The thermal management of power batteries directly determines the safety, performance, and lifespan of electric vehicles/energy storage systems.
In today's booming development of electric vehicles, the importance of thermal management technology for battery packs as the "heart" of electric vehicles/energy storage systems is self-evident. An excellent thermal management system not only enables the battery to perform at its best, but is also the key to ensuring vehicle safety.
1, Why is thermal management so important?
The thermal management of battery packs has a decisive impact on the performance and safety of electric vehicles/energy storage systems. Firstly, good thermal management can maintain stable battery performance. Temperature is a key factor affecting the performance of lithium batteries, and temperatures that are too high or too low can directly affect the charging and discharging efficiency and capacity of the battery.
In low-temperature environments, the internal resistance of the battery increases, leading to a decrease in discharge capacity and affecting the range of electric vehicles. In high-temperature environments, the chemical reaction rate of batteries accelerates, which may lead to a shortened battery life.
Thermal management also helps to extend battery life. Research has shown that when batteries operate within a suitable temperature range, their lifespan can be significantly extended. By implementing an effective thermal management system, the battery temperature can be controlled within the optimal operating range, which can reduce damage to the battery caused by high or low temperatures.
Thermal management is the key to preventing thermal runaway. Overheating of batteries may lead to uncontrolled heat generation, resulting in safety accidents such as fires. The thermal management system can timely dissipate the heat generated by the battery, avoiding overheating and causing thermal runaway. Some advanced thermal management technologies also use insulation materials and explosion-proof valves to further suppress the spread of thermal runaway.
2, The core design objective of battery thermal management is 1 Temperature control range
Ideally, the battery system should operate within the optimal temperature range of 15 ℃ -35 ℃. Under extreme working conditions, the battery temperature should remain stable below 45 ℃, with a temperature rise of less than 10 ℃, and the temperature difference between battery cells should be controlled within 5 ℃
2. System flow resistance and flow distribution
The flow resistance of the water cooling system should meet the requirements of the entire vehicle, and the flow difference between each branch should be less than 10% to ensure uniform cooling.
3. Space and weight limitations
The thermal management system needs to achieve efficient thermal management in a limited space, usually requiring a cooling system volume ratio of less than 15% and a weight increase of no more than 10% of the total weight of the battery pack
3, Main thermal management methods and their applications 1 Air cooling (air-cooled)
The air cooling system introduces cold air for convective heat dissipation, ensuring that the temperature of the battery system is suitable. Reasonable air duct layout and fan selection directly affect the heat dissipation performance of the air cooling system.
Advantages: Simple structure, low cost, and easy maintenance.
Disadvantages: Relatively low heat dissipation efficiency, suitable for low energy density battery packs.
2. Liquid cooling (liquid cooling)
The liquid cooling scheme achieves efficient thermal management through water-cooled plates and is widely used in various battery pack designs. For example, in Tesla Y, water-cooled plates are cleverly arranged between cylindrical cells and embedded in the lower frame.
Advantages: High heat dissipation efficiency and good temperature uniformity.
Disadvantages: The system is complex, the cost is high, and there is a risk of leakage.
3. Phase change material cooling (PCM)
Phase change material cooling utilizes phase change materials to absorb or release heat during the phase change process, achieving stable control of battery temperature and good temperature uniformity.
Advantages: No external power required, stable temperature control.
Disadvantages: High material cost and limited thermal conductivity.
4, Design Process and Key Technologies of Thermal Management System 1 design process
Thermal management design typically follows the following process:
Determine thermal management requirements → Estimate heating power → Preliminary design of thermal management → Simulation of flow and temperature fields → Experimental verification
2. Heat generation calculation and simulation analysis
The calculation of battery cell heat generation is an important part of battery thermal management simulation analysis. The widely used model currently is the Bernardi model, which relates the heat generation rate of batteries to factors such as charge and discharge current, battery volume, and open circuit voltage.
The modeling method for thermal management CFD analysis includes geometric model simplification and channel extraction after obtaining the battery pack model from the structural engineer. Simulate flow and temperature fields using professional simulation software such as STAR-CCM+, ANSYS Fluent, etc.
3. Material selection and structural design
The choice of material for the battery pack box directly affects its thermal management performance. For liquid cooling systems, the casing material should have good thermal conductivity; For phase change material cooling systems, the box material should have good compatibility with the phase change material.
In the structural design of battery pack casing, thermal management requirements should be fully considered. For example, designing the layout of battery modules reasonably, increasing the flow area of fluid channels, and improving heat exchange efficiency.
5, Thermal management control strategy
The intelligent thermal management strategy can achieve precise control of water pump flow and water temperature in different operating modes based on the power map characteristics of the battery cell.
Based on the power map characteristics of battery cells (lithium batteries have the highest charging and discharging power at temperatures of 15 ℃ -35 ℃), thermal management control strategies typically include three modes:
Slow charging mode: using the temperature of the battery cell to determine the on and off conditions for heating and cooling
Fast charging mode: using the temperature of the battery cell to determine the on and off conditions for heating and cooling
Discharge mode: The heating on condition is determined by the state of SOC and the temperature of the battery cell, and the off condition is determined by the temperature of the battery cell
1. Intelligent thermal management
With the development of technology, battery thermal management is evolving towards intelligence. By introducing advanced sensors and intelligent control systems, real-time monitoring of battery parameters such as temperature, current, and voltage can be achieved, and thermal management strategies can be automatically adjusted according to different operating conditions.
Based on the digital twin dynamic regulation technology, it can integrate the historical working conditions, SOH data and AI algorithm, predict the future heat load and adjust the cooling strategy in advance.
2. New Materials and Applications
New heat dissipation materials such as graphene have extremely high thermal conductivity, which can quickly dissipate the heat generated by the battery. The microchannel heat dissipation structure can improve heat dissipation efficiency by increasing the heat dissipation area and fluid flow rate.
The thermal conductivity of the Vapor Chamber technology can reach 5000 W/m · K, which can replace traditional liquid cooled plates and achieve ultra-thin design (thickness<3mm).
3. Highly integrated
With the increasing demand for space utilization and lightweight in electric vehicles/energy storage systems, battery pack thermal management technology is showing an integrated trend. The integrated thermal management system integrates functions such as heat dissipation, heating, and temperature monitoring, reducing system volume and weight.
