Design and Development of Battery Thermal Management System for New Energy Vehicles


In the near future, gasoline powered vehicles running on the roads will gradually be replaced by new energy vehicles driven by electricity. The most crucial component for the long-term endurance of new energy vehicles is the vehicle's battery. The state, temperature, and uniformity of the temperature field of a battery have a significant impact on its performance and lifespan.

 

 

Battery thermal system management is the optimization design and prediction of heat dissipation performance of battery cooling systems, which has significant practical significance for improving the maturity and reliability of hybrid vehicles and power batteries.

 

Battery thermal management system

1Battery management system

The Battery Management System (BMS) is an important link connecting the in vehicle power battery and electric vehicles. The power output of pure electric vehicles relies on the battery, and the battery management system is the core of it, responsible for controlling the charging and discharging of the battery and achieving functions such as battery state estimation.

2Thermal management system

Vehicle thermal management refers to the coordination of the relationship between the thermal management system, thermal management objects, and the entire vehicle from a system integration and overall perspective. It adopts a comprehensive control and system management approach to integrate various systems or components into an effective thermal management system, controlling and optimizing the heat transfer process of the vehicle. In other words, the system manages the thermal energy of various parts of the vehicle, which can effectively reduce waste heat emissions, improve energy utilization efficiency, and reduce environmental pollution.

3Battery thermal management system

 

 

 

Based on the above description of battery management system and thermal management system, the battery thermal management system can be regarded as the intersection of battery management system and thermal management system.

1) Necessity

Battery Thermal Management System (BTMS) refers to a closed-loop regulation system composed of thermal conductive media, measurement and control units, and temperature control equipment to ensure that the power battery operates within an appropriate temperature range and maintains its optimal usage state. The thermal related issues of batteries largely determine the performance and lifespan of battery systems.

A. Battery energy and power performance

When the temperature is low, the available capacity of the battery will rapidly decay. Charging the battery at a low temperature (such as below 0 ℃) may cause instantaneous voltage overcharging, resulting in internal short circuits.

B. Safety of batteries

Defects in the production and manufacturing process or improper operation during use may cause local overheating of the battery, leading to chain exothermic reactions and ultimately causing serious thermal runaway events such as smoke, fire, and even explosions.

C. Battery lifespan

The suitable temperature for batteries is about 10-30 ℃, and high or low temperatures will cause a rapid decline in battery life. The large-scale development of power batteries results in a relative reduction in the ratio of surface area to volume, making it difficult for internal heat to dissipate and more likely to cause uneven internal temperature and high local temperature rise, further accelerating battery degradation and shortening battery life.

2) Function

The battery thermal management system is designed to address the thermal related issues of batteries, and its main functions include:

A. Heat dissipation: Effectively dissipate heat when the battery temperature is high to prevent thermal runaway accidents;

B. Preheating: Preheating the battery when the temperature is low, increasing the battery temperature to ensure charging and discharging performance and safety at low temperatures;

C. Temperature balance: reduce the temperature difference within the battery pack, suppress the formation of local heat zones, prevent the battery from decaying too quickly at high temperature positions, and improve the overall lifespan of the battery pack.

3) Classification

A. Direct cooling system

The direct cooling system has the advantages of compact system, light weight, and good performance. However, this system is a dual evaporator system, with no battery heating, no condensate protection, difficult to control refrigerant temperature, and a short lifespan of the refrigerant system. At present, direct cooling is mainly used in electric passenger cars, with the most typical example being the BMW i3 (which has two cooling options: liquid cooling and direct cooling).

 

 

B. Low temperature radiator cooling system

The low-temperature radiator cooling system is a separate system for batteries, consisting of a radiator, water pump, and heater. This cooling system has the advantages of simple system, low cost, and economic and energy-saving in low-temperature environments. However, this system has drawbacks such as low cooling performance, high water temperature in summer, and limited application by weather conditions.

 

C. Direct cooling water cooling system

Liquid cooling is currently the preferred solution for many electric passenger cars, with typical products at home and abroad such as BMW i3, Tesla, General Motors Volt, Brilliance BMW Zhinuo, and Geely Emgrand EV. The direct cooling water cooling system has advantages such as compact system, good cooling performance, and wide industrial application range. However, this system has more components than direct cooling, is more complex, has poor fuel economy, and has a higher compressor load. This type of cooling system is currently one of the most commonly used battery thermal management systems.

 

 

D. Air cooling/water cooling mixed cooling system

There are two key components in the air-cooled/water-cooled hybrid cooling system, namely the water-cooled battery cooler and the air-cooled battery radiator. The air cooling/water cooling hybrid cooling system has the advantages of compact system, good performance, and economic and energy-saving in low-temperature environments. However, this system is complex, costly, complex to control, and requires high reliability.

 

 

E. Direct air cooling system

This system utilizes the low-temperature air in the cockpit to cool the battery. In the early days, electric passenger cars were widely used, such as the Nissan Leaf, Kia Soul EV, etc. They are also widely adopted in current electric buses and electric logistics vehicles. Direct air cooling systems have the advantages of simple system, controllable air temperature, and low cost. However, this system is not suitable for all types of battery cells, and there is a risk of contamination inside the battery due to slow recovery after soaking.

 

 

Design of battery thermal management system

1Design process

 

2design requirement

The development process of the battery thermal management system should be consistent with the battery pack development process and must strictly follow the design process of the battery thermal management unit. The development process of the battery thermal management system should be consistent with the battery pack development process, and its performance and functional reliability must be strictly verified in accordance with the design process, component selection, and unit performance evaluation of the battery thermal management unit. A high-performance and highly reliable thermal management unit can ensure the stability and safety of the battery thermal management system. At the same time, from a practical perspective, attention should be paid to various issues such as the cost, environment, and area occupied by the thermal management system.

3key technology 

1) Determine the optimal operating temperature range of the battery

2) Battery thermal field calculation and temperature prediction

The battery pack thermal management system should ensure that the battery pack always operates within a safe temperature range and try to maintain the working temperature of the battery pack within the optimal working temperature range. Therefore, determining the optimal operating temperature range of the battery is an important step in the design process. The optimal working temperature range can be provided by the battery manufacturer or determined by the battery user through experiments.

2) Battery thermal field calculation and temperature prediction

Due to the fact that batteries are not good conductors of heat, only knowing the surface temperature distribution of the battery cannot fully explain the internal thermal state of the battery. Therefore, it is necessary to calculate the temperature field inside the battery through mathematical models and predict its thermal behavior.

 

 

3) Selection of heat transfer medium

The heat transfer medium should be determined before designing the thermal management system. The heat transfer medium determines the cooling method of the thermal management system (air cooling, liquid cooling, phase change material cooling). The heat transfer medium largely determines the efficiency of battery thermal management systems, and has a significant impact on heat exchange, heat transfer, and system environment.

4) Design of heat dissipation structure for thermal management system

Excellent heat dissipation structure design will improve the temperature difference between different battery modules inside the battery box, which will exacerbate the inconsistency of internal resistance and capacity of the battery. If accumulated for a long time, it will cause some batteries to overcharge or discharge, thereby affecting the battery's life and performance, and causing safety hazards. When arranging the battery pack structure and designing heat dissipation, it is important to ensure the uniformity of the battery pack's heat dissipation as much as possible.

            Parallel ventilation                                                                                                                              Single row ventilation

   

                                                                                                                  

 

5) Fan and temperature measurement point selection

For fans, while ensuring a certain heat dissipation effect, efforts should be made to reduce flow resistance, reduce fan noise and power consumption, and improve the efficiency of the entire system. For temperature measurement points, as the temperature distribution of the battery pack inside the battery box is generally uneven, it is necessary to determine the temperature points in the battery box that may cause danger in order to ensure the normal operation of the battery. On the one hand, temperature sensors should comprehensively reflect the operation status of the battery, and consider the situation of sensor failure. On the other hand, cost should also be considered. Therefore, the number of temperature sensors should be designed appropriately.

 

Main technologies

The main technologies of battery thermal management system include air cooling technology, liquid cooling technology, phase change material cooling technology, and thermal management cooling technology.

1Air cooling technology

At present, research on air-cooled thermal management systems mainly includes the influence of factors such as battery arrangement, battery spacing, air ducts, wind speed or air volume on the system's thermal management capability. Air cooling technology is currently the most widely used heat dissipation technology in new energy power batteries.

Forced airflow can be generated through fans, as well as by utilizing headwinds or compressed air during the movement of a car. Compared with other technologies, air-cooled technology is relatively simple, safe, and easy to maintain. Toyota's hybrid electric vehicle Prius and Honda's Insight in Japan both use air cooling, while thermal management systems developed by car companies such as Nissan and General Motors mainly use forced air cooling. But with the increasing heat load of lithium-ion batteries, traditional forced air cooling is gradually unable to meet the requirements. For large-scale lithium-ion batteries, due to their low thermal conductivity, tight battery arrangement, limited battery box space, and long relaxation time of thermal conduction, air cooling alone is no longer able to meet the expected requirements.

2Liquid cooling technology

Water cooling technology is a cooling technology based on liquid heat exchange. Water cooling technology has been studied early and applied for a long time abroad. With continuous exploration, practice, and improvement, the heat exchange coefficient and cooling and heating speed of the system have reached a good level. Moreover, through the application of new materials, the weight of foreign water cooling systems has also been reduced.

The Tesla Model S model uses water-cooling technology to cool the battery. Tesla has carried out very in-depth design in its battery layout, thermal management system, and battery management system to ensure that each battery unit is under supervision, and its status data can be feedback and processed at any time. For individual small battery cells, Tesla independently seals them in steel compartments, and the liquid cooling system can specifically cool each cell, reducing the temperature difference between them and relatively reducing the risk of battery self ignition.

3Heat pipe cooling technology

1) Heat pipe

Heat pipe, also known as heat pipe or superconducting pipe, is an artificial component with excellent heat transfer performance, divided into evaporation section, insulation section, and condensation section. In terms of rapid heat transfer, heat pipe has strong advantages.

 

 

2) Application

Heat pipes are a good type of thermal bridge, which can achieve good results in cooling by combining their diverse forms and flexible placement with other forced cooling methods. Especially with the development of small heat pipe technology, it can bring greater development space for the safe and long-term operation of power batteries.

The design of battery thermal management system is to optimize and improve its performance on the basis of traditional working methods. On the other hand, research should be conducted on new materials such as heat pipes. Effective control of heat is crucial for sustained performance improvement and market expansion, and this development will greatly affect the development and promotion of new energy vehicles.