voltage recovery curve graph. (Refer to the attached graph.) Figure 6 .

[0165] The above describes preferred embodiments and corresponding examples of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the inventive concept of the present invention, including but not limited to adjustments in proportions, processes, and dosages. These modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for detecting the self-discharge of a lithium battery, characterized in that, include: After the lithium battery is formed and discharged under different capacities, it is fully charged and then left to stand. Discharge the lithium battery after it has been left to stand. After the lithium battery is discharged, it is left to stand, and the voltage of the lithium battery is collected in real time. A voltage recovery curve is plotted based on the change of voltage over time. The time of the inflection point in the voltage recovery curve is obtained; wherein, the inflection point is the turning point in the voltage recovery curve where the voltage drops after reaching its maximum value; The self-discharge level of the lithium battery is evaluated based on the time of the inflection point of the curve.

2. The lithium battery self-discharge detection method as described in claim 1, characterized in that, The method for fully charging and then allowing the lithium battery to stand includes: The device is charged to 3.6V using a constant current and voltage of 0.01C-1.00C, with a cutoff current of 0.01-0.50C and a resting time of 0-30 minutes.

3. The lithium battery self-discharge detection method as described in claim 2, characterized in that, A constant current voltage of 0.5C is used.

4. The lithium battery self-discharge detection method as described in claim 2, characterized in that, Cut-off current 0.02C.

5. The lithium battery self-discharge detection method as described in claim 1, characterized in that, The process of discharging the lithium battery after it has been left to stand includes: Take the lithium battery and discharge it step by step from a high rate to a low rate, and allow it to stand after each discharge step.

6. The lithium battery self-discharge detection method as described in claim 5, characterized in that, The discharge rate decreases by 5.0C-0.01C during discharge. The settling time after each discharge step is 5 minutes to 300 minutes.

7. The lithium battery self-discharge detection method as described in claim 6, characterized in that, The discharge rate decreases by 0.1C-0.01C during discharge.

8. The lithium battery self-discharge detection method as described in claim 6, characterized in that, Let it stand for 30-60 minutes.

9. The lithium battery self-discharge detection method as described in claim 5, characterized in that, The step of gradually discharging the lithium battery from a high rate to a low rate, and then allowing it to stand after each discharge, includes: Take the lithium battery and perform the following step-by-step discharge process from high rate to low rate: Discharge at 1.0C to 2.0V, then let stand for 5 minutes to 300 minutes; Discharge to 2.0V at 0.5C and let stand for 5 minutes to 300 minutes; Discharge to 2.0V at 0.2C and let stand for 5 minutes to 300 minutes; Discharge to 2.0V at 0.1C and let stand for 5 minutes to 300 minutes; Discharge to 2.0V at 0.05C and let stand for 5 minutes to 300 minutes; Discharge to 2.0V at 0.02C and let stand for 5 minutes to 300 minutes; Discharge to 2.0V at 0.01C.

10. The lithium battery self-discharge detection method as described in claim 1, characterized in that, The lithium battery is left to stand after discharge, and its voltage is collected in real time. A voltage recovery curve is plotted based on the voltage change over time. The standing time of the lithium battery is 24-48 hours.

11. The lithium battery self-discharge detection method as described in claim 10, characterized in that, The lithium battery was left to stand for 36 hours.

12. The lithium battery self-discharge detection method as described in claim 1, characterized in that, The lithium batteries include lithium iron phosphate batteries, lithium manganese oxide batteries, lithium cobalt oxide batteries, ternary lithium batteries, and lithium titanate batteries.

13. The lithium battery self-discharge detection method as described in claim 1, characterized in that, The evaluation of the self-discharge level of the lithium battery based on the time of the inflection point of the curve includes: Calculate the average time of the inflection point of the curve for all lithium batteries in the same batch, and use it as the time average; The self-discharge level of the lithium battery is evaluated based on the time of the inflection point of the curve corresponding to the lithium battery and the average time of the same batch.

14. The lithium battery self-discharge detection method according to claim 13, characterized in that, The evaluation of the self-discharge level of the lithium battery includes: The time of the inflection point of the curve corresponding to the lithium battery is compared with the difference between the average time and the preset empirical constant for defective products to obtain a comparison result. The self-discharge level of the lithium battery is evaluated based on the comparison result.

15. The lithium battery self-discharge detection method as described in claim 14, characterized in that, The evaluation of the self-discharge level of the lithium battery includes: The difference between the time average and the preset empirical constant for nonconforming products is used as the evaluation value; the evaluation value is calculated using the following formula: ； Where K≥M≥0; The evaluation value is K; K is the average time of the curve inflection point of all lithium batteries in the same batch; M is a preset empirical constant for non-conforming products. Using the time of the inflection point of the curve of the lithium battery as T, compare T with... The comparisons are performed to obtain the comparison results. If the comparison result is T> If so, the lithium battery is determined to be a low self-discharge product; If the comparison result is T≤ If so, the lithium battery is determined to be a high self-discharge product; If, during the step of allowing the discharged lithium battery to stand still, collecting the voltage of the lithium battery in real time, and plotting a voltage recovery curve based on the voltage change over time, the lithium battery does not exhibit the inflection point of the curve, then the time during which the lithium battery is allowed to stand still after discharge is taken as the value of T corresponding to the lithium battery.

16. The lithium battery self-discharge detection method as described in claim 15, characterized in that, The value of M ranges from 0 to K.

17. The lithium battery self-discharge detection method as described in claim 15, characterized in that, The value of M is any natural number between 5 and 10.

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