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The issue of safety can be summarized as thermal runaway of power lithium batteries, which means that when the battery is heated to a certain temperature, it becomes uncontrollable. The temperature rises linearly, exceeding 500 ℃ or 1000 ℃, and then it will burn and explode. Why is there this problem? Firstly, overheating can cause this problem. As the temperature rises, it will trigger side reactions in the battery. As the temperature increases, a series of side reactions will occur in our battery, all of which will release heat, leading to heat loss. Another reason, triggered by electricity, such as external short circuits, internal short circuits, and overcharging, can lead to heat generation and then thermal runaway. Another reason is collisions, such as vehicle collisions and squeezing. After being squeezed, it feels like a needle pricking a battery, then a short circuit occurs, triggering electricity, generating heat, and then heat causes thermal runaway. As the temperature increases, different side reactions will be triggered, such as the reaction between the graphite negative electrode and the electrolyte, the decomposition of the electrolyte, large-scale internal short circuits, etc., leading to the final combustion of the electrolyte, and ultimately losing control of heat, resulting in a very fast heat release rate. After a thermal runaway, it will cause its propagation within a battery, and it will quickly spread throughout the entire battery pack.

The reasons and solutions for overheating include unreasonable battery selection and thermal design, temperature rise caused by external short circuits, or loose cable joints. There are two solutions: one is battery design, and the other is battery management. For example, we can develop materials to prevent thermal runaway and block reactions caused by thermal runaway. In addition, from the perspective of battery management, we can predict different temperature ranges to define different safety levels. For example, at different stages, we can calculate its temperature to determine the different safety levels and conduct graded alarms. The second issue is related to thermal runaway triggered by overcharging. For example, the combustion of electric buses in the previous stage was the cause. It was ultimately discovered that the battery management system itself did not have functional safety for the overcharged battery management system circuit, resulting in the BMS of the battery being out of control and still charging.

As the battery ages, the consistency between each battery will become worse, and of course, overcharging is more likely to occur. Another issue is that the capacity of the battery pack decreases faster than that of the individual cells. So we must understand this process, and on this basis, we need to balance the entire battery pack to maintain consistency, which is a principle of balance. You can see that for a series connected battery pack, the most common method of battery pack combination is to first connect in parallel and then connect in series. We can see that the final result is that the capacity of the battery pack is the smallest capacity monomer, which is the maximum battery pack capacity we can achieve. That is to say, the capacity of a series battery pack is actually the same as the capacity of the smallest capacity monomer. With this consistency, our capacity has also rebounded, and overcharging can also be prevented. In order to achieve this consistency, we must estimate the capacity of each individual cell, and there must be a method to estimate the capacity. This is to estimate the state of all battery packs based on the similarity of the charging curve. This means that as long as we understand the curve of one cell's battery, the other curves should be similar to it. After the curve changes, they can approximately overlap, The differences in the process of curve changes are easy to calculate, which means that other monomers can be inferred from one monomer. With this method, we can perform the consistency balancing we just mentioned, but of course, this algorithm takes too long, so it needs to be simplified.

Regarding the thermal runaway triggered by internal short circuits, such as the Boeing 787 incident, the final cause was found to be metal objects on the electrodes and membranes. With internal short circuits, it cannot be 100% confirmed that this thermal runaway was triggered by internal short circuits, but it is the most likely cause because no other cause can be found and internal short circuits cannot emerge. What are the reasons for internal short circuits? There are three types: one is impurities in battery manufacturing, metal particles, and the other is contraction caused by charging and discharging expansion. Internal short circuits occur slowly and for a very long time, and you don't know when it will cause thermal runaway. Moreover, this experiment cannot be repeated, and we have not yet found a way to replicate the process of internal short circuits caused by impurities.

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