What is battery thermal management system and in-depth research

1. Battery thermal management system requirements
2. Common battery thermal management system means
   1. Battery cooling technology
   2. Battery heating technology
3. Research on battery thermal management system
   1. Research on battery operating conditions
   2. Analysis and design of heat pipes
4. Summary and outlook

The power battery is an important part of the electric vehicle, and its performance directly restricts the power, safety and economy of the whole vehicle. The energy density of the power battery determines the cruising range of the electric vehicle, the power density determines the maximum grade and the maximum speed, and the cycle life and economical use of the vehicle.

The electrical/thermal safety and environmental adaptability of the power battery determine the key factor of vehicle safety and environmental adaptability. It is necessary to design a reasonable system structure of the battery thermal management system and develop an advanced thermal management control strategy to make the power battery work within a suitable temperature range and effectively control the cells.

The lithium-ion battery management system is an electrochemical power source with complex flow and heat transfer processes, and temperature is a key factor affecting its performance, which is mainly reflected in three aspects:

1. The increase in temperature will aggravate the decline of battery capacity, and excessive temperature may even cause thermal runaway;
2. If the temperature is too low, the battery power and capacity will be significantly attenuated, and the charging and discharging efficiency will decrease;
3. The temperature difference between different cells in the battery pack will lead to inconsistency and uneven speed of the internal resistance and capacity of the battery. Aging will form a short board in the performance and life of the entire battery system.

Therefore, the working performance of the power battery is greatly affected by temperature. The temperature difference between the two, thereby improving the performance of the power battery.

Battery thermal management system requirements

The essence of the charging and discharging process of lithium-ion batteries is ion migration and chemical reaction, and Li+ is intercalated and deintercalated in layered carbon materials and metal oxides. Under normal working conditions, the heat source of the battery includes ohmic heat, electrochemical reaction heat and polarization heat.

As the temperature increases, a series of exothermic chemical reactions occur inside the battery, including the decomposition of the electrolyte, the thermal decomposition of the negative electrode, the reaction between the negative electrode and the electrolyte, and the decomposition of the SEI film. Excessive temperature may lead to thermal runaway.

Temperature causes changes in electrochemical performance, which affects battery performance and life. When the temperature increases, the electrochemical reaction rate increases, which intensifies the battery capacity decay. The low temperature environment will also cause the performance of the battery to deteriorate, the mobility of lithium ions in the electrode active material is weakened, and the charge and discharge capacity decreases rapidly.

In addition, too high or too low temperature will accelerate battery aging and affect battery life. Especially in the case of high rate charge and discharge, the effect of temperature on battery life is more significant.

According to the data, the capacity loss of Sony 18650 lithium battery is 30% after 800 cycles at 25°C, while the capacity loss is close to 60% after 800 cycles at 50°C. Too high or too low storage temperature will also cause the capacity of lithium batteries to decay and accelerate aging.

Vehicle battery systems usually consist of hundreds or thousands of battery cells, and battery packs face more severe thermal problems. Affected by factors such as heat transfer structure, series-parallel mode, operating conditions, etc., the temperature of each single cell in the battery pack presents strong inconsistency during operation, resulting in inconsistent battery internal resistance, capacity attenuation and depth of discharge, which in turn leads to the entire battery pack.

Common battery thermal management system means

The battery thermal management system includes high temperature heat dissipation and low temperature heating. Commonly used battery cooling methods include battery cooling technologies based on gas (air), liquid, solid phase change materials (PCM) and heat pipes. The low-temperature heating methods of the battery module mainly include external heating based on fluid or thermistor element (PTC) and internal heating based on the heat generated by the battery itself.

Battery cooling technology

The applied air mainly includes forced convection and natural cooling. The researchers studied the heat transfer characteristics of the battery pack by means of cooling air duct structure design, battery arrangement design, and ventilation control strategy optimization, and proposed measures to enhance heat transfer and improve temperature uniformity.

Due to the advantages of low cost, simple system structure, and easy maintenance, the air-cooling system is applied to some models with short cruising range and the main cost-effectiveness.

For example, Nissan LEAF adopts passive battery thermal management system for its lithium-ion soft pack battery pack. heat dissipation. However, for large-scale lithium-ion battery packs, due to the large thermal load of the battery and the long relaxation time of thermal conduction, air cooling cannot meet the heat dissipation requirements.

Especially in the high temperature environment, the heat exchange efficiency of the air-cooled thermal management technology is low, and the inconsistency is large, and it is difficult to meet the battery thermal management system needs.

Due to the low heat transfer coefficient of air convection, the use of liquid instead of air has become an inevitable means to enhance heat transfer. In research, liquid cooling plates are usually arranged at the bottom of the battery pack or between the cells to dissipate heat.

At present, most of the research of liquid cooling system focuses on the design of cooling channels: by increasing the number of cooling liquid channels, improving the structure of cooling channels, arranging fins in channels, and designing connected combined cold plates to improve heat dissipation capacity and temperature uniformity.

In recent years, the use of new refrigerants as thermal management coolants has also become more common, such as the use of liquid metals, nano-metal fluids, etc. to achieve enhanced heat dissipation.

At present, different car companies have different application methods for liquid cooling. The Tesla liquid cooling system uses a mixture of water and ethylene glycol with a mass ratio of 1:1, and the cooling pipes are meanderingly arranged in the 18650 battery stack.

Dissipate heat for each cell. The Chevrolet Volt pouch battery module also adopts liquid cooling for heat dissipation. Every two pouch cells constitutes a unit, and an aluminum plate with a liquid cooling flow channel is arranged between the two cells.

In addition, there is also a battery cooling method based on the principle of liquid phase change, that is, the evaporator of the air conditioning system is installed at the bottom of the battery system, and the refrigerant evaporation is used to take away the heat generated by the battery, also known as direct cooling.

Typical applications are BMW i3 series. Liquid cooling and heat management is a common method in current engineering applications. However, the system is more complicated, the quality is larger, and there is a possibility of leakage.

Battery heating technology

The charge-discharge performance of lithium-ion batteries is significantly reduced in a low temperature environment. Therefore, it is necessary to preheat the battery to improve its performance. The current heating technology is mainly divided into two categories: internal heating and external heating.

Internal heating refers to the heating method in which the battery generates heat through its internal resistance, including external alternating current heating, mutual pulse charge and discharge heating between batteries, and battery self-discharge heating.

In addition, the scientists designed a three-electrode battery with the addition of nickel electrodes and rapid heating-up of the fast battery through electrode switching.

External heating mainly includes air heating method and liquid heating method. The former uses electric heating wire to heat the air and then heat the battery, the temperature is uniform but the energy consumption is high. In this process, energy storage BMS is the best choice to solve this problem. Here is a list of top 10 energy storage BMS manufacturers for you to know more about this industry.

The latter heats the battery pack by heating the liquid in the flow channel, and has a complex structure and a slow heating rate. In addition to the above-mentioned heating method based on convection, PTC or low-power heating film can also be used to directly heat the surface of the battery, which has a certain impact on the heat dissipation of the battery.

The use of heat pipes as heat transfer elements for high temperature heat dissipation/low temperature heating of batteries is a new thermal management method. The heat pipe is a high-efficiency heat exchange element based on the principle of gas-liquid phase change.

The liquid working medium evaporates and vaporizes at the heated end, flows to the other end under the pressure difference, and condenses and releases heat in the condensation section. The liquid working medium returns to the evaporation section along the porous material through capillary force, which has the advantages of high heat transfer efficiency and good temperature uniformity.

Research on battery thermal management system

The main factors affecting the heat transfer performance of the system include three aspects:

1. The operating conditions and heat production of the power battery, that is, the influence of the working conditions of the heat source on the performance of the battery thermal management system;
2. The heat transfer characteristics of the heat pipe mainly involve the influence of the internal structure design of the heat pipe and its arrangement in the power battery pack on the heat dissipation performance of the system;
3. The heat dissipation at the cold end of the heat pipe mainly includes two forms of direct air cooling and water cooling secondary heat exchange. In addition, in the case of low temperature, the heat pipe needs to be locally heated by PTC or electric heating film, and then transferred to the battery in the form of heat conduction.

Research on battery operating conditions

The operating condition of the system determines the heat generation characteristics of the battery, which is a key factor affecting the heat transfer of the system. Before the battery temperature rises to the start-up temperature of the heat pipe, the heat pipe transfers heat in the form of heat conduction through the shell and tube.

When the temperature rises to the start-up temperature, the working medium in the tube begins to absorb heat by using the latent heat of phase change, thereby increasing its thermal conductivity and making the battery temperature gradually increase. in stability.

Studies have shown that the time required for the battery to reach a stable temperature from initial discharge under the condition of constant rate discharge is about 400-2000s, which is related to factors such as the battery discharge rate and the heat dissipation conditions of the cold end of the heat pipe.

The heat production rate of the battery increases nonlinearly with the discharge rate. The heat production rate of a 10 Ah square battery is about 10.5, 25.4, and 54.4 W at 3, 5, and 8C, respectively. The temperature distribution is also different. In addition, the greater the heat exchange at the cold end, the shorter the time required for the heat pipe to stabilize and the lower the stable temperature.

Analysis and design of heat pipes

Design and optimization of heat pipe based on power battery

Heat pipe design is an important factor affecting heat transfer performance, and its heat transfer effect is closely related to the channel size, wick structure, liquid filling rate and other factors. A reasonable heat pipe design is very important to improve the efficiency of the battery thermal management system.

Different types of heat pipes, such as gravity heat pipes, sintered heat pipes, pulsating heat pipes, flat loop heat pipes, flat micro heat pipes, etc., have been used in the existing power battery thermal management system research, and there is no unified selection or design method.

From the structural point of view, the plate type heat pipe shows superiority in the power battery thermal management system, and is expected to become the first choice for the power battery thermal management system. However, there are few design studies on the plate heat pipe at present.

Design of heat pipe layout scheme

The layout of the battery thermal management system is another key factor affecting the thermal conductivity of the heat pipe. Scientists compared the thermal conductivity of heat pipes when placed horizontally and vertically. Applying a 38 W heat source to simulate the heat generation of the battery pack, the temperature at the evaporating end of the heat pipe reaches 61°C when it is horizontally arranged, and only 51°C when it is vertically arranged.

In addition, the tilt angle of the device also affects the heat transfer. When the heat pipe is installed horizontally, the temperature difference on the surface of the battery is greatly affected by the inclination angle. When the heat pipe is installed vertically, the dual action of gravity and capillary force reduces the heat transfer resistance of the heat pipe, and the road slope has little effect on the local temperature difference.

In order to ensure the heat transfer performance of the heat pipe, the structural design of the battery thermal management system should fully consider the influence of the heat pipe arrangement on its thermal conductivity.

Summary and outlook

Temperature is a key factor affecting the performance of power batteries, and efficient thermal management systems are of great significance to electric vehicles. Heat pipes have strong heat exchange and temperature uniformity capabilities, and are an important research direction for future battery thermal management system.

Significant progress has been made in the study of using heat pipes as battery cooling/heating elements. However, with the increasing requirements for thermal management systems in electric vehicles, there are still several problems in the application of heat pipes that need to be solved:

• • The temperature of the power battery is closely related to its dynamic heat production conditions. Further research should be combined with the actual vehicle operating conditions to formulate an effective real-time control strategy, so as to realize an efficient and low-energy battery thermal management system.

• • In terms of heat transfer of heat pipes, due to many factors affecting the heat transfer performance of heat pipes, it is necessary to comprehensively consider the internal structure design of heat pipes and their arrangement in the battery pack to optimize their heat transfer performance during use, especially for flat plate type. Heat transfer characteristic analysis and optimization design research of heat pipe.

• In terms of heat pipe heat dissipation, most of the current system designs focus on reducing the temperature rise and temperature difference of the battery pack, and less consideration is given to system energy consumption and weight. Further research on enhanced heat dissipation of heat pipes should focus on multi-objective optimization of the system, comprehensive system thermal and electrical characteristics, system energy consumption and light weight and other indicators, and propose thermal management system heat dissipation solutions.

• Heating using heat pipes, especially the research on heating strategies in low temperature environment.

With the development of electric vehicles and the continuous advancement of power battery technology and heat pipe technology, heat pipes will be more widely used in battery thermal management system.

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