Decreasing charging time of Lithium-Ion Battery by controlling temperature during the process

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Master Thesis

HALMSTAD

UNIVERSITY

Master's Programme in Mechanical Engineering, 60 credits

Decreasing charging time of Lithium-Ion Battery by controlling temperature during the process

Mechanical Engineering, 15 credits

Goteborg 2020-05-27

Seyedhafez Sadeghi Meykola

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Abstract

In this report, which is written for Halmstad University and on behalf of Semcon, a mechanical design process was implemented to evaluate the possibility of executing a low-cost cooling system for one of the battery company’s products. The report started from the background of former studies to clarify the problem and finding existing solutions in the first step and following up with developing concepts based on gathered information from the literature study.

The battery company provides a high-performance battery based on customer requirements because of the flexible lithium battery technology. There are two similar conditions in the operating period of the battery. One in the high- performance usage of the battery and two in the charging period by adding power to the battery cells. Both of these actions generate heat which is the result of resistance of the material in the cells and the chemical reaction of the cells. The battery company wants to decrease the time of charging period in order to reach the operative condition as fast as possible by enabling fast charging. According to the battery management system (BMS), at high temperatures the efficiency of charging rate decreases and also it has a negative influence on the battery cells performance and lifetime. A cooling system is required in order to fulfil the requirement of having high performance and making fast charging possible for the battery.

The goal of this thesis is to develop a cooling system based on literature study with consideration of applying to the lithium battery in order to reduce charging time by decreasing temperature in the modules during the operating period. This report contains the whole developing conceptual design process of the cooling system by starting a wide range of information gathering in the purpose of determining the main variables which have an influence on different cooling techniques. In specific, after deciding about the type of cooling method for the product, the study has been focused on the fluid dynamics and heat transfer by forced convection with airflow.

The concept generation executed on the information which was gathered through the information retrieval.

The final developed concept has been introduced in this thesis work which has the potential of fulfilling the requirement of designing a cooling system to decrease the temperature of the battery’s modules without making any serious changes in the product. The final concept has been developed by various changes in its components and evaluating those changes by executing simulation for each change.

The final concept contains twelve similar heat sinks which have been installed over connectors to provide more surface in the purpose of exchanging heat through the airflow provided by three high-speed fans. In order to force the airflow over the heat sinks, a cover has been designed.

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Acknowledgement

This master thesis project carried out in the Department of Mechanical Engineering in collaboration with the Division of Product Design Engineering at Semcon AB for a battery company as a client.

Thanks to Fredrik Borg from the department of Product Design Engineering at Semcon for giving the opportunity to me and his trust that he put on me to cooperate on this project. He was always kind with me and spent his time to answer my questions and lead me to do a better job. I learnt a lot from him and his experience of how to manage a job for a company like Semcon. He introduced me the product that I worked on it in this project which was totally new for me at the first place.

Also, I want to thank Fredrik Carlsson and Federico Ghirelli from simulation department of Semcon for being generous helping me and sharing their knowledge with me. Thanks to my supervisor Håkan Petersson from Halmstad University for helping me through this project and also giving me enough space and freedom to develop this project in my way.

At the end and most importantly for me, I would like to thank my best friends, Jafar who introduced Semcon to me and his constant support for all these years, and Maryam for being kind and supportive to me. Thank you for being my friends.

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List of Figures

Figure 1-1 Battery module parts (Karlsson and Nilsson Bornhall 2019) ... 3

Figure 2-1 An overview of the mechanical design and product support process (D. G. Ullman 2016) ... 5

Figure 2-2 Ulrich and Eppinger - The product development process (Ulrich and Eppinger 2012) ... 7

Figure 2-3Overall view of Project Flow ... 8

Figure 3-1Velocity regime through a surface (Cengel, Cimbala and Turner 2017) ... 14

Figure 4-1 the liquid cooling system in the battery’s module (Martin Karlsson 2019) ... 16

Figure 4-2 a single tower heatsink with an individual fan (CRYORIG 2018) ... 16

Figure 4-3 vapor compression cycle (Sinopoli 2010) ... 16

Figure 4-4 simple heatsink (ElectronicsBeliever n.d.) ... 17

Figure 4-5 the relation between requirements and mechanical specification in QFD house ... 17

Figure 4-6 simplified model of battery pack ... 18

Figure 4-7 fluid domain and connectors in the meshing environment of ANSYS 19 Figure 4-8 heat flux sources in connectors ... 19

Figure 4-9 temperature contour of connectors in firs simulation ... 19

Figure 4-10 3D contours of air velocity and streamline ... 20

Figure 4-11 CES Edupack material selection ... 22

Figure 4-12 heat sink concept for the product with dimension ... 23

Figure 4-13 heat sink position and assembly in the product ... 24

Figure 4-14 designed cover for heat sinks ... 24

Figure 4-15 designed cover position in the final concept ... 25

Figure 6-1Maximum potential US lithium demand and supply from recycling (G. Pistoia 2014). ... 29

Figure 6-2Schematic flow chart for the production of lithium-ion cell materials (G. Pistoia 2014) ... 30

Figure 6-3Main criteria of critical raw material (G. Pistoia 2014) ... 30

Figure 6-4Schematic of the system approach to lithium-ion battery design and use (G. Pistoia 2014) ... 31

List of Abbreviations

Acronym Abbreviation

QFD Quality function deployment

TRIZ Theory of inventive problem solving

CAD Computer Aided Design

CFD Computational fluid dynamic

BMS Battery Management System

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7 References ... 33 Appendix I – Gantt chart ... I Appendix II – Project Flow Chart ... II Appendix III – Fluid Dynamics related to the Fin heat reduction ... III Appendix IV – Requirement and Specification (QFD) ... IX Appendix V – Final Concept Development ... X Appendix VI – Simulation Result table ... XXIV Appendix VII – Heat Reduction Figure ... XXVII

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Appendix VIII – Heat sink material properties ... XXVIII Appendix IX – Fan Specification ... XXIX Appendix X – Overall Concepts ... XXXI

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1 Introduction

In this report, the search for a design solution on the cooling system for a lithium battery is discussed. The section includes a background, purpose, demarcations and a clarification of the issue. The customer of Semcon for which this work is carried out, develops and manufactures batteries and will be named the battery company in this report.

1.1 Introduction Battery company

The battery company specializes in flexible lithium battery technology and offers its customers high-performance battery solutions, tailored to the customer's needs.

The company has a standard module that is filled with battery cells, where the number of cells and modules can then be configured and the characteristics of the battery adapted to the customer's request.

The battery company wants to prevent the problem that arises when recharging lithium batteries. To charge a battery, power is supplied to the battery. In turn, an increase in power supply results in a reduced battery charge time, but unfortunately also increases the temperature. In order for a lithium battery to maintain optimum performance during its lifetime, fast charging is required under controlled temperatures. Therefore, the company uses a safety system that turns off the battery if the temperature exceeds a dangerous level. The company values innovative customer-adapted battery solutions and therefore wants us to develop cooling equipment that can in some way be implemented in their current product and thus create a more complete product range and become a more attractive supplier. The work on developing a solution to the problem has been carried out at the company Semcon on their premises.

1.2 Aim of the study

The purpose of the work is to develop cooling equipment for a lithium battery module and thus minimize the charging time by limiting the temperature in the modules. In this project, terms of heat transfer related to the fluid dynamic will be discussed to consider the possibility and capacity of fluid in reducing heat in the charging process.

1.2.1 Research questions

The purpose of the work is to answer the following questions.

 1. How can cooling equipment for the battery company's lithium battery module be designed and integrated with as few changes as possible to the existing product?

 2. How should the cooling equipment be designed to minimize charging time and transport the heat which is generated in the lithium battery module during the charging process?

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For this purpose, the amount of heat which batteries produce during charging and temperature of it is important. Besides these, the capability of cooling systems to transfer the heat and reducing temperature is important.

1.2.2 Work structure

According to the product development process, the process has a defined structure and writing down the steps which should be taken, will lead to the final results. In order to have an organized time table for the project and clarify steps and activities, a Gantt Scheme has been generated which can be found in Appendix I.

 Literature study to understand various aspect of the problem and possible solution

 Customer requirements and understanding the final goal of the project

 Concept developing process

 Evaluating the final concept and detail design

 Report and presentation the process details and results 1.3 Limitations

The limitations may influence the process and district the parameter’s value is dimension, temperature, costs. Defining customer expectation and technical specifications as well as evaluating concepts and taking steps needs appropriate access to a different stage of data which has some access limitation and in some levels, assumptions have been taken stemming from the industrial experience of the supervisor and the author. So, all these will be considered in the product development and concept design process.

1.4 Study environment

The study has been executed in Semcon as a consultant for the battery company.

They have been provided with the possible equipment and data which was needed such as a computer, CAD software and CAE software. The main part of studying literature, benchmarking and product development process has been done in the Semcon office with the support of the “Product Design Engineering” team. Some short meetings and visits to the battery company also have been carried out to get some information related to the project.

1.5 Problem Determination

After the first visit to the company where the problem was discussed, questions could be set up and answered according to the question method. This meant that the final description of the main problem where the purpose of the work could also be specified.

The final problem statement reads as follows: The battery company specializes in flexible lithium battery technology and offers its customers high-performance battery solutions, tailored to the customer's needs. When the battery modules are used, the energy stored in the battery is consumed, which means that a charging

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process must sometimes be performed by the user. This charging process results in a supply of power to the battery cells, which in turn means that heat generation, due to resistance heat and chemical reaction heat, occurs in each battery cell. The company wants to be able to offer a faster charging process, a so-called quick charge. To achieve this, an increase in the amount of power supplied per unit of time is required. This increase results in an increase in heat generation and leads to a higher risk of reaching critical temperatures. The problem is that if these critical temperature levels are reached, the amount of current supplied is gradually reduced and the charge becomes less efficient due to the battery's BMS system. Battery performance and service life are also adversely affected. To solve the problem and enable quick charging, a cooling device is required to be integrated into the existing product.

1.6 Product Description

This section describes the product that the cooling equipment is designed for and explains what is meant by fast charging.

The battery company manufactures lithium-ion battery modules and is currently available in a few different sizes. The battery module affected by the work has a capacity of 210 Ah and is available in two variants, one A and one B module, where the only difference is that the components inside the outer casing are rotated half a revolution. This rotation allows multiple plus and minus coils of several modules to be easily connected to a larger battery pack with a voltage of, for example, 48 V (corresponding to 14 battery modules). The battery module weighs 3.45 kg and consists of a total of seven main components: battery cell, cell holder, aluminum plate, outer casing, top cover, circuit board and bottom cover which can be seen below.

Figure 1-1 Battery module parts (Karlsson and Nilsson Bornhall 2019)

In total, there are 52 battery cells in this model that are placed in 13 rows with four cells in each row. The battery cells are connected in parallel, which gives a total

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voltage for the battery of 3.7 V (which varies between 2.8 - 4.2 V depending on the amount of charge). The two poles of the cells, minus and plus, are welded together with the two plates, which results in a minus plate and a plus plate which each have a connection in the top cover of the battery module. The battery module has an IP- rating of 65, which is obtained by sealing the battery module with silicone and ultrasonic welding. The inner parts, the battery pack (with cells, cell holders and plates) are fixed with an injection of silicone at selected locations. The battery module is also designed to withstand a transport temperature between -35 to 65 ° C. However, the battery life will deteriorate if exposed to extreme temperatures.

The basis for this work is, as mentioned earlier, that the battery company wants to enable quick recharging of its lithium batteries. By fast charging, it means that the charging time is reduced by an increase in the power supply. The increase in power supply, in turn, increases the temperature of the heat sources: battery cells and aluminum sheets. The temperature of the battery module is distributed in such a way that the highest temperature occurs at the top of the module. This is a result of all current to all battery cells passing through the plates and thus more current flows at the top of these. Currently, the battery is charged with 0.8 times the capacity of the battery C, which results in charging time of about 114 minutes. The company aims to charge the battery in 45 minutes, which means a power supply of twice the capacity of the battery C, also called 2C charging. According to the company, when charging twice the battery capacity, about 50 W of heat per unit of time is generated which needs to be cooled. The charging process is currently done by manually switching the charging equipment.

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2 Methods

This chapter describes the method choice of the work, which is based on the purpose and question of the work. The section is divided into three parts: problem determination, problem solving and evaluation, assessment and choice of solutions.

All the elements represent the stages of the product development process and include the work's applied methods for product development. In connection with the presentation of each part, the execution of the work will also be reported.

2.1 Chosen methodology

In this project, Ullman's book has been used as a guideline for the process of doing the project. As it is shown in the figure below, the steps which need to be taken from the start of project definition to the product development phase with considering limitations has been determined.

Figure 2-1 An overview of the mechanical design and product support process (D. G. Ullman 2016)

2.1.1 Literature research methodology

Some different keywords have been used to get information about Lithium-ion batteries working situation and the cooling system related to the batteries which are used right now or trying to develop. There are a lot of articles about batteries and cooling systems which after reading the abstract of related articles have been filtered for use at the needed time. Articles which some part of them have been mentioned in this report addressed in the references part.

2.1.2 Benchmarking

The essence of benchmarking is the process of identifying the highest standards of excellence for products, services, or processes, and then making the improvements necessary to reach those standards, commonly called “best practices” (Elmuti and Kathawala 1997).

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In this case, some actions have been taken to find a way to tackle a product problem.

For benchmarking two ways have been taken to reach the best practices. First one is to search in other industries and their solution and mechanism which could help the conceptual design phase to solve the cooling problem of the batteries, and the second is to search out of the batteries industries and try to find similar problems in other production and their solution for this. It is one of the best ways of reaching the best concept of the product by comparing these data and information.

2.1.3 Quality function deployment (QFD)

There are many techniques used to generate engineering specifications. One of the best and currently and most popular is called Quality Function Deployment (QFD).

What is good about the QFD method is that it is organized to develop the major pieces of information necessary for understanding the problem: (D. G. Ullman 2010)

1. Hearing the voice of the customers

2. Developing the specifications or goals for the product

3. Finding out how the specifications measure the customers’ desires 4. Determining how well the competition meets the goals

5. Developing numerical targets to work toward

Customer requirements and the engineering specifications are the two vital and critical parts of the QFD house. To measure each requirement and reach the target value for our design, there is a need for having appropriate parameters named as engineering specifications consisting of exact units. Without such information, the engineers cannot know if the system being developed will satisfy the customers.

Engineering specifications consist of parameters of interest and targets for parameters. Engineering specification is the voice of customers in engineering language. Also, these parameters are showing the relation between the impacts of engineers' decisions on satisfying customer requirements.

2.1.4 Importance matrix

After determining the relation between requirements and specifications, there should be a comparison between the values of the importance of each specification.

To measure the value of specification, the weight of each specification is related to all requirements that should be considered. Then the value of specification could give the designer which target is more valuable and vital than others to satisfy customer requirements.

2.1.5 Pugh matrix

The Pugh matrix used to evaluate the concepts as a tool to use to facilitate a disciplined respecting the team method process for concept generation and

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selection. According to the strengths and weaknesses against a reference concept, it is recognized as a concept comparison. (D. G. Ullman 2010)

2.2 Alternative methods

Here are different methods which could have been applied to the project but no need forced to use them. However, they have been inspired by some solutions that have been considered in the process.

2.2.1 Generic product development

Ulrich and Eppinger have released a multidisciplinary model of the product development process with some changes which have been made in steps of the usual task. As it has been shown below, it is the whole process of Ulrich and Eppinger method. (Ulrich and Eppinger 2012) The schematic of the whole process is shown in figure 2.2.

Figure 2-2 Ulrich and Eppinger - The product development process (Ulrich and Eppinger 2012)

2.2.2 TRIZ

Theory of inventive problem solution or TRIZ “Teoriya Resheniya Izobretatelskikh Zadach” is a Russian creativity method in the theory of inventive problem-solving.

It is defining almost 40 dimensions of a system with the capability of improvement based on contradictions. Founder of the method also has presented 40 principles due to the inventions and established a method which by using these principles and determining contradiction will solve the problems (Rantanen and Domb 2008). The most important tool of TRIZ is the contradiction matrix which rows are showing improvement required features and columns illustrating worsening features. Cross-

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section of each row and column would contain TRIZ principles that solve this certain contradiction (Hipple 2012).

2.2.3 Biomimicry

Designing with inspiration from nature is one of the methods for developing solutions for engineering problems (Helms, Vattan and Goel 2009). This method is also known as the intersection of engineering and biology which contains various aspects to increase the ability of problem-solving for different problems.

Biologically inspired design is a system to develop solutions for engineering problems (Helms, Vattan and Goel 2009). Or in another definition it is the intersection of biology and engineering and contains many different areas which engineering would enhance the solutions for biological problems and vice versa.

Some of the areas which have the ability to influence and change in the process are bioengineering, Biomechanics, biomedical engineering, biophysics, bionics and biomimicry (Shu, et al. 2011). The aim of biomimicry is to be responsible and sustainable in using resources and taking inspiration from nature to increase the efficiency of products and processes (Primlani 2013).

Reducing heat from an organ is a common act in nature which can possibly be observed between animals. But in this study, biomimicry methodology was not followed as a direction and just had a roll of inspiration to reach the solutions.

2.3 Project work flow

At end, the following chart shows the project flow and the steps has been taken based on integrating studied methodology and the project condition to achieve best concept for the product (Appendix II).

Figure 2-3Overall view of Project Flow

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3 Theory

The following chapter presents the theory that underlies the work's implementation and results. Initially, the lithium battery and its use and significance are explained.

Thereafter, the heat generation in lithium batteries is explained and how cooling of these can proceed. Finally, the chapter describes how a coolant for a cooling system can be selected and what the stock exchange should have in mind when choosing, as well as some important parameters for heat transfer.

3.1 Introduction for Lithium Battery

A battery can be defined as a system where an electrochemical reaction converts the chemical energy of material into electrical energy. The lithium battery is one of many different types of rechargeable batteries on the market and is used in various electrical products. Common areas of use are energy storage systems in, for example, small electric portable units or different types of electric vehicles. The reason why lithium batteries are common and have changed battery cell technology is that lithium is the metal with the lowest density and high energy density. These properties make the lithium battery a suitable candidate for applications requiring a compact lightweight design with high performance and fast charging (Park 2012).

3.1.1 Heat development in lithium batteries

In a lithium battery, heat generation occurs as a result of charging the battery. This heat development mainly consists of resistance heat and chemical reaction heat (G.

Pistoia 2013). The resistance heat is a result of electrons colliding and forming frictional heat as they move in the same direction due to the voltage field. It is important to limit this heat generation since performance, service life and safety can be adversely affected by the operating temperature of a lithium battery (Bandhauer, Garimella and Fuller 2011), (Genies and Glaize 2013).

3.1.2 Cooling of lithium batteries

Two techniques could follow in the cooling system. Tab cooling and surface cooling are two different categories of cooling techniques. Selecting each of these techniques depends on the condition of batteries and the possibility of applying.

One of the concerns in the batteries is temperature gradient and its effects on the cells and their lifetime. The temperature gradient between the core and the outer layer of the cell can exacerbate unbalanced discharging. In comparison to surface cooling approaches, tab cooling has the potential to be more compact given those cooling mechanisms in between cells are not required. Furthermore, tab cooling approaches for battery cells can extend their service life by providing an in-plane temperature gradient through the cells (Worwood, Kellner and Wojtala 2017).

There are different ways to cool lithium batteries: you either cool with a liquid or a gas. The cooling method you use is mainly based on how much heat needs to be cooled, what it is that needs to be cooled, and how much space you have available

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for your cooling system. Using liquid needs more equipment and consideration than using gas in most cases.

3.2 Introduction to heat transfer 3.2.1 Heat transfer

The performance of a cooling system is directly linked to its heat transfer. Heat transfer is defined as the amount of heat, in the form of energy, transferred from one system to another, as a result of a temperature difference. The driving force for the heat transfer is the temperature difference and its various mechanisms are conduction (conduction), convection and radiation. The conduction and convection mechanisms are similar to each other as they both need the presence of a solid material to function. However, they differ in that convection requires the presence of a moving fluid. Heat transfer through a solid always takes place with conduction while it is more complicated for a liquid or gas. For a liquid or a gas, the heat transfer includes both convection and conduction, where the movement of the fluid improves the transfer of energy. This is because the fluid initiates a higher conduction rate at a greater number of locations in the fluid. Thus, an increase in the velocity of the fluid also gives a higher heat transfer rate. The heat transfer rate of convection depends on the properties of the fluid such as thermal conductivity k, dynamic viscosity, density and specific heat capacity Cp. It also depends on the surface fineness and geometry of the fluid channel, as well as the type of flow (laminar or turbulent) that prevails. The type of flow that is commonly used is turbulent as it usually results in higher heat transfer coefficients. The heat transfer rate can in some cases be increased up to 400% by using a turbulent flow instead of laminar. This can be applied by manufacturing the fluid channels with a coarser surface finish or with fins (Cengel, Cimbala and Turner 2017).

3.2.2 The heat transfer capacity

The heat transfer is dependent on the thermal conductivity of the cooling system (denoted by k) and is measured in W / mK or W / m ° C, as well as the heat storage capacity that can be expressed in how much energy can be stored in a material per unit of mass or volume. The heat storage capacity is represented by the specific heat capacity number Cp and is measured in J / kgK or the heat capacity Cp multiplied by the density, with the unit J / m3K. The heat transfer is thus dependent on the material of the cooling equipment and how it is constructed. Different materials conduct and store heat differently, for example, heat conductivity varies by a factor of 104 between wool and pure metals (Cengel, Cimbala and Turner 2017).

3.2.3 Thermal diffusivity

Thermal diffusivity is a material property that is significant for how well a cooling system performs and represents how quickly heat dissipates in a material. This property is defined as the thermal conductivity in relation to the thermal capacity, α is the ratio between the thermal conductivity and the thermal storage capacity, α is measured in m2 / s, which means that the heat propagates faster in a material with

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a high thermal diffusivity while a material with a low value mostly absorbs the heat (Cengel, Cimbala and Turner 2017).

3.2.4 Thermal resistance

The resistance to heat transfer between two systems is thermal resistance and can be divided into convection resistance, conduction resistance, radiation resistance and also contact resistance. These resistances can easily be added to an equivalent thermal resistance. The greater this thermal resistance, the slower the heat transfer.

For example, the thermal conductivity resistance increases with additional layers of insulation at a solid surface (Cengel, Cimbala and Turner 2017).

Thermal contact resistance is the resistance that inhibits the heat transfer between two materials' contact surfaces. This resistance is mainly due to surface finesse and material properties but also to the temperature, the type of fluid and the pressure at the interface between the surfaces. With a fine surface finish and an increase in the contact pressure, this resistance decreases. A thin layer of any type of foil can be placed between the materials at the contact surface, thus reducing the resistance.

Thermal contact resistance has been found to be significant and can dominate the heat transfer for a good thermal conductor (Cengel, Cimbala and Turner 2017).

3.2.5 Fouling factor

The performance of systems that include some form of heat transfer between fluids or between a fluid and a heat source deteriorates over time. This is because deposits are formed on the heat transfer surfaces. The heat transfer is reduced as the extra layer on the surface helps to increase the thermal resistance of the system. The type of fluid and the choice of material in a system affect how extensive this factor becomes. The net effect of these accumulations is represented by a fouling factor and is measured in the unit m2K / W and therefore depends on the operating temperature and speed used on the fluid. The factor increases with increasing temperature and decreasing the speed of the fluid (Cengel, Cimbala and Turner 2017).

3.2.6 Temperature expansion coefficient

When an object is heated, the material will be stretched due to the temperature change temperature ∆T (° C), which means that its volume will change. This elongation can be determined using the formula ϵ = α∆T, where α is the temperature expansion coefficient of the material (also known as the length expansion coefficient). The elongation of the object δ which becomes due to this elongation can be calculated as: δ = Loα∆T, where Lo represents the original length 1 / ° C (Cengel, Cimbala and Turner 2017).

3.3 Radiation

Radiation is one of the energy transfer ways in the form of electromagnetic waves caused by a change in atoms’ electronic configuration. The transfer of heat by this method does not need the presence of an intervening medium and because of this,

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radiation is the fastest way of heat transfer. All bodies at a temperature above absolute zero emit thermal radiation.

𝑞̇ = 𝜀𝜎𝐴_𝑠𝑇_𝑠⁴ (3-1)

Which is the Stefan-Boltzmann constant and ε is the emissivity of a surface that is defined as a similarity of the surface with the blackbody (Cengel, Cimbala and Turner 2017).

3.4 Conduction

Conduction is the way of transferring energy through interaction between particles with a higher level of energy to the particles with a lower level of energy. It can happen in solids, liquids, and gases. In liquids and gases, it is more based on random motion and interaction between particles. In solids, it is based on vibration and the free electrons energy transportation. Heat transfer by conduction depends on the material, geometry, and temperature differences between two points (Cengel, Cimbala and Turner 2017).

𝑄̇ = 𝑘𝐴^𝑑𝑇

𝑑𝑥 (3-2)

In this equation, k is the thermal conductivity of the material (Cengel, Cimbala and Turner 2017).

3.5 Convection

Transferring energy between solid surface and liquid or gas in motion is happening through convection. It is a combination of conduction and fluid motion. The motion of fluid makes the transfer faster and the speed of fluid has a positive influence on it. Convection is caused by buoyancy forces which are the result of density differences caused by variation of temperature is called natural convection. If the fluid is forced to move over the surface by external forces, it is called forced convection (Cengel, Cimbala and Turner 2017).

𝑄̇ = ℎ𝐴_𝑠(𝑇_𝑠− 𝑇_∞) (3-3)

In this equation, h is the convection heat transfer coefficient in W/m2K. This coefficient is not the property of the fluid contacted to the surface. The value of this parameter depends on various variables such as the geometry of the surface, speed of the fluid, the nature of fluid motion, and the properties of the fluid (Cengel, Cimbala and Turner 2017).

3.6 Heat transfer from finned surfaces

Based on Newton’s law, the rate of heat transfer from a surface at the temperature Ts to the surrounding at T∞ is:

𝑄̇_{𝑐𝑜𝑛𝑣}= ℎ𝐴_𝑠(𝑇_𝑠 − 𝑇_∞) (3-4)

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Which A is the surface for transferring heat and h is convection heat transfer coefficient.

There are two ways to increase the rate of heat transfer through the convection which are increasing the convection heat transfer coefficient h or increasing surface area A. To increase h, it is possible to enhance liquid flow with adding pump or fan but it could depend on other parameters such as space and other possibilities. The second way is to increase the surface area by adding some extra surfaces and make more contact between surface and flow. Using fins is not a new method in nature.

In the Jurassic era, some sort of dinosaur had two rows of big bony plates down its back. After a long time studying these plates, scientists have figured out that a lot of blood flowed through the plates and may act as a radiator for it. In the analysis of fins, some assumptions have been considered to study it such as no heat generation in the fin, constant thermal conductivity, constant and uniform convection heat transfer coefficient (Cengel, Cimbala and Turner 2017). (Appendix III)

3.7 Forced Convection

Heat transfer by convection is a bit complex because of fluid motion and heat conduction together. Fluid motion brings warmer and cooler chunks of fluid in contact and increases the heat transfer. Because of that, the heat transfer rate with fluid is much higher by convection than by conduction. It means the higher velocity of fluid provides a higher heat transfer rate. Convection highly depends on the properties of fluid dynamic viscosity, thermal conductivity, density, specific heat, and fluid velocity. Furthermore, geometry and roughness of the solid surface and type of fluid have their influences on heat transfer by convection. Dependency on various variables and conditions makes convection the most complex way of heat transfer. Due to Newton’s law of cooling, the rate of convection heat transfer is:

𝑄̇ = ℎ𝐴_𝑠(𝑇_𝑠− 𝑇_∞) (3-5)

From the formula, the convection heat transfer coefficient h can be introduced as the rate of heat transfer between a solid surface and a fluid per unit surface area per unit temperature difference. The convection heat transfer coefficient is hard to determine because of its dependency to various variables. Consider the fluid flow near a surface. The relevant speed of the motion fluid near the surface is zero. It means that the fluid is in direct contact with a solid due to the viscous effect with no-slip which is known as the no-slip condition. The no-slip condition makes the velocity profile near the solid surface which is called a boundary layer.

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Figure 3-1Velocity regime through a surface (Cengel, Cimbala and Turner 2017)

As a result of the no-slip condition near the surface and zero velocity of the fluid contacting the surface, the heat transfer from solid surface to the fluid layer near the surface is by pure conduction (Cengel, Cimbala and Turner 2017).

𝑄̇_{𝑐𝑜𝑛𝑣}= 𝑄̇_{𝑐𝑜𝑛𝑑}= −𝑘_{𝑓𝑙𝑢𝑖𝑑}^𝜕𝑇

𝜕𝑦 𝑎𝑡 𝑦 = 0 ^𝑊

𝑚² (3-6) T is the representative of the temperature distribution in the fluid and ^𝜕𝑇

𝜕𝑦 is the temperature gradient at the surface. (Appendix III)

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4 Results

Due to the previous knowledge and information from the subject which mostly has been on experiences, following goals and results has been presented:

 Literature study and identification of battery cooling system and optimization

 Benchmarking of other methods within the industry

 Development of measures and evaluation for new concepts and products

 Evaluating concepts and their subsystems

 Final concept

The first two goals have been presented during three previous chapters. Thus, others will be introduced here.

4.1 Requirements and Specifications

The major goal of this project is to increase the heat reduction from the system as it is possible with having as small changes as possible in the original product. To have a proper technical evaluation for the new design for the system, specifications of the product have been developed. There are some general parameters from the analytical part of the cooling system which has been presented previously in the report and some other measurements which can evaluate other qualities of the final concept. To score the final concept, a Quality Function Deployment (QFD) house has been developed. (Appendix IV)

4.2 Conceptual design

Four different concepts have been considered in the design process. Depending on fulfilling requirements and providing more engineering specification according to QFD house, the final concept has been chosen to improve its details and measurements.

4.2.1 Sub concepts

1. Liquid cooling system for modules

This concept contains individual cooling radiators for each module which should be installed inside the modules in one side of the cells. It means that it needs some changes inside the modules block. Furthermore, being sure about sealing the component and providing standard safety in order to prevent unexpected damage to the system is important. This cooling system also needs different sorts of equipment to provide liquid circulation such as pump and evaporator which make this method very expensive. While this method has the advantage of the efficient and high rate of heat reduction, it needs a lot of changes in the product, various types of equipment, subcomponent maintenance, and a lot of expenses.

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Figure 4-1 the liquid cooling system in the battery’s module (Martin Karlsson 2019)

2. Heat sink with individual fan

Using a heat sink with an individual fan is a common way of reducing heat in the electrical component in industries. This method usually is using the systems with one or two heat sources. Applying too many numbers to this component will make the controlling system difficult. Furthermore, each of the fans needs an energy source and consume more power overall. In order to use a heat sink with an individual fan, more space between them and also inside the product will be needed.

The cost of implementing this method and cost of maintenance of components should be considered.

Figure 4-2 a single tower heatsink with an individual fan (CRYORIG 2018)

3. Individual liquid radiator

This concept is similar to the first concept in some aspects with a major difference which is the place of implementing the cooling radiator. In this method, a small radiator which works with liquid will be attached to each connector with the help of the three fans which are generating airflow, and are going to cool the modules.

This method also needs various types of equipment such as pump, evaporator, and the consideration of sealing and safety of components.

Figure 4-3 vapor compression cycle (Sinopoli 2010)

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4. Heat sink with three fans

Through the study and observing different products in the market, it has been conducted that an appropriate heat sink (shape, material, and used condition) has the ability to decrease the temperature of a component even in high-temperature performance conditions. It is easy to assemble, quite cheap, and no need for any advance changes in the system. Because of its simple working condition and having no advanced equipment, it is easy to control and even easy maintenance.

Figure 4-4 simple heatsink (ElectronicsBeliever n.d.)

4.2.2 Overall concepts

Comparison between these four initial concepts has been done in QFD house and the best concept due to the requirements has been chosen to develop and improve.

(Appendix IV)

Figure 4-5 the relation between requirements and mechanical specification in QFD house

Regarding the final concept of the product, an individual process for each subsystem has been implemented. Subsystems based on their functions should be presented before introducing the whole concept and its functionality.

Inner cover: This subsystem is responsible for leading the airflow generated from fans going over the heat sinks as it is possible to prevent wasting the capability of airflow to reduce the heat.

Heat Sink: to help the modules reduce their heat as it is possible through their connectors, heat sinks will provide more surfaces against the airflow.

Fans: the responsibility of generating airflow in the system is on the fans.

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Before starting to introduce the final concept for the product, the process of developing the product and its results which is the final concept will be explained here.

4.3 Concept developing process

This part of the report will introduce step by step changes and evaluation on the concept and the reason for taking these steps with the CAD model and their analysis in the ANSYS. Between different cooling systems which could be possible to consider in the system such as liquid cooling system, individual fan, heat sink with water circulation which need various equipment changes, and will force more cost for the product, using heat sink with some changes in the product has been chosen.

4.3.1 Preparation

In the first step, the full CAD model of the battery pack has been simplified and extra parts which have no direct connection with the cooling system have been deleted. The remaining parts of the package contain the fans, connectors and the channel space which in following steps has been called as airflow domain.

Figure 4-6 simplified model of battery pack

In order to determine the heat generation of the modules and the temperature of connectors in the system and also having a view of the initial condition of the cooling system, a simulation in the ANSYS has been executed. Here is some general condition of the system:

 Three fans provide 8m/s airflow speed

 12 copper connector

 50w heat generate by each module

 Ambient temperature 20 degrees Celsius

After importing the model in ANSYS and applying mesh to the whole model (number of elements is between 450000 to 511000 for the whole model in different following cases), the following setup due to the problem definition has been considered for the simulation.

 Steady-state

 The energy equation for the model is on.

 Viscous model: Realizable k-epsilon, Enhanced wall Fn.

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 Air: incompressible ideal gas

 Solution Method: SIMPLEC, Least Squares Cell-Based.

 Controls: Default

Figure 4-7 fluid domain and connectors in the meshing environment of ANSYS

To define the heat flux in the system, heat source has been defined at the bottom of each connector and because each of them is connected to two modules in the package, the maximum heat generation of each connector due to the surface of the connected part to the modules would be 220000 watt per square meter.

Figure 4-8 heat flux sources in connectors

4.3.2 Simulations and evaluations

The first simulation without any changes and applying parts to the system has been executed and the following result has been extracted from the contours from the simulation software. The temperature of the connectors is quietly high in the first simulation with no changes. The difference between the first and last connector in the raw also is very high. (All temperature are in Kelvin)

Heat sink_1: 363±10 Heat sink_6: 382±10 ΔHeat sink1_6: 19

Air:330±37 Outlet: 302±8

Figure 4-9 temperature contour of connectors in firs simulation

The interesting part of the simulation was about the airflow which has been provided with three individual fans. The generated flow by the middle fan has no

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effect on the connectors and mostly goes in the middle of the domain with no interaction with any connectors.

Figure 4-10 3D contours of air velocity and streamline

In order to prevent wasting of generated airflow by the middle fan and guide the air to go over connectors apart as a block has been added to the domain. Due to the airflow streamline, the block helps the flow to follow its pattern and guides the airflow from the middle fan to cover some part of the connectors. In the following developing process, as it is shown in the table, it has been tried to reduce the temperature of the connectors by adding different heatsinks in order to enhance the surface for convection heat transfer. It has been chosen to use aluminum heat sink as it is common in the market and cheaper than other materials with acceptable heat specification. In order to analyses the effect of different materials for the heat sink, a simulation has been run with a copper heat sink which has shown a small change in the temperature reduction in comparison to the aluminum one, so, it has been decided to carry on with the aluminum. Furthermore, it has been discovered that using thinner fins in the heat sink makes no change in the temperature of connectors than the thicker fin. To use less material in the product, it has been decided to use fins with thinner thickness in the following simulations. The difference between the temperature of the first connector and the last one in a row was unacceptable. This high difference has made the use of similar heat sinks for all connectors pointless.

This difference is a result of the changing temperature of the air which is going over the heat sinks in its way. In the purpose of decreasing this high difference of temperature, it has been decided to provide more airflow for the last three heat sinks by leading the generated airflow by the middle fan. Thus, it has been tried to find the best shape of possible block in front of air flow and also limiting the space for air flow to force it goes over the added heatsinks. Then, in order to cover more flow by heatsink and make a better contact between its surface and flow, some fins have been added to the both sides of the heatsinks. Furthermore, after some simulations, inlet velocity modules have been replaced by intake-fan modules which have been taken from the fan curve based on the pressure differences between the rear and front of the fan. In this case, any pressure drop shows its effects on the temperature reduction through the airflow speed over the heatsinks. The type of fan and its performance curve will be shown in the following section.

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It is better to simulate heat sinks with more thin fans and look at the possibility of having smaller distances between them. It also needs quite fine and smaller mesh in ANSYS to give an accurate answer from the small distances between fins in the domain and heat transfer between these two-parts. In order to have a better and accurate simulation, it is possible to have half body simulation because the model is symmetry in one direction. All the condition has remained as the previous simulation and just the heat sink has been changed to find the better model of the heat sink for the cooling system. Various shapes of the heat sink have been simulated to study their influences on the cooling system and reducing the temperature of connectors. Some of these heat sinks have acted similarly as a block in front of the airflow and prevented having suitable airflow. Thus, the fans could not provide enough airflow over the components in the system. On the other hand, some of these heat sinks have made no difference in the system in order to decrease the temperature. (Appendix V)

Due to the component temperature in the simulations, the temperature of the connectors has decreased almost 150 degrees after adding heatsinks to the system.

Thus, according to the theory and simulation, it could be concluded that applying suitable heat sinks to the system could lead us to the satisfactory result of reducing heat from the modules.

According to the table, simulation No. 7, the temperature of the connectors has dropped almost 270 degrees. The temperature difference between the first and last connector has been decreased from 137 to 20 degree. It means that the heat reduction between the first connector and last connector with air is going to be similar. The table shows the developing process in summary and illustrates the pattern that has been followed to achieve a satisfying result. It has been planned to reach the temperature below 40 degrees Celsius for connectors in the first place, despite that it has not been delivered through this study, but the temperature reduction of the last connector between first and last simulation has been reached to 350 degrees.

The temperature difference between the first and last connector has been decreased from 137 to 7 degrees. This achievement is giving the possibility of using the same heat sink for all connectors in the cooling system with no change in dimension or shape. Results has been shown that by increasing in heat reduction, the temperature has decreased. In appendix VII you can see the changes in hA related to the heat sink property and its effect in reducing the temperature of the connectors during the developing process.

The main problem here for the continuing study is, the Student version of ANSYS has a limitation which prevents generating more than 512000 elements. Thus, it is not possible to simulate and reach reliable results with having a dimension less than 2mm in this case. (It is not possible to have a fin thickness less than 2mm or having a distance between them less than a particular amount). (Appendix VI)

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4.4 Final concepts

According to the simulations, the last successful simulation gave us promising results. Because of the limitation which has been mentioned previously, it was not possible to execute more simulations. But from the experiences from simulation and the pattern that is shown and also from the theoretical part of study, it is possible to put some changes and assumptions to the final concept. After finalizing the result from simulation and deciding about the final component, it is time to make the decisions about the details of sub concepts such as material, manufacturing process and the type of them.

4.4.1 Material Specifications

Selecting materials is one of the vital aspects in the development process. The questions are, how it is possible to choose the best one and which the best material for a specific product design is. Many industries are struggling with selecting best materials for their components in different aspects of manufacturing such as surfacing, strength, costs. CES Edu pack is a software which has a massive database of materials and gathers the data of them centralized. With the tools which this software provides, users can select the best material in its structured data base due to the application requirements and its process in a cost-effective way.

Figure 4-11 CES Edupack material selection

It is important to make sure that this product is reliable for safety and operational purposes. In the design part of the platform, dimension and shape of most parts of the product has been selected due to the availability in the market. The selected materials have been tested in the simulation of the product and showed satisfying results.

4.4.2 Sub Concepts

In the purpose of organizing the final concept and explaining its details in separate parts, sub concepts will be introduced separately. Furthermore, some consideration such as keeping the initial product the way as it is with no change, has been considered here.

1. Heat sink

According to the simulation, the type of heat sink which is needed in the system should have the ability of heat reduction between 15 to 25 W/k at the best range.

The shape of heat sink in this type of system is important because of air flow

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resistance in the system. From the designer's opinion and result from simulation, applying a heat sink which can play a role like a channel for the flow is the best option. Finding this specific type of heat sink in the market could be difficult. Thus, the concept of heat sink with its specifications and dimension will be introduced here. Figure below shows the heat sink and the dimension of it that has been used in the simulation and final answer.

Figure 4-12 heat sink concept for the product with dimension

As it has been cleared before selecting material is one of the important parts of designing a product. With the help of CES Edupack, some materials have been considered for the heat sink by limiting some specifications such as high thermal capacity, heat transfer coefficient, heat expansion, and low electrical conductivity.

Furthermore, the cost of material, manufacturing process and its cost, and availability in the market has been considered. Finally, it has been decided to use Aluminum for the heat sinks because of its specifications and being common in the market for this kind of components. The table of mechanical and thermal specification of the Aluminum is available in Appendix VIII.

There are different methods and techniques of manufacturing to produce heat sink from aluminum material.

 Stamping: The dimensions of the part are relatively small. Fins are created by cutting and bending the coil

 Forging: widely used by heat sink manufacturers in the production of light source heat sinks

 Die Casting: For huge amounts of parts, tooling costs for die is very high, the freedom of 3D design

 Extrusion: shaping by applying plastic deformation pushed by a press

 Machining from full block: cut and remove the material from full blocks, CNC machine, high cost

 Brazing: thermal performance and design flexibility, the maximum performance with low pressure drop, best thermal-electrical continuity, low machining costs

Between the various techniques of manufacturing, considering extrusion and brazing has suggested. For manufacturing the heat sink it is suggested that it should be manufactured by applying extrusion to produce the cubic part of the heat sink and by brazing technique pure Al fins can be joined with metallic continuity to the

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part. Because brazing brings high thermal performance in a heat sink with lower pressure drop in the system. As it has been designed, the heat sink can be assembled on the connectors easily by two nuts on the connectors and for the assurance of the good connection between two parts for transferring heat, it is better to apply thermal tape or epoxy adhesive between them.

Figure 4-13 heat sink position and assembly in the product

2. Inner cover

The role of this component in the product is limiting the space of the upper part of the product to force the flow to follow the pattern that is more helpful and efficient for the cooling system. In order to provide this aim, the concept of this subsystem will be introduced here.

Figure 4-14 designed cover for heat sinks

There is no load on this component and it should just lead the flow over the heat sinks. Furthermore, it should have enough resistance to heat and higher temperature for the safety of the other components. Between the different materials which could be suitable for the cover it has been decided to consider galvanized sheets to be the selected material for the prototyping. In order to produce this part for the prototype, there is just a need of using cutting equipment to cut the sheet and also add the block part with simple weld. There is no need for an assembly process, because of the design and dimension of the cover, it will be placed in the correct position.

For the mass production of this part, it is cheaper and also suitable enough to use an injection molding process. But as it is just some number of this product and also for the prototype and product test, investing on manufacturing mold for this purpose could be very expensive.

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25 Figure 4-15 designed cover position in the final concept

3. Selected Fan

From the theoretical part of the study, it has been conducted that more velocity of flow will bring more convection heat transfer for the system. But the convection heat transfer coefficient depends on various variables which in some cases improving the velocity of flow could not provide more heat transfer. Thus, from the manufacture selection of the fan supplier, just two different fans have been selected for the cooling system. The selected fan has been introduced from the supplier catalogue which is compatible with product and its requirements. Due to the product and the voltage provided for the fan, two models of fan 8318NHH and 8318NH3, have been chosen for the final concept. While 8318NHH is promising for the cooling system due to the performance curve, but the 8318 NH3 is the better choice for the cooling system because of its ability of performing under more pressure drop. Specification of the fan is available in Appendix IX.

4.5 Discussion

The result can be discussed in two aspects, developing the knowledge and concept development based on the knowledge.

4.5.1 Knowledge development

From the literature study and study on the similar problem and solution, the principle of the future design has been determined. However the important variables have been found and identified and furthermore for each parameter there are some experiments and simulations, but the differences between the condition of each problem enhance the uncertainty of the similar solution which has been found. On the other hand, a similar solution has been found in various products which has been implemented by manufacturers to tackle this kind of problems with their product, mostly in cooling solutions for computer processors and high performance electrical parts.

Furthermore, the lithium-ion battery cooling system, which is common in the automotive industries and the management system of it is a complicated area which needs to be studied in a separate investigation. The result of this kind of study could be helpful to have a better understanding of the condition of the modules and battery with the influence of the cooling system and the needs of such a system.

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4.5.2 Concept development

From the investigation and simulation result, sub concepts and the final concept for the product improvement has been introduced in this study. First, the study itself, with both theoretical parts and simulations and suggestions for the prototype can be the base of further studies and also the next product. Second, from the possible and potential solution for the problem of the product, the prototype can be tested experimentally in purpose of validating the result of the conceptual design part and its simulation results.

The results and final concept have presented to the client which would be a supportive and reliable document for their further product improvements. Because of the considerations and the situation at the time of implementing this study, the prototyping could not be possible to perform. Thus introducing the prototyping was the last part of the study (Appendix X).

5 Conclusion

The study is a development project to investigate the possibility of using air flow as a coolant part of cooling system for a specific lithium-ion battery module which has increased the scientific knowledge about the cooling systems and fluid dynamics and also introducing conceptual design processes could be follow by the manufacturer.

5.1 Conclusion

This project has been executed by the request of the battery company to investigate and study on the low cost solution for reducing heat from the battery modules during operation and charging period. This study was a product development project but because of the wide area of the science in this field and limited information about the product, it was needed to build the base structure of product development. The wide knowledge of the cooling systems, fluid dynamics, and batteries have been discovered and achieved through the literature study and up on these data and knowledge concept development has been implemented, but in purpose of proving the function of the concept, it was not possible to execute prototyping as the final step. But the investigation has shown that using airflow as a coolant and heat sink to help the heat reduction from the system during the fast charging could be the low cost solution to deliver such a goal. The list below shows the outcome of the study:

 Study of various literature in order to define important variables which have influence on the performance of cooling systems operating with forced air flow (forced convection).

 Investigating the influence of geometry of the component on the cooling system.

 Identifying customer requirements and the importance of them.

 Developing technical specifications to measure and comparison between concepts.

Decreasing charging time of Lithium-Ion Battery by controlling temperature during the process

Master Thesis

UNIVERSITY

Master's Programme in Mechanical Engineering, 60 credits

Decreasing charging time of Lithium-Ion Battery by controlling temperature during the process

Mechanical Engineering, 15 credits

Goteborg 2020-05-27

Seyedhafez Sadeghi Meykola

Abstract

Acknowledgement

List of Figures

List of Abbreviations

Contents

1 Introduction

2 Methods

3 Theory

4 Results

5 Conclusion