Who "frozen" my battery? Talking about the New Energy Vehicle Battery Thermal Management System

When it comes to pure electric vehicles, the biggest problem is probably the range. Friends who drive new energy electric vehicles in the north know that the mileage will be greatly reduced in winter. As a result, he did not dare to turn on the air conditioner for heating, and could only endure the severe cold and shiver in the compartment. This directly leads to an extremely poor experience with new energy vehicles. To solve this problem, we have to rely on the battery management system. Today I will talk to you about this topic.    

What is a battery management system

 

The main task of the battery management system is to ensure that the battery pack works in a safe area, provide the necessary information for vehicle control, respond to processing in the event of an abnormality, and determine the charge and discharge power of the battery based on ambient temperature, battery status, and vehicle requirements Wait. The main functions of BMS are battery parameter monitoring, battery status estimation, online fault diagnosis, charge control, automatic equalization, and thermal management.  

  The importance of thermal management systems

  Thermal issues of a battery are key factors that determine its performance, safety, life, and cost of use. First, the temperature level of a lithium-ion battery directly affects its energy and power performance in use. When the temperature is low, the available capacity of the battery will rapidly decay. Charging the battery at an excessively low temperature (such as below 0 ° C) may cause an instantaneous voltage overcharge phenomenon, causing internal lithium evolution and further short circuit. . Second, thermal issues related to lithium-ion batteries directly affect battery safety. Defects in the manufacturing process or improper operation during use may cause local overheating of the battery, and then cause a chain of exothermic reactions, eventually causing serious thermal runaway events such as smoke, fire, and even explosion, threatening the lives of vehicle drivers and passengers Safety. In addition, the operating or storage temperature of a lithium-ion battery affects its service life. The proper temperature of the battery is between 10 ~ 30 ° C. Too high or too low temperature will cause the battery life to decay faster. The large size of the power battery makes the ratio of its surface area to volume relatively small. The internal heat of the battery is not easy to dissipate, and the problems of uneven internal temperature and excessive local temperature rise are more likely. Total cost of ownership.  

  The battery thermal management system is one of the key technologies to deal with the thermal related problems of the battery and ensure the performance, safety and life of the power battery. The main functions of the thermal management system include:

 

1. Effective heat dissipation when the battery temperature is high to prevent thermal runaway accidents;

 

2. Warm up when the battery temperature is low to increase the battery temperature to ensure charging and discharging performance and safety at low temperatures;

 

3. Reduce the temperature difference in the battery pack, suppress the formation of local hot zones, prevent excessive decay of the battery at high temperature locations, and reduce the overall battery life.

  The temperature environment in the battery pack (PACK) has a great impact on the reliability, life and performance of the battery cell. Therefore, it is particularly important to maintain the temperature within the PACK within a certain temperature range. This is mainly achieved through cooling and heating, and its cooling methods are mainly divided into three categories:     Air-cooled:   Air-cooled is a kind of heat dissipation method that uses low-temperature air as a medium to reduce the temperature of the battery by using hot convection, which is divided into natural cooling and forced cooling (using fans, etc.). This technology uses natural wind or fans to cool the battery with the evaporator that comes with the car. The system has a simple structure and is easy to maintain. It is also widely adopted in current electric buses and electric logistics vehicles.    

Liquid cooling:

  Liquid cooling technology uses liquid convection heat transfer to remove the heat generated by the battery and reduce the battery temperature. The liquid medium has a high heat exchange coefficient, a large heat capacity, and a fast cooling speed. It has a significant effect on reducing the maximum temperature and improving the consistency of the temperature field of the battery pack. At the same time, the volume of the thermal management system is relatively small. Liquid cooling systems are more flexible: you can immerse battery cells or modules in liquid, you can also set cooling channels between battery modules, or use cooling plates at the bottom of the battery. When the battery is in direct contact with the liquid, the liquid must be insulated (such as mineral oil) to avoid short circuits. At the same time, the air-tightness requirements of the liquid cooling system are also high. In addition, it is mechanical strength, vibration resistance, and life requirements. Liquid cooling is currently the preferred solution for many electric passenger cars. Typical products at home and abroad such as BMW i3, Tesla, GM Volanda, Geely Emgrand EV.  

  Direct cooling:

  Direct cooling (direct cooling of refrigerant): Use the principle of refrigerant (R134a, etc.) to evaporate latent heat, establish an air-conditioning system in the entire vehicle or battery system, install the evaporator of the air-conditioning system in the battery system, and cool The agent evaporates in the evaporator and quickly and efficiently removes the heat from the battery system, thereby completing the cooling of the battery system. At present, the direct cooling method is basically used on electric passenger cars. The most typical one is the BMW i3 (i3 has two cooling solutions: liquid cooling and direct cooling).  

Write at the end  

The temperature of the battery in a pure electric vehicle directly affects the safety of the battery, so the thermal management system of the battery is the link we should pay most attention to. This will also become one of the focuses of future optimization of electric vehicles.