Research On The Key Technologies Of Electric Vehicle Battery Management In High Latitude And Cold Region

Power battery system is the power source of the Electric Vehicles(EVs), and is the key part of EVs. The complex battery internal electrochemical reaction is sensitive to temperature. However, the climate of plateau-cold region in winter is cold, and the temperature varies obviously between summer and winter, which is a challenge for EV battery management. Reasonable and effective battery management is the foundation for service efficiency improvement, battery life extending, operation safety, and system energy management strategy optimization. Nevertheless, in high temperature environment battery ages accelerately and causes capacity loss. And in low temperature environment, electrochemical reaction rate decreases, the amount of lithium ion reduces, the internal resistance increases, which results in the degeneration of pack capacity and power characteristics. Moreover, lithium deposition will penetrate the membrane to lead to interna l short circuit during charing, which threatens the safety. Meanwhile, along with cycling, there inevitably will be SOC imbalance between cells caused by inconsistency, which affects power and capacity characteristics, as well as thermal and temperature consistency that leads to aging and coulomb efficiency inconsistency and further limit the battery performance. The above problems greatly restrict the performance of battery system and increase the risk and cost of usage.Aiming the problem above, this thesis is meant to solve the problem of pack performance degeneration caused by inherent characteristic inconsistency and external environmental factors, as well as the heat generation and temperature inconsistency caused by SOC inconsistency. This thesis studies the attenuation mechanism theoretically and experimentally, and the low temperature thermal management strategy is developed according to the operation characteristics. The variable power control method is applied in low temperature preheating mode to enhance battery performance, and the internal temperature and peak power state are monitored in working conditions after preheating. Aiming at the problem of pack performance degeneration, as well as the heat generation and temperature inconsistency, various kinds of inconsistency are simulated to study the impact on pack power and capacity. Among them, SOC inconsistency influences the pack performance significantly. To solve this problem, the battery equalization system and strategy are designed to improve the SOC consistency and alleviate the impact on pack performance and thermal consistency. Specific research contents are as follows:For the performance degradation mechanism of EV battery pack, the impact of operation temperature and battery characteristic inconsistency on pack performance is analyzed from the view of theory, and the experimental results verify the influence of temperature on internal resistance, open circuit voltage, discharge capacity and peak power. And the results serve as a basis for the design of low temperature thermal management system. It also establishes battery characteristics inconsistency analysis model to theoretically analyze the influence factor of inconsistency such as, resistance, open circuit voltage and SOC on terminal voltage, pack capacity and peak power degeneration. Apart from this, pack capacity and power fading rate are defined to evaluate the influence of different kinds of inconsistency. Besides, the heat generation and temperature difference caused by SOC imbalance are analyzed, and the impact of temperature inconsistency on cell aging and pack capacity decreases is explored.The peak power and internal temperature and peak power are not measurable in the process of operation. To solve the problem of peak power estimation, battery voltage and current extended Kalman filter are applied to online estimation of the equivalent circuit model parameters. And the battery peak power is estimated according to the voltage and current limit of battery. To solve the problem o f internal temperature estimation, the internal temperature estimation model is established, the impact of entropy change and overpotential on heat generation is analyzed. Moreover, the influence of the two kind of heat generation in constant rate charging and discharging is analyzed. Furthermore, the internal temperature estimation method based on self heat generation is proposed to achieve accura te estimation using battery surface and ambient temperature.For the low temperature thermal management control strategy of series connected batteries, this thesis proposes preheating mode and operation condition monitoring mode according to the working state analysis. The preheating model is analyzed, and the target preheating temperature determination method is given. Based on the analysis above, the fuzzy control method of variable power preheating based on proportional factor self correction is proposed, which can improve battery performance and decrease energy consumption by adjusting preheating power according to the maximum surface temperature difference and average battery temperature. In the working state monitoring mode, the internal temperature and peak power are estimated during operation. The peak power estimation method can achieve accurate estimation for battery peak power ability, and the internal temperature estimation can accurately reflect the change of battery internal temperature.At last, for the problem of pack performance degeneration and thermal inconsistency, as well as the thermal and temperature inconsistency that caused by the SOC imbalance in series connected batteries, this thesis proposes a bidirectional active equalization structure based on SOC, which achieves large current and bidirectional SOC equalization between single cell and battery module. On the basis of this, an optimal equalization strategy considering consumed time and SOC uniformity is proposed, which can formulate equalization steps and time according to the initial SOCs of the batteries. The optimal path, time and SOC dis tribution are selected, which avoids energy consumption owing to back and forth equalization.