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With the growing advanced battery technology, lithium-ion battery with excellent battery performance and longevity is getting a large amount of preference in the market. In order to provide high quality batteries, the consistency of Lithium-ion battery cells is what manufacturers need to primary focus on. Although some solutions like the thermal management can ensure the safe operation of lithium-ion battery modules to some extent, consistency of battery cells is still the key factor that affects the performance and service life of the battery module.
Actually, consistency is a relative concept. If each characteristic parameter of multiple lithium battery cells in the same module is similar within a certain standard range, it means that the lithium-ion battery cells are in good consistency.
So, what are the main characteristics of consistency in Lithium-ion battery cells?
1. Capacity Consistency
Just like a chain, which is only as strong as its weakest link, the capacity of the battery module is determined not by the battery cell with the highest capacity, but by the lowest. That is, the inconsistent capacity of battery cells will lead to the over discharge of battery cells with low capacity, accelerating the degradation of battery cell performance and finally affecting the capacity performance of the entire battery module. Therefore, the capacity of battery, closely related to the battery service life, is one of the most vital parameters that influences the battery consistency. Fortunately, there are always ways for things to get better. The initial battery sorting process is able to ensure the consistency of new Lithium-ion battery cells. It is also possible to adjust the initial capacity and narrow the capacity gap by charging and discharging the given battery cells.
2. Voltage Consistency
The reflected voltage differences when testing the open circuit voltage of the battery cells that are charged to the same SOC under the same ambient temperature, is referred as voltage inconsistency of lithium-ion battery cells. As a result, the voltage value of each battery cell in the lithium-ion battery module cannot be exactly the same due to the influence of various production processes, the factors like current, temperature, time difference during the charging and discharging process. So, the improvement of voltage consistency is to improve the concentration of parameters of battery cells.
3. Internal Resistance Consistency
The internal resistance is not only an important characteristic of battery's power, but also one of the reasons for the further different parameters of the lithium battery cells after they are assembled. Because of the existence of internal resistance, part of the electrical power is converted into heat. Hence, the inconsistency of internal resistance of battery will lead to the differences in temperature, which is also the reason causing the differences of other parameters of lithium-ion battery cells.
While flowing through the same current, the battery cell with the higher internal resistance will generate more heat than others, so the deterioration of battery cells will be accelerated with the temperature rise in the battery cells. On the other hand, the deterioration of battery cells will further result in the rise of internal resistance, speeding up the degradation of battery cells with high internal resistance.
4. Temperature Consistency
In addition to the internal resistance factors that directly affect the heating, the inconsistencies formed in the manufacturing process of the electrochemical substances in the lithium battery cells also affect the heat generation. Different positions of each lithium-ion battery cell in the lithium-ion battery module lead to different heat dissipation conditions, which will eventually lead to inconsistent temperature rise of the lithium-ion battery cell. Besides, the temperature distribution within a lithium battery module has an impact on its performance and cycle life. If the imbalance degree of temperature is higher, the inconsistency of depth of discharge will be higher and the cycle life of battery cells will be shorter.
The temperature distribution within a lithium battery module has an influence on its performance and cycle life. The lower the average temperature is, the higher the temperature heterogeneity is, and the higher the discordant discharge depth of the lithium battery cell within the lithium battery module is. The higher the average temperature and the higher the temperature non-uniformity, the shorter the cycle life of lithium battery modules. It is worth noting that uneven temperature distribution will lead to uneven current distribution among parallel branches, which will worsen the consistency of the aging rate of lithium battery cells.
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