A new sorting method for the cascade utilization of power batteries

Abstract

The present invention is a new sorting method for the cascade utilization of power batteries. Traditional sorting methods use parameter differences as sorting indicators, and it is difficult to directly evaluate the sorting effect. The present invention is based on the parallel shunt characteristics of the module. By analyzing the dynamic change process of each branch current in the parallel case, the relationship between the capacity utilization efficiency and the terminal load current is directly established. The maximum capacity utilization efficiency of the module and the corresponding working state can be observed, and then the sorting effect can be evaluated. This sorting method can select cells with large parameter differences for grouping, with higher resolution accuracy, providing different ideas for flexible grouping, while providing a reference for evaluating the sorting effect through the Pack capacity utilization efficiency.

Description

Technical Field

[0001] The present invention belongs to the field of battery sorting, and specifically relates to a novel sorting method for cascade utilization of power batteries. Background Art

[0002] While a large number of electric vehicles are being scrapped, retired lithium-ion batteries still retain 70-80% of their rated capacity and can also be used in energy storage methods, power tools, mobile power supplies, low-speed electric vehicles, uninterruptible power supplies and other related fields that have low requirements for battery performance. However, at present, cascade utilization is still in its early stages, mainly due to the inconsistency between retired batteries. Battery sorting is a key link in cascade utilization. Traditional methods are mainly based on differences in battery cell parameters. It is difficult to directly evaluate the sorting effect, and it is impossible to take into account both timeliness and economy, and further improvement is still needed. Summary of the invention

[0003] The purpose of the present invention is to provide a novel sorting method which has a simple and effective model and exhibits high economy and accuracy.

[0004] The present invention proposes a new sorting method for the cascade utilization of power batteries. Different from the traditional sorting, it is based on the first principle and uses the module capacity utilization efficiency as the sorting index. The method analyzes the dynamic change process of each branch current in parallel. The sorting process directly establishes the relationship between the capacity utilization efficiency and the terminal load current to determine whether the highest capacity utilization efficiency is greater than 99% to achieve rapid sorting. The specific steps are as follows:

[0005] (1) Analysis of the dynamic change process of each branch current in parallel

[0006] Based on Kirchhoff's law, the current distribution of each branch in parallel is obtained:

[0007]

[0008] Where: is the branch current of the i-th cell, I L is the total branch current, R i is the DC resistance of the ith cell, U OCVi is the open circuit voltage of the ith cell, k OCVi is the open circuit voltage U of the ith cell OCVi With SOC i The slope between two variables, SOC i The SOC state of the i-th battery cell, U i0 is the initial voltage of the ith cell, C i is the rated capacity of the ith battery cell;

[0009] By solving formula (1), the branch current dynamic equation is obtained, which is as follows:

[0010]

[0011] Where: I1(t) and I2(t) are the branch currents of the first and second cells, C1 and C2 are the rated capacities of the first and second cells, k OCV1 and k OCV2 The open circuit voltage U of the first and second cells are OCVi With SOC i The slope between two variables, I L is the total branch current, U 10 and U 20 are the initial voltages of the first and second cells, respectively, and R1 and R2 are the DC resistances of the first and second cells, respectively;

[0012] Using the limit calculation branch current dynamic equation, assuming that U 10 =U 20 ,k OCV1 =k OCV2 , and the current distribution at the starting point and steady-state point is obtained:

[0013]

[0014]

[0015] Where: I i (0) is the starting point current of the ith cell branch, I i (∞) is the steady-state point current of the ith cell branch;

[0016] (2) Sorting process

[0017] Establish the relationship between capacity utilization efficiency and terminal load current, determine whether the maximum capacity utilization efficiency meets the standard, and achieve rapid sorting:

[0018] The current distribution at the starting point of each branch can be controlled by setting the load current, and the dynamic change process always follows I1·R1+U 10 =I2·R2+U 20 In the steady state, the current of each branch tends to be distributed in a capacity ratio relationship; therefore, under the given dynamic change trend and steady state, different module capacity utilization efficiencies η can be achieved by adjusting the starting point current and the terminal load current, which is defined as:

[0019]

[0020] in: Refers to the SOC state of the ith cell when any cell in the module is discharged to the cut-off voltage, C iRefers to the rated capacity of the ith battery cell, C total Refers to the sum of the rated capacities of the modules;

[0021] Ideally, the highest capacity utilization efficiency of the module is that the ratio of the current of each branch is always consistent with the ratio of the capacity, which can ensure that each branch reaches the cut-off voltage at the same time, reduce the current balancing process and loss, and achieve the highest pack capacity utilization efficiency; the starting point current is distributed according to the capacity ratio, then the load working current under the condition of I1(0) / I2(0)=C1 / C2 is

[0022]

[0023] According to the highest module capacity utilization efficiency corresponding to the working current, it is judged whether it is greater than 99%. If it is, the grouping conditions are met, sorting is achieved, and the sorting effect can be evaluated by the capacity utilization efficiency.

[0024] The present invention uses the capacity utilization efficiency η of the module after the cells are connected in parallel to determine whether it is suitable for grouping and evaluate the sorting effect.

[0025] The present invention regulates the dynamic process of each branch current after the cells are connected in parallel into groups through load current, establishes the relationship between capacity utilization efficiency and terminal load current, judges whether the highest capacity utilization efficiency meets the standard, and realizes rapid sorting.

[0026] The beneficial effect of the present invention is that the sorting method can select cells with large parameter differences for grouping, with higher resolution accuracy, providing different ideas for flexible grouping and providing a reference for evaluating the sorting effect through the Pack capacity utilization efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram comparing the traditional and new sorting methods.

[0028] Figure 2 The results of module capacity utilization efficiency under different working conditions are compared; among them: (a) is the capacity utilization efficiency corresponding to other typical working point currents of 0.65C and 0.2C to 2.0C, and (b) is the capacity utilization efficiency of a small range of working points near 0.65C (0.55C, 0.75C). DETAILED DESCRIPTION

[0029] The present invention is further illustrated by the following examples.

[0030] Embodiment 1: According to Figure 1As shown, a comparison between the traditional and new sorting methods is shown. The traditional sorting method evaluates the parameter differences between the cells and sets the difference threshold to select cells with small parameter differences for grouping to improve utilization efficiency. This new method starts from the first principle of sorting, that is, the maximum capacity utilization efficiency of the Pack, and uses it as an important indicator to determine whether it is suitable for grouping and evaluate the sorting effect. By analyzing the dynamic change process of each branch current in parallel, the relationship between capacity utilization efficiency and terminal load current is established, and it is judged whether the maximum capacity utilization efficiency meets the standard, so as to achieve rapid sorting and effect evaluation.

[0031] In order to compare the advantages and disadvantages of this new method with the traditional sorting method, two cells (ΔC = 12.5Ah, ΔR = 10mΩ) that do not conform to the traditional sorting logic were set up, and the monomers were quite different. The cells were fully charged by constant current-constant voltage method, and after standing for 1 hour, they were discharged to the cut-off voltage at 0.5C. The load operating point current that achieves the highest pack capacity utilization efficiency is calculated to be 40.63A, that is, 0.65C.

[0032] Set different working conditions to observe the branch current distribution and calculate the module capacity utilization efficiency. The module capacity utilization efficiency results are as follows: Figure 2 shown. Figure 2 (a) shows the capacity utilization efficiency corresponding to 0.65C and other typical operating point currents between 0.2C and 2.0C. Figure 2 (b) focuses on the capacity utilization efficiency of a small range of operating points (0.55C, 0.75C) around 0.65C.

[0033] In this embodiment, if the traditional method using parameter difference as an indicator is used for sorting, the cells with large parameter differences are not suitable for grouping. Figure 2 As shown in the figure, the module capacity utilization efficiency is the highest when the load current is 0.70C, which can reach η=99.7%, indicating that the two cells can be grouped and the effect is good. The terminal load current can be used to directly observe the module capacity utilization efficiency state, and then sort and evaluate the effect. This reflects that the new method proposed in this paper has higher sorting accuracy. At the same time, this example also provides different ideas for flexible grouping and effect evaluation, which is of positive significance.

Claims

1. A novel sorting method for cascade utilization of power batteries, which analyzes the dynamic change process of each branch current in parallel, and directly establishes the relationship between capacity utilization efficiency and terminal load current during the sorting process to determine whether the highest capacity utilization efficiency is greater than 99% to achieve rapid sorting; it is characterized by: The specific steps are as follows: (1) Analysis of the dynamic change process of each branch current in parallel Based on Kirchhoff's law, the current distribution of each branch in parallel is obtained: Where: I i is the branch current of the ith cell, I L is the total branch current, R i is the DC resistance of the ith cell, U OCVi is the open circuit voltage of the ith cell, k OCVi is the open circuit voltage U of the ith cell OCVi With SOC i The slope between two variables, SOC i The SOC state of the i-th battery cell, U i0 is the initial voltage of the ith cell, C i is the rated capacity of the ith battery cell; By solving formula (1), the branch current dynamic equation is obtained, which is as follows: Where: I1(t) and I2(t) are the branch currents of the first and second cells, C1 and C2 are the rated capacities of the first and second cells, k OCV1 and k OCV2 are the open circuit voltage U of the first and second cells respectively. OCVi With SOC i The slope between two variables, I L is the total branch current, U 10 and U 20 are the initial voltages of the first and second cells, respectively, and R1 and R2 are the DC resistances of the first and second cells, respectively; Using the limit calculation branch current dynamic equation, assuming that U 10 =U 20 ,k OCV1 =k OCV2 , and the current distribution at the starting point and steady-state point is obtained: Where: I i (0) is the starting point current of the ith cell branch, R1 and R2 are the DC resistances of the first and second cells, C1 and C2 are the rated capacities of the first and second cells, I i (∞) is the steady-state point current of the ith cell branch; (2) Sorting process Establish the relationship between capacity utilization efficiency and terminal load current, determine whether the maximum capacity utilization efficiency meets the standard, and achieve rapid sorting: The current distribution at the starting point of each branch can be controlled by setting the load current, and the dynamic change process always follows I1·R1+U 10 =I2·R2+U 20 In the steady state, the current of each branch tends to be distributed in a capacity ratio relationship; therefore, under the given dynamic change trend and steady state, different module capacity utilization efficiencies η are achieved by adjusting the starting point current and the terminal load current, which are defined as: in: Refers to the SOC state of the ith cell when any cell in the module is discharged to the cut-off voltage, C i Refers to the rated capacity of the ith battery cell, C total Refers to the sum of the rated capacities of the modules; The highest capacity utilization efficiency of the module is that the ratio of the current of each branch is always consistent with the ratio of the capacity, which can ensure that each branch reaches the cut-off voltage at the same time, reduce the current balancing process and loss, and achieve the highest capacity utilization efficiency of the Pack; the starting point current is distributed according to the capacity ratio, then the load working current when I1(0) / I2(0)=C1 / C2 is: According to the highest module capacity utilization efficiency corresponding to the working current, it is judged whether it is greater than 99%. If it is, the grouping conditions are met, sorting is achieved, and the sorting effect can be evaluated by the capacity utilization efficiency.

2. A novel sorting method for recycling power batteries according to claim 1, characterized in that: The capacity utilization efficiency η of the module after the cells are connected in parallel is an important indicator to determine whether it is suitable for grouping and evaluate the sorting effect.

3. According to claim 1, a novel sorting method for recycling power batteries is characterized in that: By adjusting the dynamic process of the current in each branch after the cells are connected in parallel, the relationship between the capacity utilization efficiency and the terminal load current is established, and it is determined whether the maximum capacity utilization efficiency meets the standard, so as to achieve rapid sorting.

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