03 JUN

Is Rapid Battery Pack Decay Caused by Battery Cell Issues?

  • Life Style
  • Angelina
  • Nov 08,2023
  • 0


Have you noticed that your Li-ion battery pack is degrading faster than expected? Many Li-ion battery packs experience rapid degradation in the middle of their lifespan, resulting in reduced capacity,Battery recycling machine insufficient backup time for energy storage products, and inadequate mileage for power products. These issues often surface after the lithium battery pack leaves the factory, as they are challenging to detect during the initial factory tests.

Battery packs typically undergo aging for about 2 cycles during the factory test to ensure the BMS protection function and battery capacity are normal. However, problems related to the mid-life degradation tend to become apparent while in the hands of the customer.cylindrical battery pack mahcine When the capacity of a battery pack decreases rapidly, the first suspicion is often related to the battery cells. But is it always a problem with the battery cells when this happens? In most cases, it's due to poor grouping consistency control.

If the grouping consistency isn't well-maintained before the battery pack is assembled (PACK), Li-ion ternary battery packs tend to exhibit faster degradation in about 150 cycles, while Li-FePO4 battery packs may show more rapid degradation in approximately 500 cycles.

Cycle Life

Battery cells' cycle life must have been tested before the assembly of the battery pack, including tests under various charge-discharge rates and temperatures. Whether the PACK factory manufactures its own cells or sources them from suppliers,cell stacking machine data regarding the cells' cycle life is available, so the expected cell life is well-known.

Experienced battery PACK factories are familiar with the cells they use, and they avoid using unfamiliar cells without a certain understanding of their basic performance and cycle life. Lithium ternary cell cycle life is typically around 500 cycles (1C, 25°C), but after being incorporated into a battery pack, it might last only 300 to 400 cycles. LiFePO4 cells have a cycle life of approximately 2000 cycles (1C, 25°C), but this number decreases to 1500 to 1800 cycles when used in a PACK. These cycle life figures are based on the end of life being defined as 80% of the capacity remaining.

Matching BMS Protection Boards

To address battery cell inconsistencies, manufacturers often include BMS protection boards with equalization functions. However, most equalization functions are passive, consuming power and having minimal effect on correcting severe cell inconsistencies.

What Is Cell Consistency, and How Is It Controlled?

Cell consistency refers to the differences between individual cells in a battery pack. The smaller these differences, the better the cell consistency and overall battery pack performance. For example, a 36V13Ah battery pack assembled from 10 series and 5 parallel lithium ternary cells requires 50 cells. The better the consistency among these 50 cells in terms of capacity, internal resistance, voltage, batch (if optional grouping is employed), and K value, the higher the overall battery pack performance.

Cell differences can be categorized as follows:

1. Capacity difference: Recommended within 1%.

2. Internal resistance difference: Recommended within 3mΩ.

3. Voltage difference: Recommended within 5mV (15% of the power in the case of optional grouping).

4. Batch: It is essential to use cells from the same factory and the same batch to form a PACK. Mixing cells from different factories or batches is not recommended.

5. K value: This represents the cell's self-discharge. It's measured accurately after one month of storage, and the K value is determined based on the voltage drop or capacity decay.

As the number of cells in series and parallel increases, the consistency requirements for cells become more stringent. Therefore, control over the above five points should be tightened appropriately.

Failure to control cell consistency during the core sub-electro-mixing group before PACK assembly might not be noticeable during factory testing when the capacity appears normal. However, the impacts of poor consistency become more pronounced over time, typically surfacing as mid-life degradation issues.

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