Cell inconsistency is a common problem in battery packs, which mainly refers to the differences in performance parameters of each cell in the battery pack, such as capacity, voltage, internal resistance, SOC, etc. This difference may be caused by process problems in the cell manufacturing process, material differences, or environmental changes during use. The hazards of inconsistent battery packs include reduced capacity, increased system failure rate, reduced performance, and shortened service life.
For example, the inconsistency of cells is that some cells in the battery pack are old, weak, sick, and disabled, while others are young and strong. When discharging, the old, weak, sick, and disabled cells are discharged first and cannot continue to discharge, otherwise they will be over-discharged and reduce chemical activity. The entire battery pack has to stop discharging and wait for charging. When charging, the cells with small capacity will be fully charged first. If charging continues, there may be a risk of overcharging the battery, while other cells with large capacity are not fully charged.
In addition to the inconsistency of capacity and voltage, there is also inconsistency in resistance. Cells with large internal resistance generate more heat, which leads to overheating of the battery. Overheating accelerates the aging of the cell in a vicious cycle. Refurbished disassembled batteries are more harmful because the cells are more different, so the battery life is shorter, the safety is poorer, and the service life is also shorter.
Causes of cell inconsistency:
1. Produced during the production process. After various processes such as batching, coating, rolling, winding, assembly, injection, and capacity division, the cumulative errors in each process are formed. It is best to select cells from the same batch, classify and screen them, and combine similar cells together.
2. Generated during use. Since each battery cell is located in a different position in the battery pack, the environment of the middle and edge cells will definitely be different. For example, the temperature will be slightly different. After long-term accumulation, the inconsistency of the cells will occur.
To address inconsistencies, the BMS is required to actively or passively balance the battery cells, as well as a good thermal management system.