Inconsistent lithium battery packs mainly refer to inconsistent parameters of capacity, internal resistance, and open circuit voltage.
Inconsistent performance of the cells is formed during the production process. The batteries in the same lithium battery pack are weak and weak, and the acceleration is weak. The degree of dispersion of the parameters between the individual cells increases as the degree of aging increases. Inconsistent battery strings are used together and the following problems occur.
Lithium battery packs inconsistency performance
1. Capacity loss, the battery cells constitute a lithium battery pack, the capacity is in line with the barrel principle, and the capacity of the worst battery cell determines the capacity of the entire lithium battery pack. Take two batteries in series for example. One battery has a capacity of C and the other has a capacity of only 0.9C. In series, two batteries pass the same amount of current.
When charging, the battery with a small capacity must be fully charged first, and the charging cut-off condition is reached, and the system does not continue charging. When discharging, the battery with a small capacity must first emit all available energy, and the system immediately stops discharging. In this way, the cell with a small capacity is always fully charged, and the cell with a large capacity has been using part of the capacity. The total capacity of the entire lithium battery pack is always idle.
2. The loss of life is determined by the battery with the shortest life. Small-capacity batteries, each time full charge and full discharge, the output is too strong, it is likely to reach the focus of life first. At the end of the life of the battery, a group of batteries that are welded together will follow the end of life.
3. The internal resistance increases, different internal resistances, the same current flows, and the electric core with large internal resistance has relatively more heat. If the battery temperature is too high, the rate of deterioration will increase and the internal resistance will increase further. The internal resistance and temperature rise form a pair of negative feedback, which accelerates the degradation of the high internal resistance battery. Of course, the internal resistance of the battery with a deep aging degree is relatively large, and the capacity attenuation is also more.
Lithium battery packs inconsistency sorting method
The individual cells are inconsistent and are mainly considered from three aspects. Single cell sorting, thermal management after grouping, battery management system provides equalization function when there is a small amount of inconsistency; different batches of batteries are not used together; even if the same batch of batteries, because the parameters are inconsistent, Need to be screened; lithium battery packs are mainly divided into two categories: static sorting and dynamic sorting:
1. Static sorting, screening the characteristic parameters such as open circuit voltage, internal resistance and capacity of the cell, selecting the target parameters, introducing statistical algorithms, setting the screening criteria, and finally dividing the cells of the same batch into several groups.
2. Dynamic screening is to screen the characteristics of the battery during charging and discharging. Some choose constant current and constant voltage charging process, some select pulse impulse charging and discharging process, and some compare their own charging and discharging curves. Relationship between.
Dynamic and static combined sorting, static screening for preliminary grouping, based on this dynamic screening, so that more groups are divided, the screening accuracy is higher, but the cost will increase accordingly.
Lithium battery pack security configuration method
In order to ensure the safety of the lithium battery pack, a firm security protection will be set.
1. Safety valve
When the temperature rises, it should be outside the normal range, the safety valve will automatically open, the pressure will be released, and the battery will be completely disabled to ensure safety.
A sharp increase in resistance prevents further temperature rise
In the event of an overcurrent risk, the circuit is disconnected to avoid a malignant accident.