As the main energy storage form of electric vehicles, the performance of power battery directly restricts the power, economy and safety of electric vehicles. Compared with other types of batteries, lithium-ion power batteries have great advantages in terms of energy density, power density and service life, making them the mainstream of current vehicle power batteries. But its performance, life and safety are closely related to temperature. As many studies have pointed out, temperature is one of the most important factors in battery design and operation. If the temperature is too high, the side reactions of the battery will be accelerated and the performance of the battery will be attenuated, and even lead to safety accidents. Therefore, it is very necessary to study the thermal management system (BTMS) of the battery.
The battery thermal management system obtains the temperature of battery cells at different locations through temperature measuring elements. Accordingly, the control circuit of the thermal management system needs to make the action decisions of the cooling actuators such as fans and water/oil pumps. At present, the temperature sensors of common power battery packs are mostly attached to the inner surface of the battery box or the outer surface of the battery cell. For example, in the third-generation Prius battery pack in 2010, part of the temperature sensor is arranged in the flow channel inside the battery pack; the other part is directly attached to the middle of the upper surface of the cells in some typical positions, and these cells are located in the front of the battery pack. top, middle and rear. The battery thermal management system usually performs hierarchical management according to the temperature region where the battery is located. Volt plug-in hybrid battery thermal management is divided into active (active), passive (passive) and non-cooling (bypass) three modes: when the power battery temperature exceeds a preset passive cooling target temperature, passive cooling mode starts ; and when the temperature continues to rise above the active cooling target temperature, the active cooling mode is activated. However, this is still an extensive control strategy, which leads to a larger safety margin of the battery and reduces the efficiency of the battery.
Next, an efficient and sophisticated battery thermal management system is investigated. The maximum operating time or accumulated time that the battery management system can provide the required power after being fully charged is used as the evaluation index of efficiency. as an indicator of reliability. The hardware and software relationship for integrating and coordinating battery temperature management is shown in Figure 1. For the hardware part, on the basis of understanding the influence of battery thermal properties and external temperature and pressure conditions on battery performance, by calculating these nonlinear physical properties and abstracting these features in cyberspace, an ideal solution to reduce the safety margin is developed accordingly. temperature management system, thereby improving the efficiency of the entire battery system of pure electric vehicles.
Compared with the common temperature control system, the temperature is selected as the control node to realize dynamic control, and the influence of the thermal properties of the battery and the electrical state of the battery on the efficiency and reliability is analyzed. The cell-level thermal control is adopted as the control strategy of the battery thermal management system. Temporarily boost the performance of the cell layer when high power is required, while sleeping other layers to reduce stress, and the associated model is validated.
The main research contents include the following three points:
(1) Using cyberspace to summarize thermal properties to solve problems related to the efficiency and reliability of temperature management systems.
(2) Design the battery temperature management system, and systematically study the use of temperature as the control core of the temperature management system.
(3) The temperature management system used in the in-depth evaluation proves that it can effectively improve efficiency without sacrificing reliability.