The consistency of individual cells in a lithium battery pack needs to be assessed using specialized tools to measure capacity, internal resistance, and charge-discharge curves. This requires measuring the capacity, internal resistance, and discharge plateau of each individual cell, as well as considering the manufacturing process of the battery pack and the specific product's performance requirements. Inconsistencies in battery performance (within a certain range) can only be addressed through an external battery management system; this is essential.
Consistency includes the following common types:
Consistency of aging:
As the number of uses increases, consistency factors between individual battery cells will change, such as cell capacity decreasing, but the degree of change varies among different cells. Even battery packs with excellent consistency in the initial assembly stage will show differences after a period of cycle aging.
Consistency of open-circuit voltage:
Overcharging can cause some batteries to overcharge, leading to safety issues. The greater the voltage difference, the more serious the safety problem.
Internal resistance consistency:
In a series-connected battery pack, the internal resistance of each cell is inconsistent. During charging, the cell with higher internal resistance will be overcharged, and during discharging, the cell with higher internal resistance will be over-discharged.
Inconsistent capacity:
In a series-connected battery pack, the cells have inconsistent capacities. During charging, the cells with lower capacity will be overcharged, and during discharging, they will be over-discharged.
Voltage inconsistency occurs when, after formation, the cells undergo the same charge-discharge process, are left to stand for a sufficient time, and charged to the same state of charge (SOC) under the same ambient temperature. Measuring their open-circuit voltage reveals a voltage difference, indicating voltage inconsistency within individual cells. Studies show that the open-circuit voltage of individual cells follows a normal distribution. This means that all efforts to improve consistency can only change the concentration of parameters.
Inconsistent internal resistance is a crucial indicator of a battery cell's power characteristics and one of the reasons for the further discretization of cell performance parameters after cell assembly. Inconsistent internal resistance can also cause inconsistent temperature rise, which is another reason for the further discretization of other parameters.
Internal resistance is also a testing indicator after module assembly. From individual battery cells to modules, there are assembly processes such as welding or mechanical clamping. The consistency of the assembly process is reflected in the module's internal resistance after assembly.
Capacity and lifespan may differ, but according to current lifespan measurement standards, usable capacity and lifespan are closely linked, and will be explained together here.
Capacity is generally used as an initial criterion for cell grouping and is the most important parameter reflecting cell inconsistencies. There are many reasons for capacity inconsistencies, and most of them are the result of inconsistencies in the manufacturing process.
Besides meeting lifespan metrics such as capacity and internal resistance, another implication of inconsistent lifespan is that batteries fail at different times. Research indicates that the cell with the smallest capacity or the one operating under the harshest conditions is not necessarily the first to reach the end of its lifespan. From the moment they are produced, each cell exhibits different resistance to aging.
Inconsistent temperature rise is caused by factors beyond just internal resistance, which directly affects heat generation. Inconsistencies in the electrochemical substances produced during the manufacturing process also influence heat generation within each cell. Furthermore, the different locations of each cell within the battery pack lead to variations in heat dissipation conditions, ultimately resulting in inconsistent cell temperature rises.
Consistency evaluation
Researchers use various statistical methods to evaluate the consistency. Some manufacturers use standard deviation to measure the concentration of a set of cell parameters, which is statistically reasonable.
Some manufacturers directly use the range of a set of data, the difference between the maximum and minimum values. Although this cannot fully describe the distribution of all parameters of a set of batteries, it is a reasonable choice for the control logic of the current Battery Management System (BMS). Moreover, using the range is the simplest method.
