DEVELOPMENT OF BATTERY MANAGEMENT SYSTEM IN EV APPLICATION USING WIRELESS

COMMUNICATION

BY

MD MIZANUR RAHMAN

A thesis submitted in fulfilment of the requirement for the degree of Doctor of Philosophy (Engineering)

Kulliyyah of Engineering International Islamic University Malaysia

MARCH 2020

ii

ABSTRACT

The lithium-ion battery pack is a very important part of an electric vehicle (EV) and is an expensive component. If there is no proper battery management system (BMS) for the lithium-ion battery pack, the overall performance could be affected in several ways, including the lifecycle, charging-discharging behaviour, safety and ambient temperature. Because a battery pack is exposed to different conditions, such as ambient temperature, aging and manufacturing variation, over state of charge (SOC) and under depth of discharge (DOD), the charge of the series connected cells becomes unbalanced. During the rapid charge balancing (transferring) process, the internal temperature of the cells may exceed its allowable limit (46°C) which results in unstable balancing behaviour. Besides this, communication makes the BMS convenient and even smarter by connecting all the sensors, including the sensors for voltage, current, SOC and temperature, of the battery pack. However, a large number of wire terminations in the BMS, including among the sensors, are liable to safety failure and are not fully reliable. To help address these issues, this research focuses on developing a BMS that includes a charge balancing system and wireless communication system for three series connected battery cells. Several local control units and one central controller are used to achieve this. The charge balancing system uses a DC to DC converter and a controlled algorithm, which considers internal ambient temperature, to overcome the challenges associated with the charge balancing process. With this approach, the increasing internal temperatures in the battery cells are maintained within the range of 27°C to 35°C. Real time information is monitored and used to control the functionality of the battery pack using wireless communication. The wireless communication system, using the ZigBee communication protocol and point-to-point topology, has reduced wiring problems, as well as size and cost, compared to the conventional communication system. The simulation results of this system have been verified by the experimental results. The wireless communication and control management system developed in this research can be applied to large battery packs to improve their overall performance.

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خلاصة البحث ABSTRACT IN ARABIC

انبعاث من تقلل أ�ا مثل العديدة لمزا�ها نظرا ، النقل مستقبل وتعتبر (EVs) بسرعة الكهربائية المركبات تطوير يتم أصبحت ، الفعالة الطاقة مصادر على الطلب تزايد مع .كبير بشكل للمركبات والأوزان والتكاليف السامة الغازات أيون ليثيوم بطار�ت .(EV) الكهربائية السيارة مكو�ت أغلى وأحد للتطبيق قابلية الأجزاء أكثر البطار�ت مجموعة

، ذلك ومع .الشحن لإعادة القابلة الأخرى بالبطار�ت مقارنة العالية وكثافتها طاقتها بسبب EVs ل ملاءمة أكثر والتفريغ الشحن وسلوك الحياة ودورة الوظيفي والأداء الكلي الأداء على كبير تأثير لها البطارية حزمة حرارة درجة فإن

وتغير والشيخوخة ، المحيطة الحرارة درجة :مختلفة لظروف تتعرض البطار�ت مجموعة أن بما .السلامة إلى بالإضافة العادي غير المتسلسلة المتسلسلة الخلا� تصبح ، (DOD) التصريف عمق وتحت (SOC) الشحن حالة وفوق ، التصنيع درجة 46) به المسموح الحد الداخلية الحرارة درجة تتجاوز قد ، (النقل ) السريع الشحن موازنة عملية خلال .متوازنة مستوى لتحقيق البطارية لمجموعة بالنسبة أساسية أهمية وذات للغاية مهمة مشكلة التحكم مع الشحن توازن يعتبر .(مئوية ربط بسبب ذكاءً أكثر البطارية إدارة نظام الاتصال يجعل ، ذلك جانب إلى .طويلة حياة ودورة والسلامة الأداء من عالٍ الخلا� من كبير عدد وهناك .البطارية حرارة ودرجة SOC و والجهد للجهد الاستشعار أجهزة مثل المستشعرات جميع

البطار�ت إدارة نظام في .السلامة لفشل عرضة تكون التي الأسلاك إ�اء من العديد لديها البطارية حزمة في الفردية .(CAN) التحكم منطقة شبكة اتصال خلال من ومراقبتها المعلومات هذه إلى الوصول يتم ، (BMS) التقليدي

من وطبولوجيا ZigBee الاتصال بروتوكول يستخدم سلكيلا بطارية إدارة نظام تقديم تم ، الأسلاك مشاكل لمعالجة كعقدة المستشعر عقدة) اللاسلكية والوحدات الدقيقة التحكم وحدات استخدام يتم .البحث هذا في نقطة إلى نقطة

المعلومات هذه ونقل (SOC و الحرارة ودرجة الجهد) مستشعرات عدة من المعلومات لمعالجة (كمستقبل والمنسق للمرسل طاقة محول باستخدام البطارية حزمة شحن توازن في التحكم تم ، ذلك إلى بالإضافة .التوالي على ، العرض أجهزة إلى

. اللاسلكي الاتصال عبر (وإيقاف تشغيل ) تشغيله يتم رقمي

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APPROVAL PAGE

The thesis of Md Mizanur Rahman has been approved by the following:

_____________________________ Muhammad Mahbubur Rashid

Supervisor

_____________________________ Sany Izan Ihsan Co-Supervisor

_____________________________ A.H.M Zahirul Alam

Co-Supervisor

_____________________________ M.A Hannan

External Examiner

_____________________________ Saad Mekhilef

External Examiner

_____________________________ Tanveer Saleh

Internal Examiner

_____________________________ Fouad Mahmoud Mohamed Rawash

Chairman

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DECLARATION

I hereby declare that this thesis is the result of my own investigations, except where

otherwise stated. I also declare that it has not been previously or concurrently submitted

as a whole for any other degrees at IIUM or other institutions.

MD MIZANUR RAHMAN

Signature ........................................................... Date .........................................

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INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA

DECLARATION OF COPYRIGHT AND AFFIRMATION OF FAIR USE OF UNPUBLISHED RESEARCH

DEVELOPMENT OF BATTERY MANAGEMENT SYSTEM IN EV APPLICATION USING WIRELESS COMMUNICATION

I declare that the copyright holders of this thesis are jointly owned by the student and IIUM.

Copyright © 2020 Md Mizanur Rahman and International Islamic University Malaysia. All rights

reserved.

No part of this unpublished research may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without prior written permission of the copyright holder except as provided below

1. Any material contained in or derived from this unpublished research may be used by others in their writing with due acknowledgement.

2. IIUM or its library will have the right to make and transmit copies (print

or electronic) for institutional and academic purposes.

3. The IIUM library will have the right to make, store in a retrieved system and supply copies of this unpublished research if requested by other universities and research libraries.

By signing this form, I acknowledged that I have read and understand the IIUM Intellectual Property Right and Commercialization policy.

Affirmed by Md Mizanur Rahman ……..…………………….. ……………………….. Signature Date

ACKNOWLEDGEMENTS

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All glory is due to Allah, the Almighty, whose Grace and Mercies have been with me throughout the duration of my programme. Although, it has been difficult, His Mercies and Blessings on me ease the enormous task of completing this thesis.

It is my utmost pleasure to dedicate this work to my dear parents and my family, who granted me the gift of their unwavering belief in my ability to accomplish this goal: thank you for your support and patience. My gratitude goes to my beloved wife and lovely child; for their prayers, understanding and endurance. I wish to express my appreciation and thanks to Dr. Ataur Rahman, Dr. Amanullah, Dr. Rafia Afroz, Sadakatul Bari, Brother Marwan, Alal Hossain and Niaz Morshed who provided their time, effort and support for this work.

Finally, a special thanks to supervisor Assoc. Prof Dr Muhammad Mahbubur

Rashid and co-supervisors Dr. Sany Izan Ihsan and Dr. A.H.M Zahirul Alam for their continuous support, encouragement and leadership, and for that, I will be forever grateful. To the members of my dissertation committee, thank you for sticking with me.

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TABLE OF CONTENTS

Abstract .................................................................................................................... ii Abstract in Arabic .................................................................................................... iii Approval Page .......................................................................................................... iv Declaration ............................................................................................................... v Copyright ................................................................................................................. vi Acknowledgements .................................................................................................. vii List of Tables ........................................................................................................... vi List of Figures .......................................................................................................... xi List of Abbreviations ............................................................................................... xv List of Symbols ........................................................................................................ xvii CHAPTER ONE : INTRODUCTION ................................................................. 1

1.1 Background of the Study ........................................................................ 1 1.2 Statement of the Problem........................................................................ 6 1.3 Research Objectives................................................................................ 7 1.4 Research Philosophy ............................................................................... 7 1.5 Research Methodology ........................................................................... 8 1.6 Chapter Summary ................................................................................... 9 1.7 Thesis Organization ................................................................................ 9

CHAPTER TWO: LITERATURE REVIEW ..................................................... 11

2.1 Introduction............................................................................................. 11 2.2 Battery Model/ Battery Pack Size........................................................... 11 2.3 Generalized BMS.................................................................................... 18

2.3.1 Charging System........................................................................... 19 2.3.1.1 Conductive Charging ....................................................... 22 2.3.1.2 Wireless Charging/Contactless Charging ........................ 26

2.4 Temperature Analysis of the Battery Cell .............................................. 27 2.4.1 Thermal Management ................................................................... 30 2.4.2 Monitoring SOC and Temperature via Communication .............. 32

2.4.2.1 Wired Communication ..................................................... 33 2.4.2.2 Wireless Communication ................................................. 35

2.4.2.2.1 Wireless Communication Technologies ............ 35 2.4.3 Charge Equalization System ......................................................... 39

2.4.3.1 Passive Balancing............................................................. 44 2.4.3.2 Current Bypassing Method .............................................. 44 2.4.3.3 Cell Bypass (Protection) Method ..................................... 44 2.4.3.4 Active Balancing .............................................................. 45

2.4.3.4.1 Cell to Cell ........................................................ 47 2.4.3.4.2 Adjacent Cell to Cell ......................................... 48 2.4.3.4.3 Direct Cell to Cell ............................................. 48 2.4.3.4.4 Cell to Module /Cell-to-Pack Methods ............. 49 2.4.3.4.5 Module to Cell ................................................... 50 2.4.3.4.6 Module to Module ............................................. 51

2.4.4 Algorithm and Control Base Charge Balancing System .............. 52

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2.5 Chapter Summary ................................................................................... 56 CHAPTER THREE : DEVELOPMENT FOR WIRELESS BATTERY MANAGEMENT SYSTEM .................................................................................. 57

3.1 Introduction............................................................................................. 57 3.2 Battery Management System (BMS) ...................................................... 58 3.3 State of Charge (SOC) Balancing System .............................................. 58 3.4 DC to DC Converter Design for Proposed Method ................................ 63 3.5 Balancing Control Algorithm ................................................................. 68 3.6 Wireless Communication ....................................................................... 70 3.7 ZigBee Technology ................................................................................ 71 3.8 Wireless Monitoring and Controlling Design Model ............................. 73 3.9 Wireless Communicatio Framework ...................................................... 74 3.10 Topology and its Configuration ............................................................ 75

3.10.1 Microcontroller and Language ................................................. 81 3.11 Chapter Summary ................................................................................. 81

CHAPTER FOUR : RESULT AND DISCUSSION ........................................... 82

4.1 Introduction............................................................................................. 82 4.2 Simulation Results .................................................................................. 82

4.2.1 Power Converter ........................................................................... 82 4.2.2 Temparature Analysis ................................................................... 86 4.2.3 Charge Balancing ......................................................................... 89

4.3 Result for Experimental Work ................................................................ 96 4.3.1 Power Converter Output ............................................................... 96 4.3.2 SOC and Voltage Measurement ................................................... 100 4.3.3 Cell Temperature Measuring and Monitoring .............................. 107 4.3.4 Current Measuring and Monitoring .............................................. 107 4.3.5 Charge Balancing Result .............................................................. 108 4.3.6 Wireless Communication ............................................................. 114

4.4 Chapter summary .................................................................................... 116 CHAPTER FIVE : CONCLUSION AND RECOMMENDATION .................. 118

5.1 Conclusion .............................................................................................. 118 5.2 Contribution of the Work ....................................................................... 118 5.3 Future Work and Recommendations ...................................................... 120

REFERENCES ....................................................................................................... 122 LIST OF PUBLICATIONS .................................................................................. 132 APPENDIX ............................................................................................................. 133

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LIST OF TABLES

Table 2.1 Balancing structure and comparison I 53

Table 2.2 Balancing structure and comparison II 54

Table 2.3 Summary of related works 54

Table 2.4 Summary of balancing system based on balancing time 55

Table 3.1 The configuration of common parameters of ZigBee Modules 76

Table 3.2 The configuration of ZigBee Modules 77

Table 3.3 The steps of sending and receiving data for microcontrollers 78

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LIST OF FIGURES

Figure 1.1 Typical battery pack for electric vehicle 2

Figure 1.2 Electric vehicle architecture with battery management system (BMS) 2

Figure 1.3 Example of a controller-area network (CAN) bus interface. 4

Figure 1.4 The flow chart of proposed system 8

Figure 2.1 Equivalent models of a non-linear battery cell 12

Figure 2.2 The chemical mechanism of a Li-ion battery during the charging and arging process 13

Figure 2.3 Nonlinear battery model 15

Figure 2.4 Open Circuit Voltage (OCV) vs State of Charge (SOC) 16

Figure 2.5 Battery pack structures (a) 1S0P (b) 1S2P (C) 2S2P 16

Figure 2.6 The safe operating limits for lithium ion batteries 17

Figure 2.7 A general energy transfer diagram for PEVs 21

Figure 2.8 A general block diagram of an OBC 23

Figure 2.9 Operation window with temperature and (a) voltage and (b) current 28

Figure 2.10 Lithium ion cell’s life cycle vs temperature 28

Figure 2.11 Battery Power and Temperature 30

Figure 2.12 Thermal management systems 32

Figure 2.13 Basic communication model 33

Figure 2.14 Message format before transmitting over network. 34

Figure 2.15 Parameters of balancing operation 41

Figure 2.16 Devices for charge balancing 41

Figure 2.17 Different types of balancing methods 43

Figure 2.18 Cell structure with protection for a single cell 45

Figure 2.19 Cell to cell structure using (a) Cuk converter (b) buck boost converter 46

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Figure 2.20 Adjacent cell to cell structure using switched capacitor 48

Figure 2.21 Module to Module structure 51

Figure 3.1 The proposed battery management system 57

Figure 3.2 Block diagram of proposed charge balancing system 59

Figure 3.3 Charge balancing during charging condition 60

Figure 3.4 Charge balancing during discharging condition 61

Figure 3.5 Charge balancing during inoperative condition 62

Figure 3.6 Proposed balancing circuit based on a buck converter 63

Figure 3.7 Current in inductor changed with duty ratio in a buck converter 64

Figure 3.8 Converter mode (a) On-state and (b) Off-state 66

Figure 3.9 Feedback loop for current control during balancing operation 68

Figure 3.10 Algorithm for proposed system 70

Figure 3.11 ZigBee protocol Stack/ Framework 72

Figure 3.12 ZigBee network Model 72

Figure 3.13 API frame structure 73

Figure 3.14 AT frame structure 73

Figure 3.15 BMS with wireless communication 75

Figure 3.16 Point to point topology base network model for BMS 76

Figure 3.17 Experimental setup for WBMS 78

Figure 3.18 Transferring data of BMS part (sensors and batteries) in AT mode 79

Figure 3.19 Receiving data in API mode 80

Figure 3.20 Monitoring information using software 80

Figure 4.1 Characteristics of proposed converter for proposed BMS 83

Figure 4.2 Voltage scenario from input and output side of the power converter 84

Figure 4.3 Output voltage with the reference voltage (4.2V) 85

Figure 4.4 Output voltage with current reference (2.5A) 85

Figure 4.5 Output voltage with current reference (2A) 86

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Figure 4.6 Internal temperature on different ambient temperature and constant discharging current (1A) 87

Figure 4.7 Internal temperature based on constant ambient temperature (25ºC) and 88

Figure 4.8 Balancing voltage during charging using PSPICE tool 90

Figure 4.9 Balancing SOC with voltage during charging using MATLAB/SIMULINK tool 90

Figure 4.10 Balancing voltage during discharging using PSPICE tool 91

Figure 4.11 Balancing voltage in normal condition using PSPICE tool. 92

Figure 4.12 Battery cell (B1): SOC, Voltage, Current and internal temperature during balancing. 93

Figure 4.13 Battery cell (B2): SOC, Voltage, Current and internal temperature during balancing. 94

Figure 4.14 Battery cell (B1 and B2): SOC of 2% variation and internal temperature during balancing. 94

Figure 4.15 Battery cell (B1 and B2): SOC of 5% variation and internal temperature during balancing. 95

Figure 4.16 Gate pulse (5v/div) at 10us window 97

Figure 4.17 Scenario I: the output voltage in light blue of the power converter at 10us window 97

Figure 4.18 Scenario II: the output voltage in light blue of the power converter at 5us window 98

Figure 4.19 Scenario III: the output voltage in light blue of the power converter at 10us window 98

Figure 4.20 Scenario IV: the output voltage in light blue of the power converter at 10us window 99

Figure 4.21 Scenario V: the output voltage in light blue of the power converter at 10us window 99

Figure 4.22 Scenario VI: MOSFET voltage in light blue of the power converter at 10us window 100

Figure 4.23 Scenario VII: MOSFET voltage in light blue of the power converter at 10us window 100

Figure 4.24 Connecting sensor between microcontroller and battery cell 101

xiv

Figure 4.25 SOC in charging condition 103

Figure 4.26 Voltage in charging condtion 104

Figure 4.27 Temperature in charging condition 104

Figure 4.28 SOC in discharging operation 105

Figure 4.29 Voltage in discharging operation 106

Figure 4.30 Temperature in discharging operation 106

Figure 4.31 Balancing result for three cells during inoperative condition 108

Figure 4.32 Charge (SOC) balancing during charging condition 109

Figure 4.33 Charge (voltage) balancing during charging condition 110

Figure 4.34 Temperature in cells during charge balancing in charging condition 111

Figure 4.35 SOCs balancing in discharging condition 112

Figure 4.36 Temperature in cells in discharging-charging condition 113

Figure 4.37 Wireless controlling traction motor with temperature 114

Figure 4.38 Wireless monitoring battary cell discharging 115

Figure 4.39 Wireless monitoring discharging profile 116

xv

LIST OF ABBREVIATIONS AES Advanced Encryption Standard AH Ampere-Hour API Application Programming Interface AT Transparent BEVs Battery-Powered Electric Vehicles BMS Battery Management System CAN Controller Area Network CCCV Constant Current Constant Voltage CLLC Capacitor Inductor-Inductor Capacitor C-rate Current Rate DOD Depth of Discharge DSSS Direct Sequence Spread Spectrum EMI Electromagnetic Interference EV Electric Vehicle ESSs Energy Storage Systems ICE Internal Combustion Engine I²R Internal Temperature (current*current*resistance) I-V Current-Voltage IWPT Inductive Wireless Power Transfer LAN Local Area Network LiCoO2 Lithium Cobalt Dioxide LiFePO4 Lithium Iron Phosphate LiMn2O4 Lithium Manganese Oxide LiNiO2 Lithium Nickelate LLC Inductor Inductor Capacitor MBWA Mobile Broadband Wireless Access MOSFET Metal-Oxide-Semiconductor Field-Effect Transistor OBC On Board Chargers OCV Open-Circuit Voltage OSI Open System Interconnection PCM Phase Change Material PFC Power Factor Correction PHEV Plug-In Hybrid Electric Vehicle QoS Quality of Service RC Resistor-Capacitor Rt Ohmic Resistance SC1 Energy Storage Capacitor SOC Sate Of Charge

xvi

SOH State-of-Health V2G Vehicle-To-Grid VOC Open Circuit Voltage WiMAX Worldwide interoperability for Microwave Access WSN Wireless Sensor Networks ZVS Zero Voltage Switching

xvii

LIST OF SYMBOLS

ºC Cell temperature

ρ Cell mass

I²R Internal temperature (current*current*resistance)

A Cell Current

V Cell Voltage

ΔIL current through the inductor

D Duty ratio

t Time

L Inductor

C Capacitor

B Battery cell

Df Diode forward

Q Converter Switch

AH Battery cell rating

T Internal temperature

Qgen the overall heat generation

Qs heat generation due to entropy changes

Rth thermal time constant

R Resistance

1

CHAPTER ONE

INTRODUCTION

1.1 BACKGROUND OF THE STUDY

Among the modern transportations, the electric vehicle (EV) is experiencing

tremendous success in the vehicle market. Battery technology is widely employed in

different applications such as space vehicles, traction, hybrid powertrains, EVs, and

electronic utilities as the standard power source. In the EV, it has had a great impact as

it has enabled the vehicle to run for a long time with reduced costs and fuel

consumption, and thus reducing CO2 emission (Rahimi-Eichi, et al., 2013).

The battery pack is one of the most important parts of the EV. The configuration

and type of battery technology are based on the power and energy demand. This chapter

briefly discusses the battery pack and its management system. With the rapid

technological developments, EVs and hybrid electric vehicles (HEV) have become

widely recognized all over the world. Due to issues such as environmental issues (the

exhaustion of the ozone layer and oil shortage), social, legal and economic concerns,

the demand for EVs is increasing among users as a reliable source of freight. As a result,

electric vehicles (EVs) including hybrid electric vehicles (HEVs), plug-in hybrid

electric vehicles (PHEVs) and battery-powered electric vehicles (BEVs) will be green,

clean and more advanced technology ferrite in a few decades. However, the battery, one

of the crucial components of EVs, is a prominent energy storage device and acts as an

energy buffer. The significance of the battery is increasing and attracting attention day

by day (Rahimi-Eichi, et al., 2013; Manenti, et al., 2011).

2

Figure 1.1 Typical battery pack for electric vehicle (Jim Camillo, 2018)

In a battery pack a number of cells are connected in series and grouped in

modules. Then these modules are connected either in series or in parallel to acquire the

desired high voltage and current. As an example, a typical battery pack of the EVs may

be constructed from 8 to 13 modules, each of them consisting of 10 or 12 cells.

Figure 1.2 Electric vehicle architecture with battery management system (BMS) (Texas instrument, 2016)

3

Currently, two major battery technologies are used in EVs- nickel metal hydride

(NiMH) and lithium ion (Li-ion), according to manufacturing companies. Nearly all

HEVs that exist in the market use NiMH batteries because of their advanced technology.

As EVs have a high-power demand, Li-ion batteries are expected to be commonly used

in EVs, particularly in PHEVs and BEVs. This is because of the significant properties

of Li-ion batteries such as strong power energy density and high capacity, long lifetime,

non-memory effect and low self-discharge rate, as well as improving the stability and

reliability of the electric vehicle. It is noted that there are several types of Li-ion batteries

which have different chemistries (Abdul Hak, et al., 2011). The terminal voltage range

for each cell varies between 2.7 to 4.3V for Li-ion batteries, such as the Lithium Iron

Phosphate (LiFePO4) battery, and 2-3.65V for the Nickel-Metal Hydride (NiMH)

battery.

Charging and discharging is a common phenomenon for the battery pack in EVs.

The battery pack is charged by supplying power externally and discharged by the

motor’s action of drawing currents to meet the power demands of the EVs, as shown in

Figure 1.1. During charging and discharging, the battery capacity and lifecycle decrease

gradually over time. There are also some other issues regarding the battery pack such

as differences in the state-of-charge (SOC), state-of-health (SOH), depth-of-discharge

(DOD), overcharging, under-discharging, ambient temperature, internal temperature

and charge unbalancing among the cells.

Temperature is a critical factor in battery performance. A crucial temperature

related problem is as follows. During charging and discharging, the battery temperature

may increase beyond 46°C (the maximum safe operating temperature for Li-ion

batteries) because of the increases in chemical reactions. This would result in a high

amount of chemical reactions occurring, especially throughout the discharging process,

4

and this increases the battery cell internal temperature. Accordingly, the SOC of the

individual cells in a battery pack becomes unbalanced, i.e. the cells have different

charge levels at a given time. Once this problem happens, some cells are charged faster

than other cells during charging and this leads to overcharging which may result in

explosions or fire. Similarly, during the discharging process, cells are under-discharged

(the Li-ion cells require a certain minimum amount of its charge to always be present,

and under-discharging occurs when more charge is withdrawn from the battery than the

minimum amount), which leads to permanent damage. Since the battery pack plays an

important role for EVs, the temperature of the battery pack has a significant impact on

the overall performance of the EV and is thus a critical parameter. The operating

temperatures of Li-ion battery are 0 to 46ºC for charging and -20ºC to 60ºC for

discharging (Hseih, et al., 2013). Usually, batteries are designed at room temperature

(around 25ºC). It is crucial to have good thermal management of the batteries for an EV

to run smoothly.

Figure 1.3 Example of a controller-area network (CAN) bus interface.

5

In general, Li-ion batteries have become increasingly focused on by researchers

in recent years due to their comparative advantages over other batteries. There is a

significant industrial need to improve the use of Li-ion batteries in EVs by addressing

the problems that happen to the Li-ion batteries during operations such as overcharging,

under-discharging, temperature intolerance and charge unbalancing, to avoid failures

and assure maximum safety and performance. These issues could be addressed by using

an effective battery management system (BMS). In recent years the demand, including

industrially, for an advanced and efficient BMS for EVs/HEVs has massively increased.

The voltage, current and temperature of the cells in a battery pack are the main

input parameters for the battery management system and are needed to monitor in real

time. All sensors are connected to microcontroller through the wire. When sensors are

connected in multi-layer, it is important to create a small size network (controlled area

network, CAN) or module to control and provide a route to transmit and receive signal

or data. Microcontroller collects raw data from the sensors, process and send to display

devices to monitor shown in Figure 1.2. CAN is a serial communication network for a

real-time distributed control system. The CAN-bus is extensively used in the electrically

controlled equipment of automobiles because of its high communication baud rate and

strong reliability (Zheng et al., 2008). However, CAN requires the installation of very

large and complicated wiring in order to interconnect a large number of communication

nodes. Consequently, this leads to an increase in cost, weight, and construction

complexity, as well as complications regarding the galvanic isolation of the cells.

Additionally, the CAN-bus limits the maximum data rate to 1 Mbps, which could turn

into a bottleneck for the development of future generations of BMS. This could be

addressed through using wireless communication methods instead of wired

communication inside the BMS. One such technology is Zigbee technology, a newly

6

developing wireless technology that has a number of advantages such as being low in

cost and power consumption and having a powerful ability to route data.

Thus, the efficient monitoring and control of the entire battery pack is essential

for the future of EVs, and for transportation as a whole. However, current research has

not been able to provide an efficient system in industry. This study attempts to address

this research gap by developing a battery management system that allows active charge

equalization with temperature analysis as well as monitoring the entire battery pack’s

information by utilizing wireless Zigbee technology.

1.2 STATEMENT OF THE PROBLEM

Since Li-ion battery cells are not 100% identical due to manufacturing tolerances, they

have different self-discharge rates, temperatures during operation, and undergo non-

uniform aging processes. When the battery cells are connected with wires to sensors

and microcontroller(s), there could be three significant problems. First, after a number

of charging and discharging cycles, the reduction of energy storage capacity results in

the charge unbalancing issue. Second, the temperature may exceed 46℃ during the

charge balancing process and this leads to unpredictable behaviour in the cells. The

increasing temperature in the battery cell has a significant impact on the battery

performance, cycle lifetime and safety. Third, due to the multiple wiring connections,

including between the user interface and the sensors for communication, there could be

the problem of short circuits. Complex installation, wiring, configuration, and cost are

the main dilemmas for wired connections.

7

1.3 RESEARCH OBJECTIVES

The main objective of this research is to develop a battery management system using

wireless (BMSW). The following issues need to be addressed to achieve the main

objectives:

1- To design an active charge balancing system based on DC-DC power

converter and analyse the internal temperature profile during balancing

operation.

2- To introduce a wireless communication to monitor battery cell information

(voltage, SOC, current, temperature).

3- To validate the overall performance of the proposed BMS.

1.4 RESEARCH PHILOSOPHY

This study focused on a wireless battery management system which includes a charge

equalization process to be performed in three different conditions which are charging,

discharging, and normal using the synchronous non-inverting buck-boost converter.

The input side of the converter would be connected to cells with higher charge and the

output side connected to a cell with lower charge. The voltage and current control loop

can be used to control charge balancing voltage and current during the balancing

operation. Instead of the controlled area network (CAN) bus, a wireless network can be

introduced with this system to deliver information from the battery unit to the user end

for display.