The accumulation of heat is an important cause of battery system failures and safety accidents. The thermal management system of the battery pack is an indispensable part of the battery management system. Its research and development is a key part of modern electric vehicles and an important means to improve the performance of the whole vehicle. The thermal management of the battery system is to control the temperature of the battery system by cooling or heating. Since the environment and its own temperature of the power battery system directly affect its normal operation, cycle life, charge acceptability, output power, available energy, safety and reliability, in order to achieve the best performance and longevity of the battery system, It is necessary to introduce low-temperature heating, high-temperature heat dissipation and heat preservation management of the battery by introducing a thermal management system, limiting the temperature rise of the battery and the temperature difference, thereby achieving uniformization of the battery pack temperature, ensuring that the battery operates in a suitable temperature range, and reducing the battery performance attenuation speed and Eliminate related potential security risks. The temperature is adjusted and controlled by the thermal management system, so that the power battery is always in the proper temperature range during operation, improving the performance and efficiency of the power battery system, prolonging its service life, reducing the cost of the electric vehicle, and securing the electric vehicle. The safe use and other aspects have important practical significance.
The temperature is very significant for the overall performance of the lithium-ion battery, including the battery's tolerance, power, charge and discharge efficiency, safety and longevity. Since the power battery has a suitable operating temperature range, the activity of the internal active material increases as the temperature increases, and the charge and discharge efficiency of the battery increases accordingly. However, when the temperature exceeds a certain range, if the temperature is too high, the internal side reaction of the battery is accelerated, and these side reactions consume lithium ions, a solvent, an electrolyte, and the like, resulting in deterioration of battery performance. Similarly, if the temperature is too low, the activity of the internal active material of the battery is significantly reduced, and the internal resistance and polarization voltage are increased, and the charge and discharge power and capacity are significantly reduced, and even the battery capacity is irreversibly attenuated, and a safety hazard is buried.
Since the lithium ion power battery has a certain internal resistance and various degrees of electrochemical reactions occur at any time, in addition to the influence of the ambient temperature, the performance of the lithium ion power battery is also affected by the temperature rise and temperature difference caused by its own heat generation. The research shows that the heat generation rate of lithium-ion battery has a quadratic curve relationship with its working current, and the heat generation rate increases sharply with the increase of current, especially when the temperature rises greatly during high-rate charging. When the ambient temperature is high or when charging or discharging at a large rate, the corresponding heat dissipation measures must be taken. At the same time, the charge and discharge power is limited, and the temperature rise level is controlled. Otherwise, the battery may be degraded due to overheating, the life is shortened, or even the life is shortened. The dangerous state of thermal runaway. In a low temperature environment, although during the small current charging and discharging process, a certain amount of heat can be generated due to the internal resistance of the battery, and the heat generated by the battery discharge is insufficient to maintain the battery at a relatively high temperature, so in a low temperature environment, the battery The charge and discharge power performance is limited, and the power battery needs to be preheated to maintain the performance of the battery.
In addition, the uneven distribution of the temperature field inside the battery case for a long time will also cause imbalance in the performance of each battery module and unit. Especially, the aging rate of the battery distributed in the high temperature region will be significantly faster than the low temperature portion, accumulating with time. The difference in physical properties between different batteries will become more apparent, resulting in poor consistency between the batteries, and even premature failure, shortening the life of the entire power battery system.
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