How to judge the consistency of the lithium battery pack?

The consistency of the lithium battery cells requires special tools to determine the capacity, internal resistance, and charge-discharge curve. This requires a single measurement of the capacity, internal resistance and discharge platform of a single battery, as well as the process level of the battery combination, and the requirements of the specific product on the battery performance. The inconsistency of the battery (within a certain range) can only be solved with an external battery pack management system, which is a must. Consistency includes the following common ones: Consistency of aging: As the number of times of use increases, the consistency factors between the cells will change, such as the capacity of the cell will become smaller, but the degree of change of different cells different. Even a battery pack with good consistency at the initial stage of assembly will experience differences after a period of cycle aging. Consistency of open circuit voltage: After charging, individual batteries will be overcharged, which will cause safety issues. The greater the voltage difference, the more serious the safety problem. Consistency of internal resistance: The internal resistances of the cells in the series battery pack are not consistent. Cells with high internal resistance will be over-voltage charged during charging, and those with high internal resistance will be over-voltage discharged during discharge. Inconsistent capacity: The capacity of each battery cell in the series battery pack is inconsistent. Cells with low capacity will be overvoltage charged during charging, and will be overvoltage during discharge. The discharge voltage will be inconsistent. After formation, they will undergo the same charge and discharge process and stand still. Enough time, under the same ambient temperature, the battery to be evaluated is charged to the same SOC. Measure its open circuit voltage, and the voltage gap reflected at this time is the voltage inconsistency of the single cell. Studies have shown that the open circuit voltage of a single cell conforms to a normal distribution. In other words, all our efforts to improve consistency can only change the concentration of parameters. The internal resistance is inconsistent. The internal resistance of the cell is an important characterization of the power characteristics of the cell, and 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. The internal resistance is also the detection index after the module is formed into a group. 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 inconsistencies in the manufacturing process. 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 during the manufacturing process of its 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. Consistency evaluation researchers use a variety of statistical methods to evaluate the consistency of good or bad. 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. choose. Moreover, using extremely poor is the most concise method.