What causes lithium batteries to overheat?

　　1, in our daily life, we will encounter the problem of battery heating, but do you know that the battery overheating is because the choice of battery and thermal design is unreasonable, or external short circuit caused by battery temperature rise and cable connector loose, should be solved from the battery design and battery management. From the point of view of battery material design, materials can be developed to prevent thermal runaway and prevent thermal runaway reactions; From the point of view of battery management, it can be predicted that different temperature ranges mean different safety levels, so as to achieve hierarchical alarm. This year's all-electric bus fires were caused by thermal runaway from overcharging. Specifically, the battery management system itself lacks the safety function of the overcharging circuit, resulting in the BMS of the battery being out of control but still charging.

　　2, for the reason of this overcharging, the first solution is to find the fault of the charger, which can be solved by the complete redundancy of the charger; The second is to see whether the battery management is reasonable. For example, the voltage of each battery is not monitored. It is worth noting that as batteries age, the consistency between each battery will become worse and worse, and overcharging is more likely to occur at this point. This requires balancing the entire battery pack to maintain consistency. For example, a series battery pack that uses the most common combination of battery packs, first in parallel and then in series, will have the same capacity as the smallest cell after solving the consistency problem. With this consistency, capacity recovery and overcharging are prevented. To achieve consistency, there must be a way to estimate the capacity of each monomer. Ouyang suggested that the state of all battery packs could be estimated based on the similarity of charging curves. In other words, as long as we know the charging curve of one battery, the others should be similar. After the curve changes, they can be approximated, and these differences in the curve change process are easy to calculate. The other monomers can be calculated from one monomer. In this way, we can balance the above consistency. Of course, this algorithm takes too long and needs to be simplified. The Boeing 787 caught fire after a battery exploded. When the cause of the accident was ascertained, it was found that metal objects were attached to the electrodes and diaphragm, causing an internal short circuit. Although experts cannot be 100% sure that the thermal runaway was triggered by an internal short circuit, it is the most likely cause because no other cause can be found and an internal short circuit will not appear. Impurities, metal particles, expansion and contraction of charge and discharge, lithium evolution, etc. in the battery manufacturing process may lead to internal short circuit. This internal short circuit occurs slowly over a long period of time, and we do not know when it will escape thermal control. If a test is performed, the validation cannot be repeated.

　　At present, experts around the world have not found a process that can repeat the internal short circuit caused by impurities, and they are all studying this process. To solve the problem of internal short circuit, first of all, we must find the battery manufacturer with good product quality, choose the battery and battery unit capacity; Secondly, the safety prediction of internal short circuit is made. Before thermal runaway occurs, the internal short-circuited monomer should be found. This means that the characteristic parameters of the monomer must be found, and the consistency can be started first. The batteries are inconsistent, the internal resistance is inconsistent. As soon as we find a monomer with a mutation in it, we can tell it apart. Specifically, the equation form of the equivalent circuit of the normal cell and the equivalent circuit of the micro short circuit are virtually the same, except that the parameters of the normal and micro short circuits are changed. These parameters can be studied to understand some characteristics of internal short-circuit variation. One of the characteristics is the potential difference of the internal short circuit monomer, and the difference of its internal resistance is compared with other monomers. Ouyang suggested that researchers use the model for monomer identification. After measuring the voltage and current of each monomer, the internal resistance of each monomer can be estimated using these data and combined with the model. After estimating all the parameters of the monomer, we can judge whether there is a significant change in consistency according to the change of parameters. Collision is a typical way that mechanical contact heating gets out of control. This is why Tesla has had repeated fire accidents. Ouyang revealed that Tsinghua University and the Massachusetts Institute of Technology jointly analyzed Tesla crashes in the United States. If you do a collision simulation in the lab, the closest thing is acupuncture. Ouyang Minggao also said that through the acupuncture test of ternary lithium ion battery and lithium iron phosphate battery, the thermal runaway process was studied, and found that the exothermic performance of lithium iron phosphate battery in the process of thermal runaway is not as serious as ternary lithium ion battery. Experiments have shown that different materials have different reactions during acupuncture, and lithium iron phosphate is relatively safe.

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