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Last time we talk about the reason why lithium-ion battery are small. The short answer is safety concern. Nowadays, lithium-ion batteries are used in energy storage system, electric vehicles, etc which requires high energy. Given that the high energy requirement of some applications, is there hope? Actually, in order to reach high capacity, lithium-ion battery cells can be assemble together as battery packs. At this point, you may wonder if the performance of one Li-ion battery cells is inconsistent from the others in the lithium-ion battery pack, will it damage the performance of Lithium-ion battery packs? The answer is yes. The consistency of lithium-ion battery cells for the battery pack is the key factor of battery performance. Let’s explain further below.
What will happen if the Li-ion cells are inconsistent?
Generally, we rarely think about the consistency of battery cells when lighting up the torch with batteries. We just connected the positive and negative electrodes. However, things could be complicated when lithium batteries are used in large scale. The inconsistency of Lithium-ion battery mainly refers that the capacity, internal resistance and open-circuit voltage of the Li-ion battery cells are not consistent. If the inconsistent battery cells are used together, it may lead to the following problems.
1. Capacity Loss
Battery cells is the key component of Lithium-ion battery module. the The capacity of the Lithium-ion battery module match the condition of“Wooden Barrel Theory”- the capacity of a barrel is determined not by the longest wooden bars, but by the shortest. That is, the capacity of the worst battery cell will determines the capacity of the entire battery pack. Just like a chain, which is only as strong as its weakest link.
At the heart of each battery pack, there is a Battery Management System(BMS) to ensure the battery safety.
In order to prevent the battery from over charging and over discharging, if one Li-ion battery cell reaches the discharge cut-off voltage first during discharging, the entire battery pack will stop discharging. On the other hand, if one Li-ion battery cell reaches the charge cut-off voltage when charging, the entire battery pack will stop charging.
For instance, when two batteries are in series, it means that the current pass through the batteries are the same. Suppose we have two battery cells with different capacity: One has a capacity of 1C, the capacity of another one is 0.9C. During the charging, the battery cell with less capacity is bound to be fully charged and reaches ahead of the charging cut off condition, so the entire battery pack will no longer charge and the battery cell with 1C will not be fully charged. Similarly, the battery with 0.9C will use up all the energy storing in the battery quicker than the battery with 1C during the discharging. Therefore, the battery cell with 0.9C is always fully charged and discharged, however, the battery cell with 1C has been in partial capacity and not fully used. As a result, the capacity of the Li-ion battery pack cannot reach the design maximum capacity.
2. Shorten Lifespan
The lifespan of the Li-ion battery pack is determined by the battery cells with the shortest lifespan. Most likely, the battery with the shortest life is the battery with the smallest capacity. If the capacity of one battery cell is smaller than the others, it is most likely to die first due to being fully charged and discharged every time. The lifespan of one battery cell has a significant effect on the entire battery module. When one battery cell end up its lifespan, the whole battery module will not work.
3. Increased Internal Resistance
When the same current flow through the battery cells with different internal resistance, the Li-ion battery cell with the big internal resistance will relatively generate more heat. With a higher temperature, it may lead to the accelerated rate of battery degradation, so the internal resistance of the battery cells will rise further. Consequently, the battery cell with high internal resistance will suffer a accelerated battery degradation and cause a bad effect on the entire battery module.
In summary, the inconsistent performance of the battery cell is happen in the production process and may get worse during use. In the same Li-ion battery module, the battery cell with weak performance will speed up getting worse and finally affect the entire battery module and shorten its life. Hence, the consistency of battery cells for the same battery module is of great importance to improve the Lithium-ion battery performance and last longer.
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