The operating environment within a lithium-ion battery pack varies for each individual cell. This leads to a gradual increase in inconsistency among individual cells during use, which in some cases accelerates the performance degradation of certain cells and ultimately causes premature failure of the lithium-ion battery pack.
Reasons for inconsistencies in lithium-ion battery packs
Inconsistency in lithium-ion battery packs is a cumulative process; the longer the time, the greater the differences between individual cells. Furthermore, lithium-ion battery packs are also affected by the usage environment. During subsequent use, the inconsistency between individual cells will be gradually amplified, leading to accelerated performance degradation of some individual cells and ultimately rendering the battery unusable.
The inconsistency of lithium-ion battery packs is significantly affected by time, mainly due to two factors:
1. Firstly, there are issues with the manufacturing process and uneven material distribution, resulting in very slight differences in the materials and properties of lithium-ion batteries. After the lithium-ion battery pack is put into use, differences in electrolyte density, temperature, ventilation conditions, self-discharge level, and charging/discharging process among the individual batteries in the pack can lead to variations in capacity and internal resistance even among batteries of the same model manufactured in the same batch.
2. When used in a vehicle, the differences in electrolyte density, temperature, ventilation conditions, self-discharge level, and charging/discharging process of each battery in the lithium-ion battery pack have an impact.
What are the dangers and problems that can arise from inconsistencies in lithium-ion battery packs?
① Capacity loss: The capacity of a lithium-ion battery pack is determined by the barrel principle, where the capacity of the worst cell determines the capacity of the entire battery pack.
② Lifespan loss: Small-capacity battery cells are often fully charged and discharged repeatedly, resulting in excessive power output. This makes them very likely to reach the end of their lifespan first. Once a single cell reaches the end of its lifespan, the entire set of cells soldered together will also reach the end of its lifespan.
③ Increased internal resistance: When the same current flows through cells with different internal resistances, cells with higher internal resistance generate more heat. Excessive battery temperature accelerates degradation, further increasing internal resistance. Internal resistance and temperature rise create a negative feedback loop, accelerating the degradation of high-internal-resistance cells.
How to address the inconsistency issue in lithium-ion battery packs?
△Sorting: Cells from different batches should not be used together in theory. Even cells from the same batch must be screened, and cells with relatively similar parameters should be placed in a lithium-ion battery pack, within the same battery pack.
△ Thermal Management: Addressing the issue of inconsistent heat generation in cells with varying internal resistance, the addition of a thermal management system regulates the temperature difference across the entire lithium-ion battery pack, keeping it within a relatively small range. Cells generating more heat will still experience higher temperature rises, but the difference won't widen significantly compared to other cells, thus preventing a noticeable difference in degradation levels.
△Battery Balancing Management: To address this issue, the Battery Management System (BMS) incorporates a balancing function. This ensures a favorable operating environment for the lithium-ion battery pack, maintaining a constant temperature, minimizing vibration, and preventing water and dust contamination of the battery terminals. It also understands the development patterns of inconsistencies among individual cells within the lithium-ion battery pack, promptly adjusting or replacing cells with extreme parameters to prevent the inconsistencies in the battery pack parameters from increasing over time.
△Energy Management Strategy: In terms of energy management, minimize battery discharge depth as much as possible, provided the output power allows. Introduce practical battery pack energy management and balancing systems, formulate reasonable battery balancing strategies, and proactively intervene to reduce battery inconsistencies. Prevent both deep discharge and overcharging of the battery.
