Consistency evaluation and improvement method of self-discharge performance of lithium-ion battery packs

Abstract

The method for evaluating and improving the self-discharge performance consistency of a lithium-ion battery pack of the present invention is to conduct a self-discharge performance consistency test on the lithium-ion battery pack after a mechanical test. The difference between the maximum and minimum open-circuit voltages of each parallel block is ⊿V1. If ⊿V1≤V 01 , it means that the self-discharge performance consistency is good. On the contrary, the self-discharge performance is discrete. After replacing the bad parallel block, the mechanical test and self-discharge performance consistency test are carried out again until the requirements are met. After the self-discharge performance consistency meets the requirements, it is put on hold. The difference between the maximum and minimum open-circuit voltages of each parallel block is ⊿V2, and the difference between ⊿V2 and ⊿V1 is ⊿V. If ⊿V2≤V 01 , and ⊿V≤V 02 , it means that the self-discharge performance consistency is good. On the contrary, the self-discharge performance is discrete. After replacing the bad parallel block, re-carry out mechanical tests and self-discharge performance consistency tests until the requirements are met.

Description

Technical Field

[0001] The present invention relates to performance evaluation and improvement of lithium-ion battery packs, and in particular to a method for evaluating and improving the consistency of self-discharge performance of lithium-ion battery packs. Background Art

[0002] Lithium-ion batteries have the advantages of high specific energy, high operating voltage, low self-discharge rate, long cycle life, no memory effect, strong temperature adaptability, and simple use and maintenance. They are currently widely used in the 3C field, energy storage battery field and power battery field. These fields have put forward higher requirements for the consistency of the electrical performance of lithium-ion batteries, especially the self-discharge performance.

[0003] When a lithium-ion battery is left in an open circuit, its stored energy is spontaneously depleted, a phenomenon known as self-discharge. Theoretically, the electrodes of a charged battery are thermodynamically unstable, and physical or chemical reactions spontaneously occur within the battery, resulting in a loss of chemical energy. Self-discharge in lithium-ion batteries is unavoidable, and its presence not only reduces the battery's capacity but also severely impacts its pack consistency and service life.

[0004] Lithium-ion battery packs undergo rigorous screening for consistency between cells during assembly. Individually, lithium-ion battery performance fully meets application requirements. However, when connected in series or parallel to form a battery pack, especially after undergoing mechanical testing as required by the user, inconsistent self-discharge performance can occur within individual cells or parallel packs. Uneven battery self-discharge can lead to overcharging or over-discharging, impacting product performance and lifespan, and more seriously, posing safety risks. Therefore, developing a method to evaluate and improve the consistency of self-discharge performance during the use of lithium-ion battery packs is of great practical significance. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for evaluating and improving the consistency of the self-discharge performance of a lithium-ion battery pack, so as to solve the problem of discrete self-discharge performance of the lithium-ion battery pack during use.

[0006] In order to solve the above technical problems, the present invention provides a method for evaluating and improving the consistency of self-discharge performance of a lithium-ion battery pack, comprising the following steps:

[0007] Step 1: vibrate the lithium-ion battery pack in three mutually perpendicular directions in accordance with a vibration test spectrum, with each direction vibrating for 1 to 5 minutes. The vibration test spectrum magnitude and vibration time are determined according to the lithium-ion battery pack usage environment and user needs.

[0008] Step 2: Charge the lithium-ion battery pack at a constant current I1 to the charging cut-off voltage V0 specified by the manufacturer, and leave it for a time T0;

[0009] Step 3, charging the lithium-ion battery pack with a constant current I2 to the charging cut-off voltage V0 specified by the manufacturer, and leaving it for a time T0;

[0010] Step 4, charging the lithium-ion battery pack with a constant current I3 to the charging cut-off voltage V0 specified by the manufacturer, and leaving it for a time T0;

[0011] Step 5: Charge the lithium-ion battery pack at a constant current I4 to the charging cut-off voltage V0 specified by the manufacturer, leave it for a time T1, measure and record the open circuit voltage V1 of each parallel block, and calculate the difference ΔV1 between the maximum and minimum open circuit voltages of each parallel block;

[0012] Step 6: Interpret the voltage difference ⊿V1. If ⊿V1≤V 01 , it means that the self-discharge performance consistency of the lithium-ion battery pack is good. Otherwise, the self-discharge performance is discrete. In the case of discrete self-discharge performance, the parallel block with poor self-discharge performance needs to be replaced. After the parallel block is replaced, the lithium-ion battery pack is re-tested with a vibration level determined according to user requirements. The self-discharge performance consistency test is then re-performed according to steps 2 to 6 until the requirements are met.

[0013] Step 7: If the result of step 6 indicates that the self-discharge performance is consistent, continue to wait for time T2, then measure and record the open circuit voltage V2 of each parallel block, calculate the difference ⊿V2 between the maximum and minimum open circuit voltages of each parallel block, and calculate the difference ⊿V between ⊿V2 and ⊿V1.

[0014] Step 8: Interpret the voltage difference ⊿V2 and ⊿V. If ⊿V2≤V 01 , and ⊿V≤V 02 , it means that the self-discharge performance consistency of the lithium-ion battery pack is good. Otherwise, the self-discharge performance is discrete. In the case of discrete self-discharge performance, the parallel block with poor self-discharge performance needs to be replaced. After the parallel block is replaced, the lithium-ion battery pack is re-tested for vibration. The vibration level is determined according to user needs, and the self-discharge performance consistency test is re-performed according to steps 2 to 8 until the requirements are met.

[0015] In the above-mentioned method for evaluating and improving the consistency of self-discharge performance of a lithium-ion battery pack, in step 2, I1 is C / 5 to C / 3; V0 is 4.0V to 4.2V; and T0 is 10min to 30min.

[0016] In the above-mentioned method for evaluating and improving the consistency of self-discharge performance of a lithium-ion battery pack, in step 3, I2 is 1 / 5 to 1 / 2 of I1.

[0017] In the above-mentioned method for evaluating and improving the consistency of self-discharge performance of a lithium-ion battery pack, in step 4, I3 is 1 / 5 to 1 / 2 of I2.

[0018] In the above-mentioned method for evaluating and improving the consistency of self-discharge performance of a lithium-ion battery pack, in step 5, I4 is 1 / 10 to 1 / 4 of I3; and T1 is 4 hours to 24 hours.

[0019] The above-mentioned lithium-ion battery pack self-discharge performance consistency evaluation and improvement method, wherein, in said step 6, V 01 It is 10mV~15mV.

[0020] In the above-mentioned method for evaluating and improving the consistency of self-discharge performance of a lithium-ion battery pack, in step 7, T2 is 5 to 14 days.

[0021] The above-mentioned lithium-ion battery pack self-discharge performance consistency evaluation and improvement method, wherein, in said step 8, V 02 It is 2.5mV~5mV.

[0022] Compared with the prior art, the beneficial technical effects of the present invention are:

[0023] The method for evaluating and improving the self-discharge performance consistency of a lithium-ion battery pack of the present invention can expose and detect the problem of discrete self-discharge performance of parallel blocks of a lithium-ion battery pack, locate the parallel blocks with poor self-discharge, and replace the poor parallel blocks, thereby improving the consistency of the self-discharge performance of the lithium-ion battery pack, thereby improving safety and extending service life.

[0024] The method for evaluating and improving the self-discharge performance consistency of a lithium-ion battery pack of the present invention is simple to operate, can quickly and effectively evaluate the consistency of the self-discharge performance of parallel blocks in a lithium-ion battery pack, and improve the discreteness of the self-discharge performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The invention will be further described below with reference to the accompanying drawings:

[0026] Figure 1 This is a comparison curve of the open circuit voltage of the lithium-ion battery pack before the defective parallel blocks are replaced in an embodiment of the present invention.

[0027] Figure 2 This is a comparison curve of the open circuit voltage after the defective parallel blocks of the lithium-ion battery pack are replaced in an embodiment of the present invention.

[0028] Figure 3This is a diagram of the ablation of the electrode and diaphragm after the battery is disassembled in an embodiment of the present invention. DETAILED DESCRIPTION

[0029] The self-discharge performance consistency evaluation and improvement method of the lithium-ion battery pack proposed by the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] The method for evaluating and improving the self-discharge performance consistency of a lithium-ion battery pack according to an embodiment of the present invention includes the following steps:

[0031] Step 1: Fix the lithium-ion battery pack on a vibration table and vibrate the lithium-ion battery pack in three mutually perpendicular directions (X, Y, Z) according to the vibration test spectrum. The vibration time in each direction is 1 to 5 minutes.

[0032] In this embodiment, the vibration test spectrum is shown in Table 1, and the vibration time in each direction is 4 minutes.

[0033] Table 1 Vibration test spectrum

[0034]

[0035] Step 2: Charge the lithium-ion battery pack at a constant current I1 to the charging cut-off voltage V0 specified by the manufacturer, and leave it for a time T0;

[0036] The magnitude of the constant current I1 can be selected based on the capacity of the lithium-ion battery pack. Assuming the capacity of the lithium-ion battery pack is C (unit: Ah), the constant current I1 can be C / 5 to C / 3 (unit: A). The charge cut-off voltage V0 can be 4.0V to 4.2V, and can be selected based on the characteristics of different lithium-ion battery electrode materials to ensure that the lithium-ion battery is close to a fully charged state. The shelf time T0 can be 10 minutes to 30 minutes.

[0037] In this embodiment, the series-parallel structure of the lithium-ion battery pack is 3 in parallel and 9 in series (3 single cells are connected in parallel to form a parallel block, and 9 parallel blocks are connected in series to form a lithium-ion battery pack); the lithium-ion battery pack has a capacity of 90Ah, a constant current I1 of 18A, a charging cut-off voltage when the voltage of any parallel block reaches 4.1V, and a shelf time T0 of 10 minutes.

[0038] Step 3, charging the lithium-ion battery pack with a constant current I2 to the charging cut-off voltage V0 specified by the manufacturer, and leaving it for a time T0;

[0039] The constant current I2 is approximately 1 / 5 to 1 / 2 of I1, which charges the lithium-ion battery to a fully charged state.

[0040] In this embodiment, the constant current I2 is 9 A, the charging cut-off voltage is when the voltage of any parallel block reaches 4.1 V, and the holding time T0 is 10 min.

[0041] Step 4, charging the lithium-ion battery pack with a constant current I3 to the charging cut-off voltage V0 specified by the manufacturer, and leaving it for a time T0;

[0042] The constant current I3 is approximately 1 / 5 to 1 / 2 of I2, which charges the lithium-ion battery to a fully charged state.

[0043] In this embodiment, the constant current I3 is 4.5 A, the charging cut-off voltage is when the voltage of any parallel block reaches 4.1 V, and the holding time T0 is 10 min.

[0044] Step 5: Charge the lithium-ion battery pack at a constant current I4 to the charging cut-off voltage V0 specified by the manufacturer, leave it for a time T1, measure and record the open circuit voltage V1 of each parallel block, and calculate the difference ΔV1 between the maximum and minimum open circuit voltages of each parallel block;

[0045] The constant current I4 is approximately 1 / 10 to 1 / 4 of I3, so that the lithium-ion battery is further charged to a fully charged state; the shelf time T1 is 4 hours to 24 hours, which is specifically selected according to the different characteristics of the lithium-ion battery electrode material.

[0046] In this embodiment, the constant current I4 is 0.9 A, the charging cut-off voltage is when the voltage of any parallel block reaches 4.1 V, the holding time T1 is 24 h, and ΔV1 is 1.9 mV.

[0047] Step 6: Interpret the voltage difference ⊿V1. If ⊿V1≤V 01 , it means that the self-discharge performance consistency of the lithium-ion battery pack is good. Otherwise, the self-discharge performance is discrete. In the case of discrete self-discharge performance, the parallel block with poor self-discharge performance needs to be replaced. After the parallel block is replaced, the lithium-ion battery pack is re-tested with a vibration level determined according to user requirements. The self-discharge performance consistency test is then re-performed according to steps 2 to 6 until the requirements are met.

[0048] V 01 It can be 10mV to 15mV. In this embodiment, V 01 is 10mV, ⊿V1 is 1.9mV, which is less than V 01 (10mV), the self-discharge performance is consistent.

[0049] Step 7: If the result of step 6 indicates that the self-discharge performance is consistent, continue to hold for time T2, measure and record the open circuit voltage V2 of each parallel block, calculate the difference ΔV2 between the maximum and minimum open circuit voltages of each parallel block, and calculate the difference ΔV between ΔV2 and ΔV1;

[0050] The shelf time T2 may be 5 to 14 days to allow the open circuit voltage V2 of each parallel block to reach a stable state, and is specifically selected according to the characteristics of the lithium-ion battery electrode material.

[0051] In this embodiment, the shelf time T2 is 7 days, ΔV2 is 20.2 mV, and ΔV is 18.3 mV.

[0052] Step 8: Interpret the voltage difference ⊿V2 and ⊿V. If ⊿V2≤V 01 , and ⊿V≤V 02 , it means that the self-discharge performance consistency of the lithium-ion battery pack is good. Otherwise, the self-discharge performance is discrete. If there is discrete self-discharge performance, the parallel block with poor self-discharge performance needs to be replaced. After the parallel block is replaced, the vibration test of the lithium-ion battery pack is carried out again. The specific test conditions can be determined according to the use environment of the lithium-ion battery pack and user needs. Then, the self-discharge performance consistency test is carried out again according to steps 2 to 8 until the requirements are met.

[0053] V 02 It can be 2.5mV to 5mV. In this implementation, V 02 is 2.5mV, ⊿V2 is 20.2mV, which is greater than V 01 (10mV), ⊿V is 18.3mV, which is greater than V 02 (2.5mV), so it does not meet the self-discharge consistency requirements of lithium-ion battery packs.

[0054] Table 2 shows the open circuit voltage data of the lithium-ion battery pack before the defective parallel blocks are replaced in this embodiment. Figure 1 The open circuit voltage comparison curve of the lithium-ion battery pack before the defective parallel block is replaced in this embodiment is shown in the figure. As can be seen from the figure, the open circuit voltage of the 8th parallel block is significantly lower than that of the other parallel blocks, indicating that the 8th parallel block is a defective parallel block. Therefore, the three single cells of the 8th parallel block were disassembled, and it was found that one of the single cells had obvious ablation points inside. The ablation points on the electrode and the diaphragm corresponded to the ablation points. Figure 3 shown.

[0055] The eighth parallel block was replaced. After the parallel block replacement was completed, the vibration test of the lithium-ion battery pack was re-performed according to Table 1, and the self-discharge performance consistency test was re-performed according to Steps 2 to 8. Table 3 shows the open circuit voltage data of the lithium-ion battery pack after the defective parallel block was replaced in this embodiment. Figure 2 The open circuit voltage comparison curve of the lithium-ion battery pack after the defective parallel block is replaced in this embodiment is shown in the graph. It can be seen from the graph that ⊿V1 is 2.2mV and ⊿V2 is 3.1mV, both of which are less than V 01 (10mV), ⊿V is 0.9mV, which is less than V 02(2.5mV), so after replacing the defective parallel blocks, the self-discharge consistency of the lithium-ion battery pack meets the requirements.

[0056] Table 2 Open circuit voltage data of lithium-ion battery pack before replacement of defective parallel blocks

[0057]

[0058] Table 3 Open circuit voltage data of lithium-ion battery pack after defective parallel blocks are replaced

[0059]

[0060]

[0061] Obviously, those skilled in the art may make various modifications and variations to the present invention without departing from the spirit and scope of the present invention. Thus, if such modifications and variations fall within the scope of the claims and their equivalents, the present invention is intended to include such modifications and variations.

Claims

1. A method for evaluating and improving the self-discharge performance consistency of a lithium-ion battery pack, characterized in that: The following steps are involved: Step 1: vibrate the lithium-ion battery pack in three mutually perpendicular directions in accordance with a vibration test spectrum, with each direction vibrating for 1 to 5 minutes. The vibration test spectrum magnitude and vibration time are determined according to the lithium-ion battery pack usage environment and user needs. Step 2: Charge the lithium-ion battery pack at a constant current I1 to the charging cut-off voltage V0 specified by the manufacturer, and leave it for a time T0; Assume that the capacity of the lithium-ion battery pack is C, I1 is C / 5 to C / 3; V0 is 4.0V to 4.2V; T0 is 10min to 30min; Step 3, charging the lithium-ion battery pack with a constant current I2 to the charging cut-off voltage V0 specified by the manufacturer, and leaving it for a time T0; I2 is 1 / 5 to 1 / 2 of I1; Step 4, charging the lithium-ion battery pack at a constant current I3 to the charging cut-off voltage V0 specified by the manufacturer, and leaving it for a time T0; I3 is 1 / 5 to 1 / 2 of I2; Step 5: Charge the lithium-ion battery pack at a constant current I4 to the charging cut-off voltage V0 specified by the manufacturer, leave it for a time T1, measure and record the open circuit voltage V1 of each parallel block, and calculate the difference ΔV1 between the maximum and minimum open circuit voltages of each parallel block; I4 is 1 / 10 to 1 / 4 of I3; T1 is 4h to 24h; Step 6: Interpret the voltage difference ⊿V1. If ⊿V1≤V 01 , it means that the self-discharge performance consistency of the lithium-ion battery pack is good. Otherwise, the self-discharge performance is discrete. In the case of discrete self-discharge performance, the parallel block with poor self-discharge performance needs to be replaced. After the parallel block is replaced, the lithium-ion battery pack is re-tested with a vibration level determined according to user requirements. The self-discharge performance consistency test is then re-performed according to steps 2 to 6 until the requirements are met. V 01 10mV~15mV; Step 7: If the result of step 6 indicates that the self-discharge performance is consistent, continue to wait for time T2, measure and record the open circuit voltage V2 of each parallel block, calculate the difference ⊿V2 between the maximum and minimum open circuit voltages of each parallel block, and calculate the difference ⊿V between ⊿V2 and ⊿V1; T2 is 5 to 14 days; Step 8: Interpret the voltage difference ⊿V2 and ⊿V. If ⊿V2≤V 01 , and ⊿V≤V 02 , it means that the self-discharge performance consistency of the lithium-ion battery pack is good. Otherwise, the self-discharge performance is discrete. In the case of discrete self-discharge performance, the parallel block with poor self-discharge performance needs to be replaced. After the parallel block is replaced, the lithium-ion battery pack is re-tested for vibration. The vibration level is determined according to user needs, and the self-discharge performance consistency test is re-performed according to steps 2 to 8 until the requirements are met. V 02 is 2.5 mV to 5 mV.

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