What causes the lithium battery to overheat?

　　1. In daily life, everyone will encounter the problem of battery heating, but do you know that battery overheating is due to unreasonable battery selection and thermal design, or external short circuit causing battery temperature rise and loose cable joints, which should be solved from both battery design and battery management. From the perspective of battery material design, materials that can prevent thermal runaway and prevent thermal runaway reaction can be developed; From the perspective of battery management, it can be predicted that different temperature ranges mean different security levels, so as to achieve hierarchical alarm. This year's pure electric bus fire was caused by thermal runaway caused by over charging. Specifically, the battery management system itself lacks the safety function of the overcharge circuit, which causes the BMS of the battery to be 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 through the full 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 with the aging of batteries, the consistency between each battery will become worse and worse, and overcharge is more likely to occur at this time. This requires balancing the entire battery pack to maintain battery pack consistency. For example, a series battery pack adopts the most common battery pack combination method, which is first connected in parallel and then connected in series. After solving the problem of monomer consistency, the best situation is to have the same capacity as the smallest monomer. With this consistency, capacity recovery and overcharging can be prevented. In order to achieve consistency, there must be a method to estimate the capacity of each monomer. Ouyang Minggao suggested that the state of all battery packs can be estimated according to the similarity of the charging curve. In other words, as long as we know the charging curve of one battery, other curves should be similar to it. After the curve changes, they can be approximately identical, and these differences in the curve change process are easy to calculate. Other monomers can be calculated from one monomer. In this way, we can balance the above consistency. Of course, the algorithm takes too long and needs to be simplified. Boeing 787 passenger plane caught fire due to battery explosion. When finding out the cause of the accident, metal objects were found on the electrode and diaphragm, causing internal short circuit. Although experts cannot 100% confirm that the thermal runaway is triggered by internal short circuit, this is the most likely cause, because no other cause can be found, and internal short circuit will not occur. Impurities, metal particles, charge discharge expansion and contraction, lithium evolution, etc. during battery manufacturing may cause internal short circuits. This internal short circuit occurs slowly for a long time, and we do not know when it will leave the thermal control. If the test is conducted, the verification cannot be repeated.

　　3. At present, experts around the world have not found a process that can repeat the internal short circuit caused by impurities, and they are studying this process. To solve the problem of internal short circuit, first find the battery manufacturer with good product quality, and select the battery and battery cell capacity; Secondly, the safety prediction of internal short circuit is carried out. Before thermal runaway, find the monomer with internal short circuit. This means that the characteristic parameters of the monomer must be found, and the consistency can be started first. The batteries are inconsistent, and the internal resistance is also inconsistent. As long as we find a variant monomer in the middle, we can distinguish it. Specifically, except that the parameters of the normal cell and the micro short circuit cell change, the equation forms of the equivalent circuit of the normal battery and the micro short circuit are actually the same. These parameters can be studied to understand some characteristics of internal short-circuit changes. One of the characteristics is the potential difference of the internal short-circuit monomer, and the difference between its internal resistance and other monomers is compared. Ouyang Minggao suggested that R&D personnel use this model for monomer recognition. After measuring the voltage and current of each cell, the internal resistance of each cell can be estimated by using these data and combining with the model. After all parameters of monomer are estimated, it can be judged whether there is significant change in consistency according to the change of parameters. Collision is a typical way of mechanical contact heating out of control. This is the reason for repeated fire accidents in Tesla. Ouyang Minggao revealed that Tsinghua University and MIT jointly analyzed Tesla's collision accident in the United States. If collision simulation is carried out in the laboratory, acupuncture is the closest. Ouyang Minggao also said that he studied the process of thermal runaway through the needle test of ternary lithium ion battery and lithium iron phosphate battery, and found that the exothermic performance of lithium iron phosphate battery in this process of thermal runaway was not as serious as that of ternary lithium ion battery. The experiment shows that different materials will have different reactions during acupuncture, and lithium iron phosphate is relatively safe.

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