This article presents a novel grouping method for lithium iron phosphate batterie and ways of reducing thermal runaway. In this method, a simplified electrochemical impedance spectroscopy (EIS) model is utilized to describe the battery characteristics. Dynamic stress test (DST) and fractional joint Kalman filter (FJKF) are used to extract battery model parameters.
Here We would like to talk about an alternative battery pack assembly method. The advantage is that it can detect the status of each cell, and can remove rapidly deteriorating and unhealthy cells from the battery pack. The effect of “preventive treatment” can greatly reduce thermal runaway fire accidents. The assembly method is as follows
List of Advantages
It can record the status of each cell, especially suitable for the cascade utilization of battery packs
Because two cells are connected in series when the cells are assembled, the voltage of each cell is independent. Therefore, BMS can simply test the voltage of each cell. It can be calculated with the charging and discharging state and current. Even without testing, the battery pack can be reduced to 80% of the battery pack directly according to the BMS data. times used.
It can cut off bad cells and greatly extend the service life of the battery pack
For example, if the internal resistance of cell 2 is found to be too large, it is only necessary to cut off the electrical connection between cell 2 and cell 6, and cell 2 and cell 6 become a single cell that is electrically connected to the battery pack. In fact, the electrical connection between the bad cells 2 and 6 and the battery pack is cut off, and the bad cells 2 and 6 become empty cells. Therefore, the battery pack is maintainable and is especially suitable for swap stations with maintenance conditions. It greatly reduces the occurrence of thermal runaways.
Discharge is not affected by bad cells
For example, the capacity of battery cell 2 is reduced and scrapped. When we discharge, our strategy is to ensure that the voltage across battery cell 2 and battery cell 6 is not lower than 6.5volts. Obviously, the voltage of battery cell 2 will not be lower than 6.5-4.2=2.3 volts, while the other A healthy cell can be discharged to 6.5/2=3.75v, and since cell 2 is a scrapped cell, the damage to its life of 2.3 volts can be ignored.
Especially suitable for use in power station mode
The battery pack can be maintained and the cells that are deteriorating will be removed. Generally speaking, any system has many early failures, many late failures, and relatively stable medium-term. That is to say, a battery pack will be removed after a short period of use. After the faster cells, it will enter the stable period, which means that we can use the second-class cells, and after early use and maintenance, the stability of the battery pack entering the stable period is not inferior to using high-quality cells. The battery pack production cost will be reduced. The assembled battery pack cannot have a single cell damaged and short-circuited during the entire n-year service cycle. Even if only one cell is damaged and short-circuited, the battery pack will be scrapped in light, and the battery pack will be scrapped in severe cases. Thermal runaway fires, so the current automotive battery packs have strict requirements on the quality of the cells, and the cost of the battery packing in the existing mode is relatively high.
Especially suitable for use in energy storage stations
Generally speaking, the capacity of the energy storage station is much larger than that of the car. Normally, battery towers and battery racks are built in the warehouse. The large capacity means that once the battery cells are damaged, the consequences are often serious due to a large number of parallel cells. It is very suitable for using this patent to maintain and cut off a single cell, thereby increasing energy storage. station security.
List of Shortcomings
There are many detection lines, and the BMS has a heavy burden
If there are thousands of cells like a Tesla battery pack, BMS may not be able to afford it, but there are more and more battery packs using large single cells in China, and the number of cells is greatly reduced. On the other hand, the function of BMS is also increasing It is getting stronger and stronger, and wireless BMS has also seen publicity, and BMS should be able to bear it in the future. Even if the BMS can’t afford each cell, it can be done step by step. For example, the original 12 batteries were changed to 6 and 2 groups. The BMS was improved and then 4 and 3groups were combined. Finally, all 12 cells were tested separately. The principle is to use the BMS detection capability. Use enough, as long as the number of parallel cells is reduced, the safety is improving.
The balancing strategy is complex
Because the charging strategy is that BMS ensures that the voltage of each cell does not exceed 4.2 volts when there is an imbalance between the cells, the number of cells that need to be equalized is large, and the equalization circuit is relatively large and complex, so it is especially suitable for battery replacement mode because the power exchange station can be equipped with a charging device with a complex equalization circuit. If you charge at home, you may need to go to the charging station for equalization charging on a regular basis (one month to six months). Of course, if the battery swap station has a maintenance function, it can maintain the battery pack while balancing, remove bad cells, and improve the stability and safety of the battery pack.
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