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

What is a battery management system

The important task of the battery management system is to ensure that the battery pack works within a safe range, supply the necessary information required for vehicle control, respond in time when an abnormality occurs, and determine the charging and discharging power of the battery according to the ambient temperature, battery status, and vehicle needs. wait. The important functions of BMS include battery parameter monitoring, battery state estimation, online fault diagnosis, charging control, automatic equalization, thermal management, etc.

The Importance of Thermal Management Systems

The heat-related problems of batteries are the key factors that determine their performance, safety, life and cost. First, the temperature level of a Li-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 extremely low temperature (such as below 0°C) may cause instantaneous voltage overcharge, resulting in internal lithium precipitation and a short circuit. . Second, the heat-related issues of Li-ion batteries directly affect the safety of the batteries. Defects in the manufacturing process or improper operation during use may cause local overheating of the battery, which in turn causes a chain exothermic reaction, eventually causing serious thermal runaway events such as smoke, fire or even explosion, threatening the lives of vehicle drivers and passengers Safety. In addition, the operating or storage temperature of lithium-ion batteries affects their service life. The suitable temperature of the battery is about 10~30°C, too high or too low temperature will cause a rapid decay of battery life. The large-scale power lithium battery makes the ratio of its surface area to volume relatively smaller, and the internal heat of the battery is not easily dissipated, and problems such as uneven internal temperature and excessive local temperature rise are more likely to occur, thereby further accelerating battery attenuation and shortening battery life. increase the user's total cost of ownership.

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

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

2. Preheat the battery when the battery temperature is low, increase the battery temperature, and ensure the charging and discharging performance and safety at low temperature;

3. Reduce the temperature difference in the battery pack, suppress the formation of local hot spots, prevent the battery from decaying too quickly at high temperature locations, and reduce the overall life of the battery pack.

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 in the pACK within a certain temperature range. This is mainly achieved by cooling and heating, and its cooling methods are mainly divided into three categories:

1. Air-cooling: Air-cooling is a heat dissipation method that uses low-temperature air as the medium and uses heat convection to reduce the temperature of the battery. It is divided into natural cooling and forced cooling (using fans, etc.). This technology uses natural wind or a fan to cool down the battery with the evaporator that comes with the car. The system has a simple structure and is easy to maintain. It was widely used in early electric passenger cars, such as Nissan Leaf, Kia Soul EV, etc. It is also widely adopted in current electric buses and electric logistics vehicles.

2. Liquid cooling: Liquid cooling technology uses liquid convection heat exchange to take away the heat generated by the battery and reduce the battery temperature. The liquid medium has high heat transfer coefficient, large heat capacity, and fast cooling speed, which 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. The form of the liquid cooling system is relatively flexible:   The battery cells or modules can be immersed in liquid, cooling channels can also be set between battery modules, or cooling plates can be used 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 prevent short circuits. At the same time, the airtightness requirements of the liquid cooling system are also relatively high. In addition, it is mechanical strength, vibration resistance, and life requirements.  Liquid cooling is currently the preferred method for many electric passenger vehicles, typical products at home and abroad such as BMW i3, TSLA, GM Volt, Geely Emgrand EV.

3. Direct cooling: direct cooling (refrigerant direct cooling): use the principle of latent heat of evaporation of refrigerants (R134a, etc.) to build an air-conditioning system in the vehicle or battery system, install the evaporator of the air-conditioning system in the battery system, and refrigerate 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 cooling method through direct cooling is basically used in electric passenger cars, the most typical one is BMW i3 (i3 has two cooling methods, 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 that we should pay the most attention to. This will also become one of the key points in the optimization of electric vehicles in the future.