Guide: Because too high or too low temperature will directly affect the service life and performance of the power battery, and may lead to safety problems of the battery system, and the long-term uneven distribution of temperature field in the battery box will cause the imbalance of the performance of each battery module and individual, therefore, the battery thermal management system is necessary for the power battery system of electric vehicles.
I. Functions of Power Battery Thermal Management System
Because too high or too low temperature will directly affect the service life and performance of power batteries, and may lead to safety problems of battery system, and the long-term uneven distribution of temperature field in battery box will cause the imbalance of performance between battery modules and cells, therefore, battery thermal management system is necessary for power battery system of electric vehicles. Reliable and efficient thermal management system is of great significance for the reliable and safe application of electric vehicles.
Battery thermal management system has the following five main functions:
Accurate measurement and monitoring of battery temperature.
(2) Effective heat dissipation and ventilation when the temperature of battery pack is too high.
(3) Rapid heating at low temperature.
Effective ventilation when harmful gases are produced.
_Guarantee the uniform distribution of the temperature field of the battery pack.
2. Basic Ways of Heat Transfer in Batteries
There are three main heat transfer modes in battery: heat conduction, convection heat transfer and radiation heat transfer.
The heat exchanged between batteries and the environment is also carried out by radiation, conduction and convection. Thermal radiation mainly occurs on the surface of batteries, which is related to the properties of materials on the surface of batteries.
Heat conduction refers to the heat transfer caused by the direct contact between substances and objects. Electrodes, electrolytes and collectors inside the battery are all heat conduction media. The temperature and environmental heat conduction properties of the interface layer between the battery and the environment determine the heat conduction in the environment.
Heat convection refers to the exchange of heat on the surface of the battery through the flow of environmental medium (usually fluid), which is also proportional to the temperature difference.
For the cell interior, the influence of thermal radiation and convection is very small, and heat transfer is mainly determined by heat conduction. The endothermic capacity of the battery itself is related to the specific heat capacity of the material. The larger the specific heat capacity, the more heat dissipation, the smaller the temperature rise of the battery. If the heat dissipation is greater than or equal to the heat generated, the battery temperature will not rise. If the heat dissipation is less than the heat generated, the heat will accumulate in the battery and the temperature of the battery will rise.
3. Battery Thermal Management System
According to the heat transfer medium, the thermal management system of battery pack can be divided into three types: air cooling, liquid cooling and phase change material cooling. Considering the problems of material development and manufacturing cost, air is the most effective and commonly used heat dissipation system. According to the structure of cooling duct, air cooling system can be divided into two kinds: serial ventilation mode and parallel ventilation mode.
In the serial case, air is usually flowed from one side of the battery pack to the other side, thus achieving the effect of taking away heat. At this time, the airflow will take the heat from the first place to the next place, which will lead to the temperature difference between the two places. In parallel, the air between modules is upright updraft. In this way, the air flow can be distributed more evenly, thus ensuring the consistency of heat dissipation in all parts of the battery pack.
Thermal management system can be divided into passive type and active type according to whether there are internal heating or refrigeration devices. The passive system has low cost and relatively simple facilities. The active system is relatively complex and needs more additional power, but the effect is ideal.
Although air is cooled and heated by automotive air conditioning or heating systems, it is still considered a passive system.
With this passive system, due to the inconsistency of the temperature introduced into the ambient air, the ambient air must be in a certain temperature range (10-35 C) in order to conduct normal thermal management, and the operation of battery packs under extreme cold or extreme hot conditions may result in greater unevenness.
Thermal management system of power battery is a very important part of electric vehicle, which affects power consumption, endurance, battery life and so on.