A self-discharge test method for a lithium iron manganese phosphate battery cell

Abstract

The present application relates to a kind of manganese iron lithium phosphate battery self-discharge test method, by two-step method screening: first, after formation and when being filled, respectively record the cell voltage OCV1, OCV2 and corresponding time, calculate the first self-discharge rate K0, and preliminary screening is carried out based on batch average value X0 and standard deviation S0;Subsequently, the qualified cell of preliminary screening is charged to characteristic voltage V, after short time is deposited and high temperature accelerated aging, record voltage OCV3, OCV4 and time, calculate the second self-discharge rate K1, and accurate screening is carried out based on X1 and S1. Wherein, characteristic voltage V is determined by analyzing the differential capacity curve of cell filling process, located in the sensitive interval between two peak voltages V1, V2. The present application realizes the quick, accurate detection of lithium phosphate iron battery self-discharge under the premise of not affecting production efficiency, effectively improves product consistency and reliability.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and specifically to a self-discharge test method for lithium manganese iron phosphate cells. Background Technology

[0002] Lithium-ion batteries are widely used in various aspects of life due to their high energy density, long cycle life, and high operating voltage, such as power banks, electric vehicles, and energy storage cabinets. Self-discharge of lithium batteries refers to the phenomenon where a single cell, in an open-circuit state, experiences a natural reduction in capacity due to internal chemical reactions or physical factors, as the positive and negative electrodes are not connected. Impurities in the positive electrode material, micro-short circuits at the positive and negative electrodes, and electrolyte decomposition can all trigger self-discharge. Most lithium battery devices in daily life are powered by battery packs formed by connecting multiple individual cells in series and parallel. Self-discharge of a single cell leads to capacity loss and performance degradation, further affecting the performance of the battery pack, and in severe cases, even posing safety hazards. Therefore, self-discharge detection of lithium batteries is crucial.

[0003] Lithium iron phosphate (LFP) batteries are widely used due to their high energy density and excellent safety performance. However, because of their material characteristics, their charge-discharge curves exhibit a dual-plateau pattern, which is significantly different from lithium iron phosphate and ternary lithium batteries. Therefore, it is particularly important to identify abnormal cells caused by defects in the formation process or the materials themselves.

[0004] Existing technologies mainly employ three methods: the K-value method, dynamic detection and data modeling, and the DC internal resistance change method. The K-value method involves charging the battery cell to a specific state of charge and then allowing it to rest in an open-circuit environment at a constant temperature. At the start of the resting period, the initial voltage and time are recorded. After a predetermined resting time, the ending voltage and time are recorded. The ratio of the voltage difference to the time difference is the K-value. By comparing the K-value with a preset threshold, the self-discharge status of the battery cell is determined. Obtaining accurate voltage drop requires a resting time of several days to several weeks, severely impacting production efficiency. For cells with flat voltage platforms, such as lithium manganese iron phosphate and lithium iron phosphate, the method is insensitive, and measurement errors are easily caused by insignificant voltage changes. The dynamic detection and data modeling method applies a short-duration pulse discharge or charge to the battery cell, collecting real-time data on voltage, current, and time. From the collected dynamic data, characteristic parameters related to the cell's health status are extracted and input into a pre-established mathematical model or machine learning algorithm to calculate a predicted self-discharge rate or health score. The determination of cell qualification is based on this prediction result. This requires high-precision testing equipment and a large amount of data to build and train a reliable model, which is costly and technically complex. A model built for a certain batch or model of cells may not be applicable to another batch or cells with different aging levels, requiring continuous calibration. The DC internal resistance change method involves measuring the DC internal resistance after the cell has completed its formation and capacity testing. The cell is then left to rest in a specific environment for a relatively short period. After the rest period, another DC internal resistance measurement is performed under the same testing conditions. The rate of change between the two internal resistance measurements is calculated. If the rate of change exceeds a threshold, the cell is considered to have abnormal self-discharge. This method measures the change in internal resistance, not the self-discharge itself. Internal resistance is affected by various factors such as temperature, state of charge (SOC), contact resistance, and the SEI film stabilization process. These interferences can cause normal fluctuations in the internal resistance of qualified cells, leading to misjudgment.

[0005] Given the problems of existing testing technologies being time-consuming, having limited applicability, and having a high false positive rate, it is particularly important to find a detection method that is suitable for lithium manganese iron phosphate systems and has both high detection speed and high accuracy. Summary of the Invention

[0006] The present invention aims to provide a self-discharge testing method for lithium manganese iron phosphate (LFP) battery cells. This method utilizes the natural flow time from cell formation to capacity testing to calculate the initial self-discharge rate K0 for preliminary screening. Subsequently, the initially qualified cells are charged to the characteristic voltage V of the voltage-sensitive region determined based on the differential capacity curve. After a short period of rest and accelerated aging at high temperature, the secondary self-discharge rate K1 is calculated for precise screening. This method can quickly screen out individual cells with poor self-discharge, improving the consistency and performance of subsequent battery cell assemblies.

[0007] The solution of the present invention to the above technical problems is as follows: A self-discharge test method for lithium manganese iron phosphate battery cells, comprising the following steps: 1) After the formation process of the battery cells to be tested, after a period of rest, record the first moment T1 and the corresponding first voltage OCV1 of each battery cell to be tested at the end of the rest period; according to the production process, record the second moment T2 when each battery cell to be tested is transferred to the capacity testing process and the corresponding second voltage OCV2 at that moment; the first voltage OCV1 and the second voltage OCV2 are both read from the corresponding equipment from the formation to the capacity testing process. Based on the first voltage OCV1, the first time T1, the second voltage OCV2, and the second time T2, calculate the first self-discharge rate K0 of each cell in the batch to be tested; Calculate the average value X0 and the standard deviation S0 of the first self-discharge rate of the batch of cells to be tested; based on the first self-discharge rate K0, the average value X0 of the first self-discharge rate, and the standard deviation S0 of the first self-discharge rate, select the cells that meet the set conditions. 2) After the cells selected in step 1) are sized, they are charged at a constant current of 0.2-0.5C to the characteristic voltage V. After being left to stand for a period of time, the third moment T3 at the end of the stand for each cell is recorded and the voltage at this moment is measured and recorded as the third voltage OCV3. After the cells are left to stand under constant temperature conditions, the fourth moment T4 when the cells return to room temperature is recorded and the voltage at this moment is measured and recorded as the fourth voltage OCV4. The third voltage OCV3 and the fourth voltage OCV4 need to be obtained through testing. Based on the third voltage OCV3, the third time T3, the fourth voltage OCV4, and the fourth time T4, determine the second self-discharge rate K1 of each cell in the batch to be tested; Calculate the average value X1 of the second self-discharge rate and the standard deviation S1 of the second self-discharge rate of the batch of cells to be tested; select cells that meet the set conditions based on the second self-discharge rate K1, the average value X1 of the second self-discharge rate and the standard deviation S1 of the second self-discharge rate.

[0008] This invention constructs a two-step self-discharge testing framework. In step 1), the preliminary screening stage is embedded in the production process without taking up extra time; the first voltage OCV1 and the second voltage OCV2 can be directly read on the equipment without additional measurement. In step 2), the precise screening stage achieves rapid detection through optimized conditions, realizing high-precision screening in the shortest time and at the lowest cost. This double-checking process effectively reduces the possibility of misjudgments and omissions that might occur with a single testing method, significantly improving the reliability of the screening results. Using the average value and standard deviation calculated based on the batch data as the screening criteria, rather than a fixed threshold, allows the method to automatically adapt to minor fluctuations in raw materials and production processes across different batches, making it more universally applicable.

[0009] Preferably, in step 2), after the cells selected in step 1) are sized for capacity testing, they are charged at a constant current of 0.2-0.5C to the characteristic voltage V. Using such a relatively small charging current of 0.2C-0.5C minimizes polarization effects. This ensures that when the battery reaches the characteristic voltage V, its voltage is very close to the true, stable equilibrium voltage. Thus, the voltage drop observed during subsequent storage more accurately reflects the battery's self-discharge level, significantly improving testing accuracy while ensuring that charging time is not excessively long, meeting the efficiency requirements of large-scale industrial production.

[0010] Preferably, in step 1), the first self-discharge rate K0 is determined based on the first voltage OCV1, the first time T1, the second voltage OCV2, and the second time T2, as follows: The first self-discharge rate K0 = (OCV1-OCV2) / (T2-T1).

[0011] It directly reflects the rate of voltage drop per unit time and is one of the most intuitive and recognized electrical performance parameters for characterizing self-discharge.

[0012] Preferably, in step 1), cells that meet the set conditions are screened out based on the first self-discharge rate K0, the average value of the first self-discharge rate X0, and the standard deviation of the first self-discharge rate S0, as follows: Cells with a first self-discharge rate K0 satisfying K0≤X0 + nS0 are considered qualified, where n≤3. Within this range, abnormal individuals deviating from the normal fluctuation range of the group are screened out. Compared to a fixed threshold, this reduces misjudgments caused by normal batch fluctuations. The value of n is also crucial; a larger or smaller value increases the probability of misjudgment. A value that is too large will result in missed detections, while a value that is too small will lead to misjudgments.

[0013] Preferably, in step 1), the resting time is 50-70 minutes. After charging and discharging, the voltage of a lithium battery cell is unstable due to polarization, and it needs to be rested for a period of time before the voltage value stabilizes. If the time is too short, the voltage drop will be insignificant, leading to test errors, inaccurate data, and voltage changes that are essentially in the millivolt range, making it susceptible to environmental factors and testing methods. Appropriately increasing the time can eliminate test errors, but it will affect production efficiency.

[0014] Preferably, in step 2), the characteristic voltage V is determined in the following way: Data on the discharge of any cell under test to the cutoff voltage during the capacity grading process is obtained. The differential capacity curve of the cell under test is plotted with differential capacity dQ / dV as the vertical axis and voltage as the horizontal axis. The peak voltages V1 and V2 corresponding to the two peaks in the differential capacity curve are identified respectively, and the characteristic voltage V∈(V1, V2).

[0015] Preferably, in step 2), the characteristic voltage V = (V1 + V2) / 2.

[0016] By setting the test voltage in the rapid change region where voltage is most sensitive to capacitance changes, minute capacitance losses can be amplified into significant voltage drops, allowing for the observation of effective signals in a short time and significantly reducing the required testing time. Furthermore, determining the rapid change range using a differential capacitance curve is more accurate, and the data is directly exported from the capacitance testing process during production (in this invention, it is exported from the Xinwei test cabinet, with data points collected at 30-second intervals; too long an interval will result in missed peaks, and too short an interval will introduce noise), eliminating the need for additional discharge processes and rest periods.

[0017] Preferably, in step 2), the second self-discharge rate of each cell in the batch to be tested is determined based on the third voltage OCV3, the third time T3, the fourth voltage OCV4, and the fourth time T4, as shown below: The second self-discharge rate K1 = (OCV3-OCV4) / (T4-T3).

[0018] Preferably, in step 2), cells that meet the set conditions are selected based on the second self-discharge rate K1, the average value of the second self-discharge rate X1, and the standard deviation of the second self-discharge rate S1, as follows: Cells with a second self-discharge rate K1 that satisfies K1≤X1+mS1 are considered qualified, where m≤3.

[0019] Similar to the process described above for determining the first self-discharge rate X0, ensure the standardization of the second-stage calculations, the comparability of the results, and the accuracy of the screening results.

[0020] Preferably, in step 2), the first rest period is 50-70 minutes. This ensures that the battery cell has sufficient time to relax after being charged to the characteristic voltage V, thereby obtaining a stable open-circuit voltage when recording OCV3, which is a prerequisite for accurately calculating K1.

[0021] Preferably, in step 2), the constant temperature is 43-47℃, and the resting time is 24-50 hours. By appropriately accelerating the self-discharge process inside the cell at a high temperature, without introducing new problems, it is equivalent to extending the room temperature resting time, thus completing accurate screening within an acceptable short time. The selected temperature range and time effectively amplify the differences in self-discharge while avoiding damage to the cell structure that may be caused by excessively high temperatures or excessively long resting times, ensuring the safety of the test and the non-destructive nature of the cell performance.

[0022] The beneficial effects of this invention are as follows: This invention performs self-discharge testing on lithium manganese iron phosphate (LFP) cells through preliminary screening and precise screening. The preliminary screening stage is embedded within the necessary production flow between formation and capacity testing, without occupying additional production time, resulting in high testing efficiency and a short production cycle. In the precise screening stage, by selecting voltage-sensitive areas as test points, the time required for voltage observation is significantly reduced. The overall testing time is shortened from several days using the traditional K-value method to two days, greatly improving production cycle time and inventory turnover.

[0023] Meanwhile, by employing both preliminary and precise screening, the limitations of a single method are effectively avoided, and cross-validation enhances the reliability of the results. Based on the differential capacity curve, the characteristic voltage V of the rapidly changing voltage region is selected, enabling even minor capacity losses to generate significant voltage signals. This amplifies the signal characteristics of abnormal cells and greatly reduces missed detections due to weak signals.

[0024] Furthermore, the method of this invention is highly compatible with existing production processes, eliminating the need for expensive specialized testing equipment. It can be achieved primarily using existing capacity distribution cabinets and temperature control devices. The operation process is simple and clear, making it easy to standardize and scale up on production lines with low implementation costs.

[0025] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below. Specific embodiments of the present invention are given in detail in the following examples. Attached Figure Description

[0026] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings: Figure 1 Example 1 provides a self-discharge test method for lithium iron phosphate battery cells; Figure 2 This is the differential capacity curve of the battery cell under test prepared in Example 1. Detailed Implementation

[0027] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0028] Example 1 like Figure 1 As shown, this embodiment uses 32EA lithium manganese iron phosphate cells of the same batch and specifications for self-discharge testing. The specific steps are as follows: 1) After the formation process of the battery cells to be tested (0.05C 30min, 0.1C 45min, 0.2C 45min, 0.3C 45min), after resting for 1 hour, record the first moment T1 and the corresponding first voltage OCV1 of each battery cell to be tested at the end of the resting period; according to the production process, when each battery cell to be tested is transferred to the capacity testing process, record the second moment T2 when it is put into the cabinet and the corresponding second voltage OCV2. Based on the first voltage OCV1, the first time T1, the second voltage OCV2, and the second time T2, calculate the first self-discharge rate K0 for each cell in the batch to be tested: K0 = (OCV1-OCV2) / (T2-T1); Calculate the average value X0 and the standard deviation S0 of the first self-discharge rate of the batch of cells to be tested; cells in this batch that satisfy K0≤X0 +3S0 are judged as qualified.

[0029] 2) After the cells selected in step 1) are sized, they are not placed in the sizing cabinet. Data is read from any cell under test during the sizing process, showing the discharge to the cutoff voltage. A differential capacity curve of the cell under test is plotted with differential capacity dQ / dV as the ordinate and voltage as the abscissa. For example... Figure 2 As shown, identify the peak voltages V1 and V2 corresponding to the two peaks in the differential capacity curve, and let the characteristic voltage V = (V1 + V2) / 2 = (3.3067 + 3.8788) / 2 = 3.5928.

[0030] After the cells are sized, they are charged to the characteristic voltage V by constant current. After being left to stand for 1 hour, the third time T3 and the corresponding third voltage OCV3 of each cell under test are recorded at the end of the stand. After the cells are left to stand in a constant temperature room at 45℃ for 48 hours, the fourth time T4 and the corresponding fourth voltage OCV4 of the cells when they return to room temperature are recorded. Based on the third voltage OCV3, the third time T3, the fourth voltage OCV4, and the fourth time T4, calculate the second self-discharge rate K1 for each cell in the batch to be tested: K1 = (OCV3-OCV4) / (T4-T3).

[0031] Calculate the average value X1 of the second self-discharge rate and the standard deviation S1 of the second self-discharge rate of the batch of cells to be tested; cells in this batch that satisfy K1≤X1+3S1 are judged to be qualified, and the results are recorded as shown in Table 1.

[0032] Table 1. Test results of this batch of battery cells From the table above, we can calculate that the self-discharge standard for this batch of battery cells is K0≦0.386+3*0.0306=0.478. After step 1, it can be determined that the self-discharge of cells No. 8 and No. 16 does not meet the standard. At the same time, we can further calculate that the self-discharge standard for the screening judgment is K1≦0.0255+3*0.00742=0.0478. After step 2, it can be further determined that the self-discharge of cells No. 8 and No. 16 does not meet the standard.

[0033] It should be noted that the standard deviation mentioned in this article refers to the overall standard deviation calculated over all cell data in the stated batch. The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Those skilled in the art can readily implement the present invention according to the description and above. However, any modifications, alterations, or variations made by those skilled in the art without departing from the scope of the present invention, based on the disclosed technical content, are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, or variations made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the present invention.

Claims

1. A method for testing the self-discharge of a lithium iron phosphate battery cell, characterized in that, Includes the following steps: 1) After the formation process of the battery cells to be tested, after a period of rest, record the first moment T1 and the corresponding first voltage OCV1 of each battery cell to be tested at the end of the rest; according to the production process, when each battery cell to be tested is transferred to the capacity testing process, record the second moment T2 when it is put on the shelf and the corresponding second voltage OCV2. Based on the first voltage OCV1, the first time T1, the second voltage OCV2, and the second time T2, calculate the first self-discharge rate K0 of each cell in the batch to be tested; Calculate the average value X0 and standard deviation S0 of the first self-discharge rate of the batch of cells to be tested; based on the first self-discharge rate K0, the average value X0 of the first self-discharge rate, and the standard deviation S0 of the first self-discharge rate, select the cells that meet the set conditions, as follows: Cells with a first self-discharge rate K0 satisfying K0≤X0 + nS0 are considered qualified, where n≤3; 2) After the cells selected in step 1) are divided into different capacities, they are charged to the characteristic voltage V under constant current. After being left to stand for a period of time, the third time T3 and the corresponding third voltage OCV3 of each cell to be tested at the end of the stand are recorded. After the cells are left to stand under constant temperature conditions, the fourth time T4 and the corresponding fourth voltage OCV4 when the cells return to room temperature are recorded. The characteristic voltage V is determined in the following way: Data on the discharge of any cell under test to the cutoff voltage during the capacity grading process is obtained. The differential capacity curve of the cell under test is plotted with differential capacity dQ / dV as the vertical axis and voltage as the horizontal axis. The peak voltages V1 and V2 corresponding to the two peaks in the differential capacity curve are identified respectively. The characteristic voltage V∈(V1, V2). Based on the third voltage OCV3, the third time T3, the fourth voltage OCV4, and the fourth time T4, determine the second self-discharge rate K1 of each cell in the batch to be tested; Calculate the average value X1 of the second self-discharge rate and the standard deviation S1 of the second self-discharge rate of the batch of cells to be tested; based on the second self-discharge rate K1, the average value X1 of the second self-discharge rate, and the standard deviation S1 of the second self-discharge rate, select the cells that meet the set conditions, as follows: Cells with a second self-discharge rate K1 that satisfies K1≤X1+mS1 are considered qualified, where m≤3.

2. The self-discharge test method for a lithium iron phosphate battery cell as described in claim 1, characterized in that: In step 1), the first self-discharge rate K0 is determined based on the first voltage OCV1, the first time T1, the second voltage OCV2, and the second time T2, as follows: The first self-discharge rate K0 = (OCV1-OCV2) / (T2-T1).

3. The self-discharge test method for lithium iron phosphate cells as described in claim 1, characterized in that, In step 1), the resting time is 50-70 minutes.

4. The self-discharge test method for lithium iron phosphate cells as described in claim 1, characterized in that, In step 2), the characteristic voltage V = (V1 + V2) / 2.

5. The self-discharge test method for lithium manganese iron phosphate cells as described in claim 1, characterized in that, In step 2), the second self-discharge rate of each cell in the batch to be tested is determined based on the third voltage OCV3, the third time T3, the fourth voltage OCV4, and the fourth time T4, as shown below: The second self-discharge rate K1 = (OCV3-OCV4) / (T4-T3).

6. The self-discharge test method for a lithium iron phosphate battery cell as described in claim 1, characterized in that, In step 2), the first resting time is 50-70 minutes.

7. The self-discharge test method for a lithium iron phosphate battery cell as described in claim 1, characterized in that, In step 2), the constant temperature is 43-47℃ and the resting time is 24-50h.

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