1.Consistency of lithium batteries definition. At present, it refers to the convergence of a group of important characteristic parameters of lithium batteries. It is a relative concept. There is no most consistent, only more consistent. For multiple strings of cells in the same battery pack, each parameter should preferably be within a small range for good consistency. In addition to the time dimension, consistency refers to the consistency of all characteristic parameters of all cells in the battery pack during the entire life cycle. Also include the inconsistency of capacity decay, inconsistent internal resistance increase, and inconsistent aging rate. The life span of the entire battery pack is our ultimate focus on consistency. According to the passage of time, some scholars put the interaction between parameters on a graph, as shown in the figure below. Time is the horizontal axis, and the parameter is the vertical axis. Several parameters are put into a table following the changes of time and interwoven into a net, which is the starting point for us to think about consistency.
The purpose of pursue consistency, in addition to existed the maximum capacity of the battery pack (including maximum power, maximum current, and maximum available capacity) in the current state, it also wants to maintain this capacity for as long as possible.
2. Consistency Evaluation Scope Take the electric vehicle for example. Personal understanding is that consistency is the consistency of all batteries used as power in an electric vehicle, regardless of the series relationship or the parallel relationship. The following content is not fully discussed, only examples.
2.1 Examples of parallel connection. The battery cell with low discharge capacity (code B) is connected in parallel with other normal batteries to become a parallel module D. For example, this is a module with 10 batteries in parallel. When the system is discharged, each parallel module needs to provide the same current, such as 100A. For other normal parallel modules, each battery discharges 10A; B can only discharge a maximum current of 1A, while the other 9 batteries need to discharge 11A each. Under normal circumstances, long-term overload, compared with other parallel modules, the aging rate of these batteries is faster. One day, the overall maximum discharge capacity of this parallel module cannot reach the designed maximum capacity. This parallel battery pack has become the bottleneck of the discharge capacity of the entire battery pack.
2.2 Series connection situation According to the general situation of electric vehicles, the series relationship is mainly between the module and the module. Continuing the scenario of the previous parallel connection, a battery pack D with a deeper aging degree than other battery packs appeared in the entire battery pack. The capacity of D is small, but the internal resistance is large. Reflected on the curve of SOC and open circuit voltage, the open circuit voltage corresponding to the same SOC, the D terminal voltage is relatively high. When the entire battery pack is charged, D reaches the charging cut-off voltage first, and the battery pack stops charging. The other modules have not eaten enough, he has already broken his belly, because it is old and the belly volume has become smaller. Therefore, the unit consistency is not a matter of the inside of a certain welded module, but a requirement for all power batteries on the same car.
3 Inconsistent lithium battery performance The voltage is inconsistent. After formation, go through the same charging and discharging process, and let it stand for enough time. Under the same ambient temperature, the battery to be evaluated is charged to the same SOC. Measure the open circuit voltage, and the voltage gap reflected at this time is the voltage inconsistency of the single cell.
The internal resistance is inconsistent. The internal resistance of the cell is an important characterization of the power characteristics of the cell. It is also one of the reasons why the performance parameters of the cell are further discretized after the cells are grouped. Inconsistent internal resistance can cause inconsistent temperature rise, which is a type of reason that causes further discretization of other parameters. Internal resistance is also the detection index after the module is grouped. Assembling a single battery cell into a module requires a grouping process such as welding or mechanical structure clamping. The consistency of the grouping process is reflected on the molded module, which is the internal resistance of the module.
The capacity is inconsistent, and the life is inconsistent. According to the current life measurement standards, the available capacity and the life are closely linked, which will be explained here. Capacity is generally used as the primary content of battery cell grouping, which is the most important parameter performance of battery inconsistency.
There are many reasons for inconsistent capacity, and most of them are the result of inconsistent manufacturing processes. In addition to reaching life indicators such as capacity and internal resistance, another meaning of inconsistent life is that the battery failure time is inconsistent. Studies have shown that it is not necessarily the cell with the smallest capacity or the cell with the worst working conditions that reaches the end of its life first. Each battery cell has different anti-aging ability from birth. The temperature rise is inconsistent. In addition to the internal resistance factors that directly affect the heating of each cell, the inconsistency formed in the manufacturing process of the internal electrochemical substances will also affect the heating value. The location of each battery cell in the battery pack is different, resulting in differences in its heat dissipation conditions, and ultimately inconsistent temperature rise of the battery cells.
4 Consistency Evaluation Researchers use a variety of statistical methods to evaluate the consistency of consistency. Some manufacturers use the standard deviation to measure the concentration of a set of battery cell parameters by the size of the standard deviation, which is statistically reasonable. Some manufacturers directly use the range of a set of data and the difference between the maximum and minimum values. Although it is not possible to fully describe the distribution of all the parameters of a set of batteries, it is also reasonable for the control logic of the current battery management system BMS. select. Moreover, using extremely deviation is the most concise method.
5 Sorting method Under the existing production capacity and technological level, there are three common ways to solve the consistency problem of lithium batteries. One is reasonable sorting, using batteries with similar performance parameters in a battery pack. The initial state of the battery cells is consistent; the Second is to improve the thermal management level, to provide a more suitable and more uniform working environment temperature for the battery, and to avoid further deterioration of the initial inconsistency; the Third is to improve the overall monitoring and balance capabilities of the battery management system, and strive to improve the inconsistency that occurred.
5.1 Static sorting Traditionally, static sorting is more widely used. Static means that the battery parameters have nothing to do with the working state. Usually, the parameters used for static sorting include battery capacity, open circuit voltage and internal resistance. Some methods are to divide the interval directly according to the value of the parameter, and the cells that fall in the same interval are a group. For a long time, static sorting has been the main sorting method in the lithium battery industry.
5.2 Dynamic sorting Dynamic sorting is a method of grouping based on different battery cell parameters during battery charging and discharging. One type of method is to take the constant current charging and discharging of the cell as the research process. Some algorithms regard the voltage-time curve as the classification object, and use statistical algorithms to divide the curve characteristics into groups; some focus on the voltage, capacity, internal resistance, discharge platform, cell thickness and other parameters in the process, and classify them; another The first category is to take the constant current and constant voltage charging process as the research process. Some use the Euclidean distance between the sampling point and the mean point on the constant voltage and constant current curve as the target parameter to perform cluster analysis to achieve cell grouping; some improve the sampling rules on the basis of the previous method to make the current vs. time The sampling points are denser in areas with a larger rate of change, while ensuring that the sampling will not fall below a minimum step size.