Part 1: Research Background on Battery Thermal Management System

Lithium-ion batteries are very fragile under thermal runaway conditions. Thermal runaway occurs when the heat generation rate is greater than the loss rate. Thermal runaway will cause the lithium-ion battery to have a tendency to explode, and it is also a manifestation of the process of “thermal runaway”. The heat generated by the chemical composition of the battery, the external environment temperature, the charge/discharge rate, and the rate are all causes of thermal runaway and explosion.

The temperature of the battery and the uniformity of the temperature field have a great influence on the performance and life of the lithium battery. Therefore: the optimization design of the battery heat dissipation structure and the prediction of heat dissipation performance have important practical significance for improving the performance and life of lithium batteries, safety and reliability.

Part 2: Basic Conception

1. Thermal design concepts and goals

The thermal design concept is to solve the thermal problem of the power battery and optimize the thermal design of the single battery to improve the thermal characteristics of the battery and reduce the complexity of the thermal management system. The thermal design goal must meet the following requirements:

1. Meet product reliability requirements
2. Meet the thermal environment requirements of the expected work
3. Meet the restrictions on the cooling system
4. Compliance with other related standards, specifications/regulations

2. Heat transfer method

a.Conduction

When the substance itself or when it comes into contact with the substance, the transfer of energy is called heat conduction. The method of heat conduction is limited to solid and liquid, because the molecular composition of gas is not very tight, and the energy transfer between them is called thermal diffusion.

b.Convection

Convection refers to the heat transfer method in which fluid (gas or liquid) contacts a solid surface, causing the fluid to heat the heat away from the solid surface.

c.Radiation

Thermal radiation is a transfer method that can exchange heat without any medium without contact. Thermal radiation can achieve the purpose of heat exchange in the form of waves.

3.  Thermal simulation model

Establish a single battery temperature field analysis model that  can be used in engineering projects, simulate the changes in the surface temperature field of the battery cell under various working conditions, and compare with the numerical analysis results of the battery temperature field to obtain a simplified and reliable battery interior Numerical analysis method of temperature field.

Import grid models in Nastran and other formats through thermal simulation software, connect with CFD tools such as Fluent and StarCCM+, and connect with one-dimensional analysis tool AMESim to realize 1D-3D coupling system simulation. It has an interface with the 3D structural analysis tool ABAQUS to realize the establishment of thermal model and thermal stress calculation.

Part 3: Theoretical Analysis

1. The nature of battery chemistry: LiFePO₄+6xC⇔Li_1-xFePO₄+Li_xC₆

According to the reaction equation of lithium iron phosphate battery —

Reaction formula of anode upper electrode:  LiFePO4 ⇔ Li1-xFePO4 + xLi+ + xe-

The electrode reaction formula that occurs on the cathode:   xLi+ +xe- +6C⇔LixC6

The total reaction mode: LiFePO4+6xC⇔Li1-xFePO4+LixC6

The battery is being charged, and the process of discharging is a process of heat release and heat absorption. The exothermic reaction is a reversible process carried out at a constant temperature and a constant pressure. In addition to proceeding according to the reaction equation, the battery reaction also involves electrolyte decomposition and battery self-discharge. As any battery inevitably has resistance (internal resistance), Joule heat generated when current flows. When we are conducting thermal design, we should analyze the heat of several aspects together.

2. Theoretical analysis of battery heat generation

Battery thermal reaction: The source of reaction heat can be expressed as:

For a typical AB5 alloy, the heat of formation of MH is -29.3KJ/mol:

It can be seen that the battery reaction heat has a linear relationship with the charging current, where Ic is the charging current.

The battery reaction has a polarized heat production rate: there is impedance in the battery and the impedance due to polarization,

Polarization impedance produces polarized Joule heat when the battery is discharged as:

All the heat generated by polarization appears in the form of Joule heat, and impedance analysis must be performed to separate the polarization impedance from the battery impedance.

3. Joule heat generated by resistance

The rate of Joule heat generated when the battery is discharged can be expressed as:

In order to reduce Joule heating, materials that reduce the resistance of the battery should be used, as well as other structural designs. In practical applications, such as: battery cell connection nickel sheet, solder joint/connection point resistance, BMS loop internal resistance, output wire internal resistance, battery cell internal resistance, reducing the resistance of each link can reduce the battery pack The generated Joule heat also improves the carrying capacity of the battery pack.

Through the model, thermal imaging analysis of the lithium battery temperature streamline distribution diagram:

Part 4: Solution Method

1. Design goals
2. Ensure that the battery runs within the optimal operating temperature range
3. The temperature difference between the batteries inside the module is less than the control value
4. The battery can be warmed up to the operating temperature range when it affects the startup under low temperature
5. Quickly discharge the excess heat in the battery pack out of the battery box

Battery Thermal Management System design ranges from “simple energy balance equations” to more complex “thermodynamics and fluid dynamics models”. Designing a thermal management system must consider aspects such as how much heat is removed from the battery pack or battery, the maximum temperature and temperature difference allowed, the cooling method used, the actual cooling requirements, and the additional cost of the system.

Process

• Process analysis
• Determine the goals and requirements of the thermal management system: determine the indicators that the thermal management system must achieve under different weather conditions, such as average temperature T, temperature variation range △T, etc., and determine the space layout and size of the battery pack according to product integration requirements.
• Measure or estimate the heat generation and heat capacity of the battery module: measure or estimate the heat generation of the battery pack under the corresponding temperature conditions and charging and discharging cycle conditions.
• Preliminary design of thermal management system: According to the analysis of the temperature field of the battery pack, determine the power consumption, heat transfer medium, circulation path, flow and other parameters required by the thermal management system, and make a preliminary design plan.
• Predict the thermal behavior of the module and the battery pack: measure or estimate the heat transfer speed between the components of the battery pack, use computational fluid dynamics or experiments to obtain the heat transfer speed between the battery cell or module and the heat transfer medium, and calculate different thermal management strategies under the system performance and its impact on battery pack and product performance.
• Preliminarily design the thermal management system, and determine the parameters of the system based on the expected performance.  Design the thermal management system and conduct experiments, and optimization of thermal management system based on test data and analysis.

Features of our solution

• Mainly through phase change materials, the thermal management device has less restriction on the inside of the battery
• Analyze and measure the thermal field distribution of battery modules and battery cells. Determine the number of temperature measurement points, and find suitable temperature measurement points in different areas
• The management model of “big data” is enriched, combined with the intelligent BMS system to predict the heating of the battery pack and intervene in advance. Change from passive thermal management to active thermal management to ensure the safety of the battery pack and prevent thermal runaway.

Traditional thermal management solutions: a. Air cooling method; b. Liquid cooling method; c. Phase change material heat transfer cooling method. Each cooling method has certain advantages and disadvantages and limitations in actual implementation

Application field: energy storage products for small cars/small power energy storage products

The battery thermal management system is one of the key technologies to ensure performance. The value of the thermal management system is to allow effective heat dissipation when the battery temperature is high to prevent thermal runaway accidents; to warm up when the battery temperature is low, increase the battery temperature, and ensure the charging and discharging performance and safety at low temperatures; reduce The temperature difference in the battery pack inhibits the formation of local hot spots, prevents the battery from decaying too quickly at high temperature locations, and improves the overall life of the battery pack.

Solution node

• Thermal model: Build a thermoelectric coupling model of a battery cell or battery pack, analyze the temperature and temperature field of the battery pack, and evaluate the heat dissipation strategy of the battery pack.
• Big Data: “Big data” model obtained by collecting usage environment, usage habits, seasonal changes, and instant temperature changes.
• Thermal management: Big data supports the implementation of effective thermal management solutions through intelligent BMS management.
• Model analysis. Through the thermoelectric coupling model of the battery cell or the battery pack. The battery module analyzes the temperature field distribution analysis in the battery pack.

The field of thermal design is mainly carried out using computational fluid dynamics (CFD) methods. Thermal design software: FLUENT/Icepack, PHOENICS/Hobox, ANSYS, NATA, CINDA, NATFIN, CATS, TANS, FLOWTHERM, ICEPAK, CoolitTM, etc. The mathematical basis for thermal analysis is finite element method, finite volume method, finite difference method and boundary element method. Thermal analysis is to simplify the model based on actual engineering. Establish mathematical models, solve nonlinear equations, compile and debug analysis programs, and finally obtain a visual temperature distribution map.

• Big data analysis: collect actual working conditions, ambient temperature, weather conditions, combined with battery internal temperature changes, and other data to form “big data”.
• Thermal management: by combining the simulation analysis model, the effective measures for thermal management are obtained. Combined with BMS for system thermal management and implementation.

Part 5: Practical Application

1. CFD simulation analysis of the original model. When the heating power is 1500W in extreme working conditions, the temperature difference between the maximum temperature and the minimum temperature is about 23℃.  The maximum temperature difference in the variable working condition is 15.2℃.  This is far greater than the temperature difference within 4℃.
2. Battery pack model analysis and optimization plan: the position of the battery does not move. By adding an arc-shaped thermally conductive phase change material to fill the gap between the batteries, the thermally conductive phase change material is in full contact with the heat dissipation aluminum plate. The contact area is enlarged, and the cooling device is provided. In the output power that meets the customer’s requirements, the test result: the temperature difference in the extreme working condition is 11.6℃, and the temperature difference in the working condition is controlled within 3.8℃.
3. Determine effective thermal management measures after battery pack model analysis. Through simulation analysis under different pressure differences, it is possible to determine the installation position of the phase change material and the installation position of the refrigeration device.

 In addition, the battery pack combines “big data” and an intelligent BMS thermal management system. This improves the success rate of battery pack development.

Part 6: Future Prospect

1. With the development of battery materials, battery cells can work in a wider environment. And the heat generation of battery cells is greatly reduced;
2. Battery Thermal Management System analysis system software is a more intelligent. 
3. All working conditions of the battery pack application in the system and the thermal data are collected in real-time. The management system implements effective measures through big data. Big data is better for the early development of battery packs;
4. From the perspective of miniaturization, the traditional thermal management system using air and water as the cooling medium has limitations, and the phase change material battery management system has a better development prospect.