Lithium batteries can't be made into one large battery, so many small batteries have to be organized. Everyone can work hard and cooperate with each other, and they can also fly with electric cars. At this time, there is a problem that needs to be faced, consistency.
Our daily experience is that if the positive and negative poles of two dry batteries are connected, the flashlight can emit light. Who cares about the same and inconsistent things. The large-scale application of lithium batteries is not so simple.
The inconsistency of lithium 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 . Single cells form 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 1C, and the other has a capacity of only 0.9C. In a series connection, the 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 idle
2) Life loss , similarly, the life of a battery pack is determined by the cell with the shortest life. 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.
How to deal with inconsistencies
The inconsistency of battery core performance is formed during the production process and deepened during use. 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 mainly 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.
Different batches of batteries should not be used together in theory. Even batteries of the same batch need to 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, mainly divided into static sorting and dynamic sorting.
Static sorting is to screen for the characteristic parameters such as open circuit voltage, internal resistance and capacity 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. relation.
Combining dynamic and static sorting, static screening is used for preliminary grouping, and dynamic screening is performed 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 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 can't be equipped with a battery pack, and no matter how good the method is, it can't be used.
2) Thermal management
Aiming at the problem of inconsistent internal resistance of the cells and different heat generation. 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 still have a high temperature rise, but they will not be far apart from other cells, and there will be no significant difference in degradation level.
The inconsistency of the battery cells, the terminal voltage of some battery cells is always ahead of other battery cells, reaching the control threshold first, resulting in a reduction in 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 charge 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 discharge part of the high-voltage battery's power, or transfer the energy away and put 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 research 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.