Introduction to the harm of lithium-ion battery

Lithium-ion battery consistency problem Lithium-ion battery cannot be made into a large one, so many small batteries have to be organized. Everyone works hard and cooperates sincerely, so that electric cars can fly. At this time, we have to face a problem, consistency. Why do we need to be consistent? Our daily experience is that if two dry batteries are connected with the positive and negative electrodes, the flashlight can emit light. Who cares about the same and inconsistent things. The large-scale application of lithium-ion batteries is not so simple. The inconsistency of lithium-ion battery parameters mainly refers to the inconsistency of capacity, internal resistance, and open circuit voltage. If inconsistent batteries are used in series and used together, the following problems will occur. 1) Capacity loss. Cells make up a battery pack. The capacity conforms to the barrel principle. The capacity of the worst cell determines the capacity of the entire battery pack. In order to prevent the battery from overcharging and overdischarging, the logic of the battery management system is set as follows: when discharging, when the lowest cell voltage reaches the discharge cut-off voltage, the entire battery pack stops discharging; when charging, when the highest cell voltage touches the charge cut-off voltage To stop charging. Take two batteries in series as an example. One battery has a capacity of C, and the other has a capacity of only 0.9C. In a series connection, two batteries pass the same amount of current. When charging, the battery with small capacity must be fully charged first, and the charging cut-off condition is reached, and the system will not continue to charge. When discharging, a battery with a small capacity will inevitably emit all available energy first, and the system will stop discharging immediately. In this way, batteries with small capacity are always fully discharged, while batteries with large capacity always use part of the capacity. Part of the capacity of the entire battery pack is always in an idle state. 2) Life loss. Similarly, the life of the battery pack is determined by the cell with the shortest lifespan. It is very likely that the cell with the shortest life span is the cell with a small capacity. Small-capacity batteries are fully charged and discharged every time, and the output is too strong, which is likely to reach the focus of life first. When the battery cell's life ends all the time, a group of battery cells welded together will die. 3) The internal resistance increases, the same current flows through different internal resistances, and the cells with large internal resistance generate more heat. The battery temperature is too high, causing the deterioration rate to accelerate, and the internal resistance will further increase. Internal resistance and temperature rise form a pair of negative feedback, which accelerates the deterioration of high internal resistance cells. The above three parameters are not completely independent. The internal resistance of the aging cell is relatively large, and the capacity attenuation is also more. Separate explanations, just to express clearly their respective directions of influence. 3 How to deal with the inconsistency The inconsistency of the battery cell performance is formed during the production process and deepened during the use process. The batteries in the same battery pack are always weak for the weak, and become weaker at an accelerated rate. The degree of dispersion of the parameters between the single cells increases as the degree of aging deepens. At present, engineers must consider three aspects to deal with the inconsistency of single cells. Single battery sorting, thermal management after grouping, the battery management system provides equalization function when there is a small amount of inconsistency. 3.1 Separate batteries of different batches and theoretically do not put them together for use. Even batteries of the same batch must be screened, and batteries with relatively concentrated parameters are placed in a battery pack and in the same battery pack. The purpose of sorting is to select cells with similar parameters. The sorting method has been studied for many years, and it is mainly divided into two categories: static sorting and dynamic sorting. Static sorting is to screen the open circuit voltage, internal resistance, capacity and other characteristic parameters of the cells, select the target parameters, introduce statistical algorithms, set the screening criteria, and finally divide the cells of the same batch into several groups. Dynamic screening is based on the characteristics of the battery cell during the charging and discharging process. Some choose the constant current and constant voltage charging process, some choose the pulse shock charging and discharging process, and some compare their own charging and discharging curves. relationship. Combining dynamic and static sorting, static screening is used for preliminary grouping, and dynamic screening is carried out on this basis, so that more groups can be divided and screening accuracy is higher, but the cost will rise accordingly. Here is a small reflection of the importance of the production scale of a handful of power lithium-ion batteries. Large-scale shipments allow manufacturers to perform finer sorting and obtain battery packs with closer performance. If the output is too small and there are too many groups, one batch cannot be equipped with a battery pack, and no matter how good the method is, it will not be able to be used. 3.2 Thermal management aims at the battery cells with inconsistent internal resistance and the problem of different heat. The addition of the thermal management system can adjust the temperature difference of the entire battery pack to keep it within a small range. Cells that generate more heat will still have a high temperature rise, but will not be separated from other cells, and there will be no obvious difference in degradation level. 3.3 Balance the inconsistency of the battery cells. The terminal voltage of some cells always leads other cells and reaches the control threshold first, which leads to the reduction of the capacity of the entire system. In order to solve this problem, the battery management system BMS has designed a balancing function. A certain battery cell is the first to reach the charging cut-off voltage, while the voltage of the other cells is obviously lagging behind. The BMS activates the charge equalization function, or connects a resistor to release part of the high-voltage battery's power, or transfers the energy away and puts it to a low level. The voltage cell goes up. In this way, the charging cut-off condition is lifted, the charging process is restarted, and the battery pack is charged with more power. Until now, the inconsistency of batteries is still an important area of u200bu200bresearch in the industry. No matter how high the energy density of the battery cell is, the battery pack capacity will be greatly reduced if it encounters inconsistency to disrupt the situation. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete.