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Rapid Decay of Lithium Battery Pack: Is it a Cell Issue? No, it's Poor Control of Consistency in Grouping!
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Rapid Decay of Lithium Battery Pack: Is it a Cell Issue? No, it's Poor Control of Consistency in Grouping!

Many lithium battery packs experience rapid deterioration in the middle of their lifespan, resulting in insufficient capacity for energy storage and inadequate power and range for vehicles. This problem cannot be detected during the initial factory testing of battery packs. Typically, lithium battery packs are tested to aged around 2 cycles before leaving the factory, and if the BMS protection function and battery capacity are normal, they are shipped according to the quality criteria. However, they tend to experience mid-life issues when used by customers. When feedback about insufficient capacity and fast performance deterioration is received after using the battery pack for a while, the initial reaction is to blame the battery cells. However, is the fast capacity deterioration really due to the cells? No, most of the time it is due to poor control of the consistency matching of the battery pack.




If the consistency of the pre-packaged battery pack is not controlled well before production, a three-element lithium battery pack will experience faster degradation after about 150 cycles, while a lithium iron phosphate battery pack will experience faster degradation after about 500 cycles.

Cycle life of lithium battery packs

The cycle life of battery cells has been tested in the pack before, taking into account the different cycle life in different rates and temperatures. Whether the factory has its own cell factory or not, the lifecycle data of these cells can be obtained from the cell supplier, so the lifecycle of the cells is already known!

For matured battery pack factories, the cells used are relatively familiar, and if not familiar, they will not dare to use them casually. There is some understanding of both the basic performance and the cycle life of the cells. The cycle life of lithium cells is about 500 times (1C, 25℃), and after being packed, only 300 to 400 cycles are available. The cycle life of lithium iron phosphate cells is about 2000 times (1C, 25℃), and after packing, only 1500 to 1800 cycles are available. The lifecycle mentioned here ends with the remaining 80% of the capacity.


Cycle Life of Lithium Iron Phosphate Battery Cells


Cycle Life of Three-element Lithium Battery Cells

Battery pack matching with BMS protection boards

For the selected BMS protection board, the addition of a balancing function can slightly repair the consistency impact caused by cell differences. However, the balance function is basically passive, also known as the power-loss balance, and for battery packs with large consistency deviations, the repair effect of only a few tens of mA balance function is very small.

What is consistency? How is it controlled? Consistency refers to the difference between the cells that make up the battery pack! The smaller the difference, the better the consistency control of the lithium battery pack. For example, a 36V13Ah battery pack using three-element lithium cells to make 10-series 5-parallel connections requires 50 cells. The smaller the differences between these 50 cells, the better the consistency of this battery pack, resulting in better performance.

What are the differences between the cells?

  • Capacity difference. It is recommended to be within 1%.

  • Impedance difference. It is recommended to be within 3mΩ.

  • Voltage difference. It is recommended to be within 5mV. (Select the matching group when at 15% battery capacity)

  • Batch. Only cells within the same batch from the same factory can be combined to form a battery pack. That is to say, cells from Company A cannot be combined with cells from Company B to form a series or parallel battery pack. Do not mix different batches, and do not mix new cells with repaired cells.

  •  K-value. That is, the self-consumption of the cell, which accurately compares the voltage drop or capacity decay after being left stationary for one month to determine the K-value.

The more cells are connected in a series-parallel in the pack, the stricter the requirements for the consistency of the cells are. Therefore, the control of the above five points must be appropriately tightened.

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