Before we make a battery pack, we sort the battery cells and divide them into a group to ensure cell consistency for a pack. Inconsistency of lithium-ion cells significantly affects the life of the battery pack, thus reducing the performance of the battery pack. Lithium battery pack inconsistency refers to the difference in parameters such as capacity, voltage, internal resistance, and self-discharge rate of the individual cell. In addition, the combination structure, operating conditions, operating environment and battery management of the battery pack also cause inconsistencies in the cells.
The differences between individual battery cells include the initial differences in the cells and the different parameters during use. Improving the consistency of the cells is a prerequisite for improving the performance of the battery pack. The mutual influence of cell parameters the current parameter state is influenced by the initial state and accumulation over time.
The inconsistent capacity, voltage and self-discharge rate of the cells results in inconsistent depths of discharge for each cell in the battery pack. Cells with lower capacity and poor performance reach full state of charge sooner, while other cells with higher capacity and good performance may not be fully charged. Cell voltage inconsistency causes the individual cells in the parallel battery pack to charge each other, and the cell with the higher voltage charges the cell with the lower voltage, accelerating cell performance degradation and consuming the energy of the entire battery pack. Cells with a high self-discharge rate have large capacity losses, and the inconsistent self-discharge rate of the cells causes differences in the state of charge and voltage of the cells, which seriously affects the performance of the battery pack.
When battery packs are connected in series, the different internal resistance of each cell causes the charging voltage of each battery to be inconsistent. The cells with high internal resistance reach the upper limit of the voltage in advance, and other batteries may not be fully charged at that time. A cell with a high internal resistance has a large energy loss and a high heat generation. The temperature difference further increases the internal resistance difference, which leads to a vicious circle.
When battery packs are connected in parallel, the difference in internal resistance causes the current of each battery to be inconsistent. A high current battery will change voltage very quickly, causing the depth of charge and discharge of each battery to be inconsistent, so the actual capacity will be very different compared to the intended capacity. The working current of the battery will be different, and its performance will be different during use, which will eventually affect the battery life.
Charge and discharge affect the consistency of the cell. The charging method affects the charging efficiency and state of charge of the battery pack. Overcharging and overdischarging damages the battery, and inconsistency occurs after multiple charging and discharging. There are several charging methods for Li-ion batteries, but the most common are segmented constant current charging and constant current constant voltage charging. Constant current charging is a great way to charge safely and effectively. Constant current constant voltage charging combines the advantages of constant current charging and constant voltage charging, which solves the problem that the general constant current charging method is quickly difficult to charge exactly full, and avoids the effects of excessive current on the battery caused by the constant voltage charging method in the initial stage of charging, the operation is simple and convenient.
The temperature affect cell consistency At high temperature and high discharge rate, the performance of lithium battery is significantly degraded. This is because of li-ion battery causes decomposition of positive electrode active material and electrolyte, when the lithium ion batteries at high temperature and high current conditions, that called the exothermic process. The heat released in a short period of time can let battery have a high temperature, which means the decomposition will be accelerated, and create a vicious circle, so that battery performance will gradually decrease and lead to irreversible performance loss if the battery thermal management is improper. Cell manufacturing technology will affects cell consistency The cathode materials of lithium ion batteries include NMC materials, lithium iron phosphate, lithium cobalt oxide and lithium manganese oxide, the anode materials include graphite, silicon and lithium titanate.
The same batch of raw materials is critical to cell consistency. To obtain good consistency of the same batch of cell, it is necessary to strictly control the particle size distribution, specific surface area and contamination of raw materials during production. Li-ion battery cell production ivnolves several processes, and each can affect the consistency of the cells. To maintain good consistency of the cell, it is important to design and control each process reasonably. Design the battery production processes according to the performance requirements of the cell, analyze raw materials, electrodes and electrolytes, etc., and control the threshold of each process reasonably. Automation of the production line with less human intervention can greatly improve the consistency of the battery.
The affect of sorting on cell consistency. To reduce the impact of the initial state difference on the battery pack, it is necessary to sort each cell, and then combine them into a pack with same state parameters cell. The ways to form a battery pack include single parameter combination, multiple parameter combination and dynamic characteristic curve combination. 1, Dynamic characteristic curve combination is to compare the difference between the charge and discharge curves of different cells under the same rate, which can reflect the cell characteristics well. During the cell formation, the first few charges and discharges will reduce the discharge capacity of the cell due to the irreversible reaction. After the electrochemical state of the cell remains stable, the battery capacity becomes stable. Therefore, the purpose of multiple charge and discharge cycles is to keep the cell stable, so that required test equipment provides multiple process and cycle settings. First use low current charge and discharge, and then use high current charge and discharge. The ways to form a battery pack include single parameter combination, multiple parameter combination and dynamic characteristic curve combination. 1, Dynamic characteristic curve combination is to compare the difference between the charge and discharge curves of different cells under the same rate, which can reflect the cell characteristics well. During the cell formation, the first few charges and discharges will reduce the discharge capacity of the cell due to the irreversible reaction. After the electrochemical state of the cell remains stable, the battery capacity becomes stable. Therefore, the purpose of multiple charge and discharge cycles is to keep the cell stable, so that required test equipment provides multiple process and cycle settings. First use low current charge and discharge, and then use high current charge and discharge. PLB strict control of battery raw materials, manufacturing process, sorting system and other dimensions to ensure the initial consistency of the battery cell, the consistency of the charge and discharge dynamics and the consistency of the damping trend.
With PLB precise design, and strict control of raw materials, manufacturing process, sorting system and others to ensure the initial consistency of cell, the consistency of dynamic charge and discharge, consistency of capacity loss.