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Analysis of causes of spontaneous combustion of electric vehicles

Battery cells quality problem

First of all, the internal structure of lifepo4 battery is complex, consisting of a number of key components such as positive electrode material, negative electrode material, electrolyte and diaphragm. As an important component inside the battery, the diaphragm mainly plays the role of isolating the positive and negative electrodes and preventing the internal short circuit of the battery. However, in the battery manufacturing process, if there are defects in the diaphragm, such as uneven thickness, pinholes or damage, it may lead to an internal short circuit during the battery charging and discharging process. When there is a short circuit inside the battery, a large amount of current will pass through in a very short time, resulting in high temperature. This high temperature environment will accelerate the decomposition of the electrolyte, producing a large amount of gas and heat. If the heat inside the battery cannot be dissipated in time, it will cause the battery temperature to rise sharply, and even trigger a combustion reaction.


In addition, the unqualified quality of the cell may also be manifested as the bending deformation of the pole sheet, the uneven coating, the existence of metal or non-metallic foreign bodies and other problems. These issues can also affect battery performance and safety. For example, the bending deformation of the electrode plate may lead to poor contact between the electrode plates during the charging and discharging process, which affects the capacity and cycle life of the battery. Uneven coating may lead to uneven distribution of current on the electrode surface of the battery during charging and discharging, resulting in local overheating. The presence of metallic or non-metallic foreign bodies may directly lead to a short circuit inside the battery or puncture the diaphragm, causing serious safety problems. Therefore, in order to ensure the safety and reliability of lithium-ion batteries, manufacturers need to strictly control the production process and quality testing of batteries. This includes selecting high-quality raw materials, optimizing production processes, and strengthening quality inspection and control.


Battery pack safety structure design

If the electric vehicle is impacted by external forces, such as collisions, falls, etc., its overall structure may be seriously damaged, including key parts such as frame and battery warehouse. The impact force generated by the impact may cause the battery housing to rupture, internal electrolyte leakage, and even short circuit inside the battery. The interior is equipped with "cross reinforcement beams", plus beams at the front and rear ends of the battery pack housing, to protect the cell from being squeezed in the event of a collision. The inside of the battery is filled with batteries, there is no space to design a separate strengthening beam, and the electric vehicle is hit by external forces, and the natural risk is increased.

The larger the gap between the cell and the shell, the buffer zone can be formed. The cell is not easily squeezed.

The cooling system is an important part to ensure the normal operation of the battery, if the design of the cooling system is unreasonable or failure, can not effectively dissipate the heat generated by the battery in a timely manner, it will lead to the battery temperature is too high, causing spontaneous combustion. BMS cooling management represented by Tesla: Each battery unit is facing the coolant pipe, and the pipe is coiled around the battery like a snake to provide more heat transfer area, and high temperature and low temperature can be controlled. Today, most high-end domestic new energy vehicles are using this design.

Heat sink liquid cooling represented by the Chevy Bolt: Heat sink increases surface area to increase heat transfer rate. Heat is transferred from the battery pack to the fins by conduction, carrying the heat away by liquid and air, and the heat sink is very thin, only 1 mm thick. And when the battery gets too cold, the heating coils heat up the entire system.


3. Battery Management System (BMS)

Battery management systems (BMS) play a crucial role in electric vehicles and energy storage systems. It is responsible for monitoring the battery's voltage, temperature, current and other key parameters, and according to these parameters to adjust the battery charging and discharging process to ensure the safe and efficient operation of the battery. If the BMS fails, the consequences can be catastrophic. First, the BMS obtains information such as voltage, temperature and current of each cell in the battery pack in real time through the sensor network. This data is the basis for assessing battery status, predicting potential problems, and developing appropriate control strategies. When the BMS works normally, it can detect the abnormal state of the battery in time, such as the temperature of the single battery is too high, the charging current is too large, and take corresponding measures to prevent the problem from worsening. However, if the BMS fails (or there are fewer sensors in the battery pack), it may not be able to accurately acquire or process this data. This means that the abnormal state of the battery may not be detected in time, increasing the risk that the lifepo4 battery will enter a thermal runaway state. Thermal runaway is a runaway reaction caused by the accumulation of heat inside the battery, which may cause spontaneous combustion or explosion of the battery.


Specifically, if the BMS is unable to monitor that the temperature of a particular cell is too high, then the cell may continue to heat up until it reaches the critical point of thermal runaway. Similarly, if the BMS cannot limit the charging current to a safe range, then too much current may cause too much heat to be generated inside the battery, which in turn triggers thermal runaway. In addition, the failure of the BMS may also affect the battery balance. The performance and status of each battery pack may vary. BMS tries to keep these cells as balanced as possible by regulating the charge and discharge process to ensure the performance and safety of the entire battery pack. If the BMS fails, the battery balance can be disrupted, further increasing the risk of battery pack failure. Therefore, in order to ensure the safety and reliability of the battery system, great attention must be paid to the reliability and stability of the BMS. Manufacturers should continuously optimize the design and algorithms of BMS to improve their fault detection and handling capabilities. At the same time, the user should also check and maintain the BMS regularly to ensure that it works normally. The battery pack shell has a pressure relief explosion-proof valve. If there is thermal runaway inside the battery pack and a large amount of gas is generated, the valve will automatically burst to remove the pressure inside the battery and avoid explosion.


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