Kirk and Friedrich

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ENERGY MANAGEMENT IN ELECTRIC VEHICLE ENGINEERING

____________

A Project

Presented

to the Faculty of

California State University, Chico

____________

In Partial Fulfillment

of the Requirement for the Degree

Master of Science

in

Interdisciplinary Studies

Electric Vehicle Engineering

____________

by

Friedrich J. Kirk 2011

Spring 2011


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A Project

by

Friedrich J. Kirk

Spring 2011

APPROVED BY THE DEAN OF GRADUATE STUDIES

AND VICE PROVOST FOR RESEARCH:

Katie Milo, Ed.D.

APPROVED BY THE GRADUATE ADVISORY COMMITTEE:

_________________________________ _________________________________

Sara Trechter, Ph.D. Michael G. Ward, Ph.D., Chair

Graduate Coordinator

_________________________________

Adel Ghandakly, Ph.D.

_________________________________

Gregory A. Kallio, Ph.D.


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PUBLICATION RIGHTS

No portion of this project may be reprinted or reproduced in any manner

unacceptable to the usual copyright restrictions without the written permission of the

author.


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ACKNOWLEDGMENTS

I wish to express my deepest appreciation for the love and support given by

my wife, Wilma.

I would also like to thank Dr. Michael Ward for the support, encouragement,

and advice he has given over the years, especially within the constraints of his demanding

schedule.

I would also like to thank my parents, James and Sylvia Kirk, for instilling in

me the curiosity and passion for studying advanced technical topics. Their love, advice,

and support are invaluable for any project.


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TABLE OF CONTENTS

PAGE

Publication Rights ...................................................................................................... iii

Acknowledgments...................................................................................................... iv

List of Tables.............................................................................................................. vii

List of Figures............................................................................................................. viii

List of Nomenclature.................................................................................................. x

Abstract....................................................................................................................... xiv

CHAPTER

I. Introduction .............................................................................................. 1

Background................................................................................... 1

Literature Review......................................................................... 4Project Scope................................................................................ 6

II. The Charging System ............................................................................... 8

Safety Considerations................................................................... 8Convenience Features................................................................... 9

Charger Input Power..................................................................... 10

Charger Output Power.................................................................. 10Lithium Ion Charge Termination Methods................................... 11

Power Converters ......................................................................... 12

Design Challenges ........................................................................ 14Design Implementation ................................................................ 17

Charging System Testing ............................................................. 40

Charging System Conclusion ....................................................... 54


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CHAPTER PAGE

III. The Battery Management System ............................................................ 56

Safety............................................................................................ 57Battery Pack Status....................................................................... 58Fuel Gauge.................................................................................... 58

Cell Balancing .............................................................................. 58

Design Implementation ................................................................ 60

Battery Management System Testing........................................... 107Battery Management System Conclusion .................................... 116

IV. Conclusions and Recommendations......................................................... 118

Conclusions .................................................................................. 118

Recommendations ........................................................................ 119

References .................................................................................................................. 121


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LIST OF TABLES

TABLE PAGE

1. Charger Specifications.............................................................................. 18

2. Detailed Charger Design Specifications................................................... 32

3. Tested States of Charger Power Control................................................... 46

4. Tested States of Sleep Control.................................................................. 46

5. Tested States of Temperature Sensor........................................................ 46

6. Tested States of Inrush Protection ............................................................ 47

7. Tested States of Overvoltage/GFI Protection........................................... 48

8. Electric Drive System Shutdown Modes.................................................. 65

9. Cell Module Specifications....................................................................... 67

10. Central Control Hub Specifications.......................................................... 81

11. Tested States of Main Contractor Control ................................................ 111

12. Tested States of GFI Control .................................................................... 112

13. Tested States of Charger Power Control................................................... 112

14. Tested States of Charger Sleep Control.................................................... 113

15. Tested States of Plugged-In Sensor .......................................................... 113


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LIST OF FIGURES

FIGURE PAGE

1. Switching Buck-Mode Power Supply....................................................... 13

2. Boost Mode Switching Supply ................................................................. 13

3. Schematic of Boost Circuit with Infineon Control IC .............................. 27

4. Schematic of Charger Circuit ................................................................... 41

5. Circuit Layout for Charger Circuit Board................................................. 42

6. Complete Charger Circuit Board .............................................................. 43

7. Solid Model of Charger and BMS Control............................................... 44

8. Charger and BMS Control Circuit Assembly ........................................... 45

9. Load Removal Showing 15.5V Overshoot ............................................... 50

10. Load Addition Response........................................................................... 51

11. Transient Load Test .................................................................................. 52

12. Sleep Mode Test ....................................................................................... 53

13. Schematic Layout of BMS Module Circuit .............................................. 74

14. Circuit Layout for BMS Circuit Board..................................................... 75

15. Circuit Board after SMT Soldering .......................................................... 76

16. Complete BMS Module Circuit Board ..................................................... 77

17. Solid Drawing of Battery Management Module....................................... 78

18. Complete Battery Management Module................................................... 79


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FIGURE PAGE

19. Schematic Layout of Isolation Circuit...................................................... 90

20. Circuit Layout for Isolator Circuit Board ................................................. 91

21. Complete Isolator Circuit Board............................................................... 92

22. Human machine interface layout, operating mode ................................... 94

23. Human Machine Interface, Setup Mode ................................................... 95

24. Human Machine Interface, Cell Data Level ............................................. 96

25. Human Machine Interface, Cell Data Adjust............................................ 97

26. Solid Model of User Interface .................................................................. 98

27. Screenshot of LCD showing individual cell values.................................. 108

28. Screenshot of LCD showing Analog Values ............................................ 110

29. Low Voltage Charger Output Waveform ................................................. 115

30. LCD Screenshot of Cell Status After Charging........................................ 116


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LIST OF NOMENCLATURE

NOMENCLATURE

A microAmp

F microFarad

H microHenry

efficiency

Ohm

A Ampere

A:D Analog to Digital converter

AC Alternating Current

Ah Amp-Hour

BMS Battery Management System

C Coulomb, or Charge

Cout Output capacitance

CAN Controller Area Network

CC/CV Constant current/constant voltage

DC Direct Current

Don_switch Power switch duty cycle

DMM Digital Multi Meter

ECU Electronic Control Unit


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NOMENCLATURE

EV Electric Vehicle

Eon MOSFET turn on energy

Eoff MOSFET turn off energy

F Farad

fswitch Switching frequency

line Line frequency

HEV Hybrid Electric Vehicle

Hz Hertz

Icell Cell current

Iin_RMS RMS charger input current

Iin_peak Peak charger input current

IL_peak Inductor peak current

IL_ripple Inductor ripple current

I2C Inter IC (communication bus)

IC Integrated Circuit

IGBT Insulated Gate Bipolar Transistor

kHz kilo Hertz

kW kilo Watt

kWh kilo Watt-Hour

KL_ripple Constant, inductor ripple

Kout_ripple Constant, output ripple


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NOMENCLATURE

Lboost Boost inductance

LiCoO2 Lithium Cobalt Oxide

LiFePO4 Lithium Iron Phosphate

LiPo Lithium Polymer

LT Linear Technology

mA milli Amp

mW milli Watt

m milli Ohm

MOSFET Metal oxide semiconductor-field effect transistor

MOV Metal oxide varistor

NiMH Nickle metal hydride

Pcond Power switch conduction losses

Pswitch Power switch switching losses

PMOSFET Total power switch losses

Pdiode Diode power loss

PRsense Sense resistor power loss

Pcharger Charger power

PL_boost Boost inductor power loss

Pd_br Bridge rectifier power loss

Plosses Total power losses

Ptotal Total charger power


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NOMENCLATURE

PWM Pulse width modulation

RL_boost Boost inductor resistance

Rcell Cell impedance

Rpack Battery pack impedance

Rdson Drain-source on resistance

Rsense Sense resistor resistance

Spec Specification

SSR Solid State Relay

TVS Transient Voltage Suppressor

Vdrop Voltage drop per cell

Vcell Cell Voltage

Vcharger Charger Voltage

Vf_br Bridge rectifier forward voltage

Vfdiode Diode forward voltage

Vin_RMS Charger RMS input voltage

Vout_ripple Output ripple voltage

V Volt

W Watt

Wh Watt-hour


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ABSTRACT


by

Friedrich J. Kirk 2011

Master of Science in Interdisciplinary Studies

Electric Vehicle Engineering

California State University, Chico

Spring 2011

The world is now facing a number of important problems brought on by hu-

manitys use of carbon-based fuels. One of the primary uses of these fuels is for transpor-

tation. There is a need to make highway capable electric vehicles accessible to the aver-

age commuter. The high initial cost of storing the energy is one of the primary barriers to

entry for electric vehicles. Energy management technology has not been leveraged to

provide energy in a manner that is congruent with the users needs. A user configurable

energy storage system would allow users to find their own solution to the cost-

performance question.

The scope of this project is to design and build an integrated energy manage-

ment system that includes all necessary components to store energy for an electric

vehicle. Every aspect of the energy management system is covered. The goal is to design,

build and test a plug & play modular energy storage system.


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Power converters and issues affecting safety, efficiency, and electromagnetic

interference are discussed. Details specific to lithium ion batteries are covered, including

charging modes and cell protection. The battery management system is used for execu-

tive control of the charger and the chargers local and executive control modes are dis-

cussed. Battery pack design and requirements such as safety issues specific to lithium ion

batteries are introduced. Energy management system features such as delayed charging,

user interface design, and cell balancing are covered.


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CHAPTER I

INTRODUCTION

Background

The world is now facing a number of important problems brought on by

humanitys use of carbon-based fuels. Pollution, global warming, and the political

upheaval caused by peak oil coupled with our increasing appetite for fuels are just some

of the problems that need to be addressed.

One of the primary uses of these fuels is transportation with the internal

combustion engine as its power source. While other transportation options commonly

exist in Europe and Asia, in the United States the automobile is the principal form of

transportation. Unfortunately, all trends in the U.S. point to less efficient vehicles and

less efficient use of those vehicles [1], [2], [3]. A solution to many of the current

problems would be to switch to more efficient personal transportation methods,

specifically to electric-drive vehicles. Studies in Europe, Japan, and the United States

have all pointed to this conclusion. These studies addressed everything from total energy

(manufacturing and use), well-to-wheel analysis, and power grid analysis [4], [5], [6].

While electric vehicles are not a complete solution that will meet everyones

needs, they can provide clean transportation for the vast majority of users. It is important

to acknowledge the strengths and limitations of electric-drive technology in order to

provide a useful alternative to the internal combustion engine. According to studies by


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the Department of Transportation, the average commuter travels considerably less than

60 miles per day. In fact, the average commute distance in 1969 was 9.4 miles, and in

2009, it only increased to 13.9 miles [7]. While a vehicle range of 30 miles may be

adequate for the average persons commuting needs in the United States, doubling it to

60 miles would be far more pragmatic as it would provide extra range for errands or

allow for less-frequent charging. In any case, it appears obvious that the need for a 300-

mile range is more a marketing gimmick than a real user requirement. Other studies by

both the DOE and the 2000 census found it is common for most households in the United

States to have more than one vehicle [2]. These statistics, along with similar studies from

other countries, provide the important information necessary to devise a practical EV

energy storage system.

The need for an energy management system is crucial for the success of

electric vehicles using newer battery technology. The sheer number of cells coupled with

the high price of failure, dictate the necessity for an extremely meticulous system. These

requirements demand a system that never gets tired and is optimized for redundant work:

namely, a computer. Furthermore, success of the electric vehicle will depend on a market

much larger than enthusiasts, a market dominated by people who really dont care what is

under the hood. Manufacturers have found that battery management systems are required

even for products with only a few cells, such as cell phones, laptop computers, and power

tools. However, these systems are always designed with a fixed number of cells for a

specific product. The concept of having designs based on user-customizable batteries is

not available. Meanwhile the power-tool industry has found that offering different

options for their products can be a huge sales benefit. The needs and cost requirements of


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a professional contractor are much different from those of the average homeowner, with

professionals generally demanding much more power from their portable tools. Lessons

learned by the power-tool industry could be directly applied to the automotive industry.

Energy management technology has not been leveraged to provide energy in a

manner that is congruent with the users needs. Because the high initial cost of storing

energy is one of the primary barriers to entry for electric vehicles, this issue is one of the

most important ones facing electric vehicles.

Finding the correct energy storage capacity can best be done by the users

themselves, as it is a highly personal life-style decision. Mass-produced, one-size-fits-all

solutions have been proposed, but an option allowing for individual user requirements

has never been provided in the marketplace. A user configurable, modular energy storage

system would allow users to find their own optimum solution to the cost-performance

question.

This project provides a solution for all of these requirements, with individual

cell monitoring achieved by central data collection and management. The central

management system can both balance the battery pack and provide a fuel gauge as well

as battery pack status for the user. This system is scalable up to 120 cells and includes an

integrated charger. While it has been implemented with a model pack made of 18650

cells like those found on laptop computers, the concept can easily be scaled up to large-

format cells required by full-size electric vehicles. Similarly, the chargers control

architecture could remain as described, with only a few power components upgraded to

withstand the added current required to charge a full-scale battery pack.


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Literature Review

For electric vehicles, the battery system is one of the most expensive and most

important pieces of the electric drive train. Thus, a study of the most effective method of

implementing a charging system and battery management system was required.

The available literature has numerous examples of electric-vehicle battery

studies and a few discussing battery management. For example, an excellent long-term

study by T. Knipe et al. discussed the performance of the nickel metal hydride (NiMh)

battery used in the Toyota RAV-4 EV [8]. However, this study had no details of the

battery or the management of the battery. Another study by G. Berdichevsky et al.

discussed the battery pack used by the Tesla Roadster [9]. That study came closer to this

project in that lithium ion batteries are used, but the system uses a fixed number of cells,

and control details are left out of the paper. Similarly, a paper presented by P. Drozdz et

al. at the EVS-23 Conference discussed a replacement lithium ion battery pack for hybrid

electric vehicles [10]. Like the Tesla paper, it discussed the cells used, packaging, and

thermal issues, but contained nothing about a battery management system. A paper by N.

Ohnuma et al. presented at the EVS-23 Conference also described development of a

lithium ion pack, this time with mention of the charger. However, no details of the

management system or whether it interacted with the charger were presented [11].

A battery system for hybrid electric vehicles was presented by T. Tan et al. for

Enerdel at the EVS-23 Conference [12]. In this paper, the control architecture is briefly

discussed and cell-level controls are mentioned. Additionally, a central control that

monitors cell performance and pack performance is mentioned, but details are not given.


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A paper by B. Kennedy et al. described a lithium ion battery pack developed

for solar racecars. This paper, written in 2000, does include details of the battery

management system, which consists of independent modules protecting each cell. While

this paper came close to the goals of the system being designed, it lacks any balancing

system, charger, or fuel gauging system [13].

To find information about lithium ion battery management at the cell level

required looking for information at the web sites of integrated circuits manufacturers.

Here, more information about cell-level monitoring, communication issues, and

integration with microcontrollers was available. For example, application notes from

Maxim Engineering Journal on integrated circuits, How to design battery charger

applications that require external microcontrollers and related system-level issues

discussed interfacing one-cell battery management ICs to microcontrollers [14].

However, these application notes were generally limited to four-cell battery packs.

Because electric-vehicle battery packs use much higher voltages, issues including

isolation would still need to be addressed.

A book by D. Andrea, Battery management systems, published in 2010, was

just discovered as this paper was completed [15]. The book is an excellent overview of

battery management systems, specifically for lithium ion batteries. Nearly every topic

involved with designing a battery management system is discussed. These topics range

from basic cell performance and battery management system topologies, to specific

details on balancing systems and battery management board communication. While the

system described in this paper differs from those described by Andrea, the book is an

invaluable resource for anyone designing a battery management system.


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Project Scope

The scope of this project is to design, build, and test an integrated and

modular energy management system that includes all necessary components to store

energy for an electric vehicle. Every aspect of the energy management system is

explored, with optimums and engineering compromises discussed along the way. Safety,

thermal management, and mechanical packaging issues are considered.

The charger and battery management system are complicated systems, with

many interactions between the hardware, software, and user and the environment. Battery

properties change over time, and sensors are often temperature-sensitive. To add to the

difficulty, the voltages produced by the charger are lethal and can easily damage

measuring equipment. A testing program ensuring core functionality of subsystems has

been implemented. The goal is to design and build a modular energy storage system that

is both scalable and designed for easy user adoption.

The source of energy for the prototype system is Utility AC power. Chapter II

discusses conversion of AC energy to DC form usable by the battery pack by switch-

mode power supplies. Issues affecting safety, efficiency, and electromagnetic interference

are discussed. Control features favorable to electric vehicles, such as delayed turn-on and

vehicle-based charging, are also introduced.

To reduce weight and improve efficiency, a high-frequency switch-mode

power supply is used in this design. Charging modes specific to lithium ion batteries and

how the system controls these modes is detailed. The topics concerning the chargers

integration with the rest of the system is described in the Design Implementation section

in Chapter II. Power supply for the control circuitry, thermal management, and control of


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electromagnetic interference is addressed from a system-design perspective.

Communication with the battery management system will be developed, with hardware

and safety issues addressed. The battery management system is designed to have

executive control of the charger and the chargers local and executive control modes are

discussed. Finally, selection of components for maximum efficiency is detailed.

Chapter III discusses energy management in general and the battery

management system in particular. Battery-pack design requirements and safety issues

specific to lithium ion batteries are introduced. Finally, favorable details for the energy

management system, such as delayed charging, user-interface design, and cell balancing

are covered.

Chapter IV discusses the effectiveness of the system. Recommendations for

further experimentation and system improvements are given.


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CHAPTER II

THE CHARGING SYSTEM

A battery pack can be considered the fuel tank of an electric vehicle. In this

analogy, the charger might be considered the gas pump in a filling station. A charger for

an electric vehicle is a power converter large enough to charge a very powerful battery

pack in a short period of time. Electric vehicle battery packs range in capacity from

15kWh to 50kWh. To be usable by the battery, the power must be higher in voltage than

the battery pack. Charging current must be limited to a value based on the battery

technology.

Safety Considerations

While the average EV battery pack ranges from 15kWh to 50kWh, voltages

can range from 120VDC to over 500VDC, while some packs can supply over 1000A of

current. Safety has to be one of the highest priorities in charger design because a charger

that can process this much power will be able to deliver both high-voltage and high-

current DC. Wiring errors such as incorrect grounds, which can produce an annoying

shock under normal conditions, can be deadly with this amount of power. The system

must be completely safe with no possibility of harm, even by user error.

To avoid battery failure and user harm, the charger must sense what is

happening with the battery and the input power. Battery condition should be verified to


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ensure that the cells are in the proper state to allow charging. This check should ensure

that the battery is not overheated, overdischarged, or already fully charged. Controlled

rapid shut down must be a built in function of the power supply.

From a safety standpoint, the charger must be isolated from the power line and

there must be no chance for the user to contact a hot component. As with any electrical

device, the input must be properly grounded when plugged in. However, unlike many

electrical devices, the case must not be connected to the high power outputs low side.

The high power system in an EV should be completely isolated from other electrical

systems. Therefore, the chargers output must also float. There can be no possibility for a

current path, intentional or unintentional, through the body of the vehicle. This safety

feature can be accomplished by using an isolation transformer to provide galvanic

isolation between the output and ground, or by providing a ground fault interrupter circuit

that will disconnect power if it senses any current flow through the ground or some other

unintended path [16].

Convenience Features

For convenience, reliability, and safety, the charging system must take less

than a minute to activate and must be completely automatic. Features such as delayed

charging can significantly reduce operating expenses without affecting convenience.

With delayed charging, one can take advantage of time-of-use metering with reduced

electric rates at night.


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Charger Input Power

The typical residential AC line voltage is either 120V RMS or 240V RMS.

When simply rectified with a full bridge rectifier (unregulated DC), this works out to

169.7V and 339.4V respectively. Because the higher voltage can deliver twice as much

power for a given amount of current, the charger should be capable of handling either

voltage. Most residential houses will have a high power circuit for a water heater, stove,

or air conditioner that is rated from 30A to 50A.

The restriction posed by residential wiring presents several important

limitations. For ease of discussion, considerations of efficiency, charger, wiring, and

battery cells will be ignored here. When charged at 120VAC, 30A, 3.6kW can be

delivered. At this rate, a 20kWh pack would take over 5.5 hours to charge, and a 50kWh

pack would take almost 14 hours. Using 220VAC, 50A could deliver 11kW. The charge

time for a 20kWh pack would be reduced to 1.8 hours, and a 50kWh packs charge time

would be reduced to 4.5 hours.

Charger Output Power

To regulate power, both voltage and current must be controlled. The charger

must be capable of providing a voltage higher than the battery packs maximum. Current

is limited to that specified by an individual cell and by whether cells are connected in

parallel. The amount of current a cell can accept is based partially on its capacity and

partially on its internal resistance. Internal resistance is determined by the cell design and

is beyond the scope of this paper. The current that a cell can accept while charging is

specified by the manufacturer as C, which is a multiple of the cells capacity. For


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example, a 10Ah cell charged at the 2C rate would be receiving 20A of current. For most

cells, it is beneficial to charge at a rate lower than the manufacturers specified rate, as

charging at a higher rate will result in cell heating and reduced cell life.

Because battery packs used on electric vehicles have low impedance, they can

be considered an ideal current sink from the chargers perspective. The charger will

attempt to drive the voltage to whatever voltages the batteries are, at whatever current the

charger can produce. However, the chargers maximum output current is limited by its

power components. Because the power components, not the control components, are the

primary cost-driver in electronics, it is not economical to build one charger for all

situations and reduce the output current by controlling them.

Lithium Ion Charge Termination MethodsConstant Current/Constant Voltage Charging

The recommended method of charging lithium ion cells is to use a constant

current/constant voltage process. This is essentially the same method used to charge lead

acid cells. Using the constant current/constant voltage method, current is held constant

until the voltage reaches a point predetermined by the battery chemistry. The voltage is

then held constant while the current is decreased over time. Once the current reaches a

pre-determined low point (C/20) the cell is considered charged [17].

Pulse Charging

The concept of battery hystresis, or pulse charging, is a different method of

finish charging. Battery hystresis is the tendency of the batterys voltage to drop once a

charging voltage is removed. In pulse charging, instead of reducing the current over time,


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the current is held constant, but switched on and off based on measured battery

performance. A pulse charger must have a reasonably accurate current-limited source to

supply the on pulses [18]. When the charge voltage is turned off, the cell voltage will

drop to some lower stable voltage. When the low stable voltage is reached, the charger

will turn on again until the high voltage threshold is reached. As the battery fills, the off

pulses will become shorter because the time required to reach the low stable voltage will

decrease. At the same time, the time required to ramp the voltage to the high voltage

threshold will also decrease. In this way, pulse charging is like pulse width modulation,

and the energy delivered to the pack can be controlled.

Pulse charging provides several advantages over constant voltage charging for

finish charging lead-acid and lithium ion batteries. Charge state is determined by actual

battery condition, not by a pre-determined current reduction algorithm. It is faster than

constant voltage charging because the current is delivered at the full rate. It also extends

battery life, as shown in several studies [10], [19].

Power Converters

Switch-mode power supplies are relatively new and are beginning to dominate

the power supply market because of their advantages of low weight and high efficiency

and can be used for either CC/CV charging or pulse charging. Figure 1 shows a switch-

mode power supply used to convert 120VAC to 15VDC for the main controller.

Switch-mode power supplies operate by switching a transistor on and off at a

high frequency. Metal oxide semiconductor field effect transistors (MOSFETs) or

insulated gate bipolar transistors (IGBTs) are used for their high efficiency and current


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Fig. 1. Switching buck-mode power supply.

carrying capability. These devices are operated in the saturated region (completely on or

off) for high efficiency and their power switches are controlled by voltage not current. By

using subtle variations in circuit topology, switch-mode power supplies can be used to

provide either a higher output voltage than the source (boost mode) (Figure 2) or a lower

voltage than the source (buck mode) [20].

Fig. 2. Boost mode switching supply.


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Boost-mode switching supplies control the current by managing the

charge/discharge cycles of an inductor. The simplified schematic in Figure 2 shows only

the main power components, with the control circuitry omitted. Switching on the

MOSFET provides a low-impedance current path to ground that is used to charge the

inductor and energize its magnetic field. When the transistor is turned off, the current is

forced to the output, energized by the magnetic field [12]. The proportion of on time

versus off time is controlled or modulated to regulate the current flow through the

switch. By varying the on/off timing (pulse width), switch-mode power supplies are able

to control both output voltage and current.

Switch-mode power supplies can significantly reduce or completely eliminate

the transformers size, significantly reducing power supply weight. In fact, these power

supplies are capable of operating directly on line input power.

Design Challenges

Isolation

Line-driven boost-mode switching power supplies have no isolation between

the input and the output lines. This means that if 120VAC is used as the source power, an

uncontrolled169VDC will be present at the output! For this reason, either a form of

isolation or the capability of immediate shut down must be provided in case an electrical

fault occurs. Also, as a battery charger, the lowest voltage present in the battery pack

must be higher than 169VDC. If it is not, the current into the battery pack will not be

controlled and severe damage to the pack, or even fire, could occur.


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Voltage Control

While most power supplies are designed to supply a voltage regulated within a

few percent, the requirements for a charger are much different. Most power supplies

regulate their output voltage to a fraction of 1%. However, all high power electric vehicle

battery packs experience a large variation in voltage from the charged to discharged state.

Thus the battery determines the final voltage, as long as the charger can produce a

voltage higher than the batterys voltage [21].

Power Factor

In the United States, AC current is highly regulated and very stable. This is a

requirement for large interconnected power grids. The voltage, line frequency and phase

angle between voltage and current are carefully monitored so that they dont change over

time.

The power factor is the cosine of the phase angle between voltage and current.

A power factor of 1 indicates a purely resistive load and the number decreases to zero as

the load becomes reactive. Thus, the power factor is the relationship between the real

or usable part of the current being drawn by a system and the reactive part of the load.

The reactive component of the load can be capacitive or inductive, and can cause both

phase shifts in the frequency and system inefficiency [22].

A switch-mode power supplys high-frequency controller can regulate the

power flow so accurately that the power factor can be unity, contributing to the efficiency

of the system. The Infineon boost-mode SMPS controller used in this project is power-

factor correcting, allowing power factors of 0.98 to 1.00.


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Electromagnetic Interference

An issue that cannot be removed by control ICs is the EMI and electrical noise

caused by the chargers switching action. The main cause of electromagnetic noise is

high-frequency square wave signals, commonly used for digital computation and

communication. These signals can be created by a microcontrollers timing clock or by a

MOSFET turning on and off to regulate current. Both higher frequency and higher power

contribute to more noise. As such, a switch-mode power supplys control MOSFET is

one of the main contributors of electromagnetic noise from electronic systems [23].

Electromagnetic interference can be transmitted by both radiation and

conduction. The primary mode of broadcasting the noise is through unintentional

antennas in the circuitry or conduction out through communication and power cords.

Conducted emissions can further result in radiated emissions, as the wires can be

excellent antennas. It must be noted that DC wires are often unintentional carriers of

high-frequency signals and can be a major source of electromagnetic radiation [24], [25].

Efficiency

Efficiency is highly dependent on both design and component choice. Current

technology allows power supplies that are well over 90% efficient. Progress has been

made with silicon devices that switch very rapidly, reducing this time and improving

device efficiency. High-efficiency MOSFETs and silicon carbide diodes are available

that have very low switching losses. Another contributor to losses is the energy required

to turn a MOSFET on. It is similar to charging a capacitor and dumping the energy, and

as the power capacity of a MOSFET increases, more switching energy is required. Any


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improvements found by increasing the frequency, such as reduction in transformer size,

must be balanced against the switching losses from a system design standpoint [26].

Design Implementation

Because a vehicle-based charger would need to be light, a switch-mode power

supply would be the best choice, even though it poses the design challenges discussed

previously. This section covers the design of a vehicle-based lithium ion battery charger

as well as its support and safety circuitry. Since a high-voltage system has the advantage

of lower resistance losses for a given amount of power, the input voltage must be

increased. A boost converter can do that efficiently while providing a unity power factor.

The heart of the proposed charging system is a high frequency boost-mode

switching power supply, coupled with a battery management system that controls the

final charge. The availability of control ICs from suppliers like International Rectifier

and Infineon reduces the complexity of the problem but they need some modifications to

act as a charger. These modifications have been implemented to enhance the capabilities

as a power factor correcting constant current source. Table 1 lists the charger

requirements based on the battery pack used in this project.

System Integration

To take full advantage of the safety and monitoring functions of the battery

management system, the charger is highly integrated with it. The system shares

information such as bus voltage, current, and even charger temperature. The battery

management microcontroller is provided with optically isolated controls for the charger.


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TABLE 1

CHARGERSPECIFICATIONS

Format PFC high voltage lithium ion

Input voltage 120 VAC

Input frequency 60 Hz

Input power 630 W

Battery type Li-Ion

Bulk charge Constant current

Finish charge Pulse (hystresis)

Charge time 2.5 hours

Max output current 2.2 A

Min pack voltage 180 VDC

Max pack voltage 252 VDCOutput power 600 W

Efficiency 95%

Control Integrated with BMS

Display Integrated with BMS

Executive control of the charger is managed by the battery management

system through the optically isolated interface.

The charger is integrated with safety circuitry such as the input breaker,

ground fault protection, DC fuses, and main contactor. To reduce wiring cost and

electromagnetic interference, the charger has been directly connected to the battery pack

and battery management system. This arrangement has several advantages:

1) It eliminates an external wire from the charger to the battery, replacing it witha short bus bar from the charger and short bus bars between cells. This dramatically

reduces cost, as well as reducing DC losses from the charger to the battery pack. Noise

from EMI is greatly reduced because both the wiring inductance and the loop size are

reduced.


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2) The main contactor and DC fuse are built into the system. Because they sharethe same short bus bars with the charger and battery pack, more wiring expense and

wiring losses are eliminated. The DC contactor solenoid control is built into the system

and can be overridden by the battery management system. This provides a measure of

security because the DC output cannot be energized without the control signal from the

battery management system and activation of the on switch.

3) Sensing and control wires are all within the same enclosure. This reduces theneed for expensive sealed connectors. It also reduces EMI noise because noise traveling

on communication wires does not leave the enclosure.

4) The water cooling system used by the battery management system is alsobeing used to cool the charger. This reduces plumbing and the potential for coolant

leakage. Multiple temperature sensors in the battery pack and charger ensure that the

system does not overheat.

5) Wiring has been greatly simplified for the user. There is only an AC input, aDC output, a motor controller interface, and a user control and BMS interface.

Communication

The data flowing between the charger and battery management system is both

extremely critical and time-dependent. For this reason, the options for transmitting this

information were carefully considered. A communication bus is valuable when a lot of

data has to be transferred but time is not critical. Discrete wires are ideal when time is

critical and not much data has to be transferred so they were used in this design.

The charging system makes use of digital on/off signals and analog feedback

signals. Because of the critical safety requirements of these controls, they are set up as


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dead man signals where a 0V or wire disconnect between the battery management

system and the charger will turn off the charging subsystem. All information is isolated to

protect the battery management system microcontroller and will be covered in Chapter

III, Battery Management System.

Safety Systems

Safety features are built into the system and distributed throughout the

subsystems. These subsystems can communicate with each other to provide more

sophisticated reactions to dangerous situations than a standard system is capable of doing.

The battery management system is the core of this functionality, and provides

executive control for safety and for normal operation. Signals for output bus current, bus

voltage, and charger temperature are monitored by the battery management computer, in

addition to its individual cell monitoring capability. From this information, the battery

management system can determine the state of charge and the health of both the charger

and the battery pack. Isolated sensors monitor charger temperature, battery pack current

and battery pack voltage. These are detailed further in Chapter III. Active control of

charger functions is done by controlling the GFI shutdown circuit, and the SMPS power

through solid state relay Q1, and by forcing SMPS sleep mode with the shut down pin on

VR1. Trigger events for these controls are detailed in the Safety section of the next

chapter. All control lines are set up dead man; in other words, there must be a positive

signal from the BMS for the function to be enabled.

Further, the charger and motor controller monitor their own condition as well

as bus voltage and current. Should any of these stray outside safe values, the subsystems


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will automatically initiate limp modes or complete shut down in addition to alerting other

systems of the problem.

While software control allows flexibility, passive control can avoid several

failure modes. The disadvantage is that changes in the system require changes in the

passive control hardware. This system uses passive control for voltage, temperature, and

current control.

Ground Fault Protection. Because the boost-mode power supply connects

directly to the source AC, there is no galvanic isolation, which presents a safety problem

should anything go wrong. To provide a means of immediate shut down if any problem is

detected, a ground fault interrupter monitors all power coming into the circuit.

Ground fault systems operate by measuring the current flowing in the hot,

neutral, and ground lines of the load. Should more than 30mA flow through the ground

line or should the current in the hot and neutral lines differ by more than 30mA, the GFI

circuit will shut off. Because it is a well-proven and accepted safety method and it has

several advantages over an isolation transformer, ground fault protection has been

provided in the system with a GFI breaker.

To enhance the systems safety, a circuit has been provided that will allow the

battery management system to shut off all power flowing to the charger. A triac

controller is located after the GFI protector, with its output connected to the AC ground.

The isolated control input is provided as an immediate power shut down and is connected

to the battery management system. Should this system be activated, current is shunted

from the AC input to ground, causing the GFI system to react and shut down power.


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In-Rush Current Protection. In-rush protection is normally not a problem with

a battery charger permanently connected to a battery pack. In-rush current is caused by

the uncontrolled charging of filter capacitors that are on the power supplys input and

output stages. Such a current can be high enough to permanently damage and decrease

the capabilities of, if not destroy, the components in its path. All systems need at least

one initialization, and a reliable system requires attention to all factors that can reduce

component reliability.

Even though the power supply controller IC features a ramped output for

limiting in-rush current, a major problem with in-rush current can occur when the power

bus is below the peak voltage produced by the bridge rectifier. In this case, the capacitor

is charged very rapidly because the current can flow without control from the switching

transistor. During initial testing, this situation resulted in several components being

blown up before an in-rush current-limiting circuit was added.

Current flow is controlled by a triac placed in front of the bridge rectifier.

Triacs are devices that are activated by current flowing through their gate. When they

fire, or turn on, they cannot be turned off until another zero crossing of the AC power.

A small amount of current taken from the AC line can be used to control a triac and

ensure that it fires only once per cycle [27]. The AC line current is controlled by an in-

rush current limiter and a triac controller. Because a threshold current has to be met to

activate the triac, the resistance in the control line can be used to delay turn-on if desired.

This method is used as a simple means of controlling AC power in some motor

controllers.


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The triac is activated by a controller that only switches it on or off at zero

crossings of the AC input voltage. The zero crossing circuit reduces stress on the triac

and other components by eliminating the large DV/DT produced when the circuit is

activated at maximum voltage. The control signal for the triac controller is optically

isolated to protect the low-power control signals from the high power being controlled by

the triac. This signal is generated by voltage monitor U2 that compares an input voltage

to an internal reference of 1.3V [28]. A voltage divider is connected across the output

capacitor (the main power bus) and will provide a 1.6V signal when the bus voltage

reaches 170VDC. This is the peak voltage produced by the bridge rectifier. Thus, the

triac will be forced off as long as the bus voltage is below 170VDC [29].

In parallel with the triac is a thermistor that limits current to 1A [30]. When

the triac is turned off by the control circuit, current is allowed to slowly fill the capacitor

through the thermistor. This system eliminates all in-rush current problems.

Current Management. Current in the charger is managed primarily by the

control IC, based on the instantaneous AC input current, as described in the section on

SMPS current control below. In addition to the sense resistor that monitors charger

current, a hall-effect current sensor has been provided on the DC bus to monitor current

flowing into and out of the battery pack. This is described in more detail in the next

chapter.

The second current-sensing system provides a parallel back-up for the system.

If a fault occurs, it gives the battery management system the ability to shut down the

charging current by turning off the charger power, either forcing the charger into sleep


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mode or tripping the GFI. It also allows the battery management system to turn off the

main contactor should there be a severe overcurrent discharge fault.

Should these active systems fail, fuses are also provided on the input of the

charger and the output of the battery pack.

Voltage Management. Cell overvoltage is a major safety concern as it can

result in failed cells or even fires. Because it is so important, a passive shutdown system

has been provided in case the battery management systems end-of-charge system fails.

The difficulty in implementing passive bus voltage monitoring is that it

provides a fixed voltage limitation. While this may seem obvious and desirable, it

counters the goal of a modular battery system. With a modular battery system, the

purpose of adding more cells is to achieve a higher battery pack voltage, thereby

increasing system power output.

To allow a modular system, the bus voltage is monitored with a variable

resistance voltage divider. The lower part of the voltage divider is fixed and provides

feedback to the undervoltage and overvoltage control systems. Both of these systems use

voltage monitor IC U2 with a target voltage of 1.3V [20]. The upper resistor in the

system is made of a string of smaller resistors, each located in the battery modules. When

a module is added, both the pack voltage and the resistance of the upper half of the

voltage divider are increased as well. Using this method, it is possible to detect a pack

over-voltage no matter what the pack voltage is.

The output of the voltage monitor is used to pull down control lines for the

charger power supply and the GFI shut down. Should a bus overvoltage be encountered,

the charger will be completely shut down.


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Another danger is encountered when a cell, not necessarily the pack, reaches

too high a voltage. This can be the result of a badly balanced battery pack. The battery

management system provides active monitoring of individual cells and can shut down or

limit the output of both the charger and motor controller depending on the severity of the

problem.

Boost-Mode Switch-Mode Power Supply

The key to this systems functionality is high-frequency active control

provided by an IC manufactured by Infineon, the ICE2PSCS01 [31]. This SMPS control

IC is one of a family of ICs offered by Infineon designed specifically for switch-mode

power supply control. While the ICE2PSCS01 is an excellent switch-mode power supply

controller that can provide efficient, stable output power, it was not designed for use in a

battery charging application. Details of its operation and the modifications for use in a

battery charger are discussed here.

Controller Power. Most MOSFETs and IGBTs require 12VDC to 25VDC to

turn on. Because the switch-mode controller IC must control an external power transistor,

they require 12VDC to 25VDC to operate. Because this is a charger, it needs to run only

while it is plugged into a power source. Therefore, its power is provided by an AC/DC

converter with a 15VDC output. Providing power from a small power supply connected

to the AC mains rather than a DC to DC converter connected to the battery pack is more

efficient and allows more control options for the system. This power supply provides

power for the battery management system and the charger while the system is plugged in.

Infineon Control Circuit. The ICE2PCS01G SMPS control IC was chosen

because it was designed for boost-mode power supplies, required little external circuitry,


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and it offered features for improved efficiency and power factor correction. The

controller can handle a wide range of input voltages, allowing both 120VAC and

220VAC inputs. Additionally, the controller allows multiple external control options,

including sleep modes for extremely low power consumption. In the charger circuit

shown in Figure 3, U1 is the ICE2PCS01G.

Switching Frequency Control. The switching frequency can be adjusted to

optimize power supply performance. By carefully balancing the power components

parameters, power output, efficiency and electromagnetic interference can all be affected

by the choice of switching frequency. To explore these relationships, a spreadsheet was

developed to assess component choice and system efficiency. Generally, as switching

frequency is increased, switching losses caused by the MOSFET and the output diode

also increase while ripple current in the inductor and filter capacitor decrease. The ripple

current can have a large effect on the power-handling capability of the inductor and

losses due to the ripple current in the inductor are reduced as frequency is increased.

Because the output voltage increases while the current is held constant, an increase in the

output power is demanded of the power supply. This causes the current through the

inductor to increase as the batteries charge. If the designer is not careful, this increased

current can cause the inductor to saturate which reduces the effective inductance causing

more ripple current and inductor losses.

The switching frequency can be set using an external resistor, set at pin 4, and

can range from 50kHz to 255kHz. The relationship between frequency and resistance in

non-linear, and can be found on page 8 of the ICE2PSCS01 app note. In this system, a


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Fig. 3. Schematic of boost circuit with Infineon control IC.


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175kHz switching frequency was implemented to reduce inductor size requirements and

maintain a high efficiency of 96%.

Power Factor Correction. The ICE2PSCS01 is capable of controlling the

output voltage and current while implementing power factor correction, all through the

pulse width modulation control of a switching transistor. This is accomplished by using

an inner current control loop and an outer voltage control loop. Because the switching

frequency is so much higher than the line frequency (50Hz60Hz), the controller can

easily follow the sinusoidal input voltage, making power factor correction an automatic

process within current control. A complete description of the SMPS controller is beyond

the scope of this paper. If the reader is interested, a complete explanation of current

control on the ICE2PSCS01can be found in these sources [32], [18].

Current Control. Current will be regulated by the switch-mode power supplys

control system. This is a core functionality of any battery charger and is important for

both battery safety and life span. It is important to note that a battery charger is not a

constant power device. As the battery voltage is increased through charging, the current

must remain constant. Thus, the power must be increased through the charging cycle.

Most components are power-limited so calculations for component power capability

should be done at the fully charged state.

The following sections focus on the modifications to control loops made to

optimize their performance for a battery charger. The control IC uses two control loops to

regulate both the current and the voltage of the power supply. Regulation is achieved by

using pulse width modulation to control an external MOSFET.


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Voltage Control. Using a switch-mode power supply controller for a battery

charger is slightly different than using the controller for its intended purpose. This is

because a battery charger must have a controlled current source; however, the voltage

changes considerably. Most battery packs have low impedance requiring high current to

increase the voltage temporarily. This is impractical because the current would be much

greater than the recommended charging current and would require an extremely powerful

charger.

For example, the impedance of the 18650 lithium ion cell used for this project

is 70m. A standard lithium ion cells minimum usable voltage is 2.75V and its absolute

maximum voltage is 4.2V, a 35% difference [33]. In order to control voltage, the charger

would need to make up the 1.45V difference between the starting voltage and the target

voltage of 4.2V. The only way to do this would be to supply enough current to cause a

1.45V drop using the cell impedance, which would require a 20A charge current if the

battery were fully discharged. Considering that the cell is a 2.2Ah cell and the maximum

charge rate is 1C, this would be nearly 10 times the recommended charging rate!

The charger must be viewed more as a constant current source with limits on

voltage so that it does not overcharge the battery pack. Under normal charging

conditions, the current is limited by the ICE2PSCS01 and the voltage will follow the

battery packs state of charge. Thus, a chargers voltage does not have to be controlled,

only monitored. From the charger perspective, only the end-of-charge voltage is

important.

The switching transistors duty cycle is dependent on a nonlinear gain block

[23]. They are primarily used for controlled voltage-ramp up and rapid response to


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overvoltage. This feature targets a narrow voltage range for tight voltage control. Because

a battery packs voltage changes significantly from the empty to the charged state, it

would be detrimental to use this feature. It overrides the current control necessary for

battery charging by forcing the charger to operate entirely in the controlled ramp-up

portion of the control algorithm.

For the system discussed here, the voltage control loop will be modified so

that certain safety features of the switch-mode controller will be implemented while

dynamic voltage control features will be disabled. To override the nonlinear gain block, a

constant 3V must be supplied to the feedback pin. Note that this should be a constant 3V,

not a value proportional to the battery voltage or SMPS output voltage. This is done by

supplying a regulated 3V from power regulator IC VR1.

The power regulator IC is supplied through the 15V common to all the control

devices. It features a shutdown pin that allows external control by another device. When

the feedback pin is below 0.6V, the ICE2PSCS01 goes into sleep mode, so the regulators

shutdown pin will be used for external control of the charger.

The danger of overriding the voltage feedback is that it disables all of the

ICE2PSCS01s voltage-related safety devices. To ensure safe system operation, both

active and passive safety shut down-systems have been added to the charger. These

systems are detailed in the section on safety, above. In addition to these controls, the

battery management system has executive control and if necessary can either force the

charger into sleep mode or shut down the system completely through the ground fault

interrupt system.


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Charger voltage is passively monitored with an ICL7665 voltage monitor.

These ICs monitor two input pins and control two internal MOSFETs based on trigger

voltages. Battery-pack voltage is monitored with voltage monitor U2. In an undervoltage

situation, the current is limited by turning off triac controller U4 which forces the input

current through a current limiter. Should an overvoltage situation occur, triac controller

U5 will trip the GFI protector. Under normal conditions, the BMS limits voltage by

shutting down VR1 forcing the SMPS controller into sleep mode. An overvoltage event is

considered an extreme event, and the system is completely shut down to prevent a fire or

other catastrophic situation.

Power Component Selection. The power components used in a boost-mode

switching power supply are very simple. They include a bridge rectifier to convert line

AC to DC, a boost inductor, a transistor to control the inductor, a diode to rectify the

output, and a capacitor to filter the output [18], [34].

Target specifications are set by the maximum charging current allowed by the

cells and the maximum voltage of the battery pack, which is determined by the cells

maximum voltage and the number of cells in a string. The cells resistance must be

accounted for so that the voltage supplied by the charger can be suitably above the

maximum pack voltage.

For this system, lithium ion 18650 2200mAh cells are being used. These cells

have a maximum voltage of 4.2V and a maximum charge current of 2.2A. The cell

resistance is 70m. Because the system can be upgraded, the user determines the number

of cells in the battery-pack string but the charger must be capable of working within the

packs limitations. With this design, the number of cells is limited by the communication


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bus which allows 120 cells. Table 2 includes specifications determining the chargers

design targets:

TABLE 2

DETAILED CHARGERDESIGN SPECIFICATIONS

Type of Cell Lithium Ion 18650 2200mAh

Number of Cells Cellnum 50120Maximum Cell Voltage Vcell 4.2V

Maximum Charge Current Icell 2.2A

Cell Impedance Rcell 0.07RMS Input Voltage Vin_RMS 120VAC

Target Efficiency 95%

Switching Frequency fswitch 175kHzInductor Ripple Current Constant KL_ripple 40%

Output Ripple Voltage Constant Kout_ripple 20%

The voltage drop per cell is calculated in equation (1).

Vdrop = Icell * Rcell = 2.2 A * 0.07 = 0.154 V (1)

The maximum pack voltage is found with equation (2).

Vcharger= (Vcell + Vdrop) * Cellnum = (4.2 + 0.154) * 120 = 522.5 V (2)

The maximum number of cells for this project is 60; therefore, the chargers

output voltage will be significantly reduced, as shown in equation (3).

Vcharger= (Vcell + Vdrop) * Cellnum = (4.2 + 0.154) * 60 = 261.2 V (3)

The chargers output power is calculated in equation (4).

Pcharger= Vcharger* Icell = 261.2 V * 2.2 A = 574.7 W (4)

To select the bridge rectifier, the current carrying capability was determined

by the chargers output power. The output voltage and the cell charge current determine

the output power, which must be consistent with the input power and design efficiency.


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In a worst-case scenario, the lowest input voltage would be coupled with the highest

number of cells in the pack. Input current is calculated in equations (5) and (6).

Pcharger 574.7 W

Iin_RMS = = = 5.0 A (5)

Vin_RMS * 120 V * 0.95

Iin_peak= 2 * Iin_RMS = 7.1 A (6)

These equations determine the bridge rectifiers minimum current handling

capability. As mentioned previously, devices must be severely de-rated when their

temperature exceeds 100C. Further, should the number of cells increase or the input

voltage drop, input current will be greater. Because much more capable devices cost little

more, it might be wise to specify a bridge rectifier capable of handling 15A to 20A. An

inexpensive bridge rectifier has been sourced from Diodes, Inc. that has a very low

forward voltage of 0.95V at 10A, thus greatly improving efficiency. The bridge rectifier

losses are calculated in equation (7).

Pd_br= 2 * Vf_br* Iin_RMS = 2 * 0.95 V * 5.0 A = 9.5 W (7)

The duty cycle for the switching MOSFET is determined next in (8).

Vin_RMS 120 V

Don_switch = 1 = 1 = 54% (8)Vcharger 261.2 V

There are two forms of losses in a MOSFET: switching losses (the energy

used to turn the MOSFET on and off) and conduction losses, calculated in (9). Switching

losses, calculated in (10) are constant while conduction losses start low and increase as

the battery charges. This is because the output voltage of the charger increases and, thus,

the duty cycle increases.


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Conduction losses:Pcond = Iin_RMS

2* Don_switch * Rdson = 5.0 A

2* 0.54 * 0.42 = 5.67W (9)

Switching losses:Pswitch = (Eon + Eoff) * fswitch = (0.007 mWs + 0.015 mWs) * 175 kHz = 3.85 W (10)

Total switching losses are summed in equation (11).

PMOSFET = Pswitch + Pcond = 3.85W + 5.67W = 9.52W (11)

The boost diode can have a large effect on system efficiency, primarily

because of its reverse recovery time. While ultrafast diodes are available that can improve

performance over standard diodes, new technology is available that eliminates diode

switching losses. Silicon-carbide diodes have almost no reverse recovery time and

switching losses can be ignored [35]. This leaves only conduction losses for the boost

diode, calculated in (12).

Pdiode = Vfdiode*Iin_RMS*(1 - Don_switch) = 0.86V * 5.0A * ( 1-54% ) = 1.97W (12)

For a boost converter operating in continuous conduction mode, the boost

inductor must carry both the high-frequency ripple current and the low-frequency peak

line current.

The size of the inductor is inversely related to the ripple current and the

switching frequency. At the same time, the combined peak current the inductor must

carry is directly proportional to the ripple current. A good design is a compromise of the

two parameters, calculated in (13) and (14).

IL_ripple = KL_ripple * 2 * Iin_RMS = 40% * 1.414 * 5.0 A = 2.828 A (13)


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Iin_peak+ IL_ripple 7.1A + 2.828 A

IL_peak = = = 8.5 A (14)

2 2

Because a 50% duty cycle will result in the highest inductance, the boost

inductance, calculated in (15) must be:

Don_switch(1 Don_switch)Vcharger 0.5 * (1 0.5) * 261.2 V

Lboost > > > 131 H (15)IL_ripplefswitch 2.828 A * 175 kHz

This is barely within the capability of the largest production inductor available

from Bournes. If a problem with inductor saturation occurs, a custom inductor will have

to be fabricated.

Losses in the inductor are primarily related to copper losses in the winding

[36], calculated in (16).

PL_boost = Iin_RMS2

* RL_boost = 5 A2

* 0.041 = 1.025 W (16)

The requirements for the output capacitor on a battery charger are very

different than the requirements for a regulated power supply. The output ripple voltage

can be much higher both because the feedback voltage has been disabled and because the

battery pack will act as a low-impedance load. Additionally, there is no hold-up

requirement. Should a brownout occur, the charger does not have to continue operating.

Because there is no problem with high ripple voltage interfering with the

feedback system, parametric studies were conducted to test the effects on other charger

components. These tests indicated that up to 20% ripple voltage, calculated in (17) could

be tolerated without adversely affecting the charger. The capacitance required to produce

the required ripple voltage is calculated in equation (18).

Vout_ripple = Kout_ripple * Vcharger= 20% * 261.2V = 52V (17)


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Icel 5.0 A

Cout > > > 255 F (18)

* 2 * line * Vout_ripple * 2 * 60 Hz * 52 V

To be effective, the capacitor must have a lower ESR than the battery pack.

The battery pack resistance is found with equation (19).

Rpack = Rcell * Cellnum = 0.07 * 60 = 4.2 (19)

The ESR of the capacitor should be < 4.2; this is much higher than the ESR

of many electrolytic capacitors; the one chosen has an ESR of 0.153 [37].

Rsense is a sense resistor between ground and the bridge rectifier. If the sense

resistor reports a voltage less than 0.66V, the controller will go into soft overcurrent

control by reducing the pulse width to limit output current. If the load demands too much

current (for example, a battery pack), the output current will be limited to the value set by

the sense resistor, calculated in (20).

Rsense < 0.66 V / Iin_peak < 0.66 V / 7.1 A < 0.092 (20)

Calculating losses in the resistor is important when designing for reliability;

equation (21). When the first prototypes of this design were being tested, the triple

surface mount system advised in the application notes was implemented. With a low ESR

470F capacitor the in-rush current was so great that the sense resistors exploded! This is

one of the primary reasons for the in-rush current limiting circuit. Under ideal conditions,

surface mount resistors can handle a maximum of 1W. If there is any variation among

resistors in a parallel set-up, the one with the lowest resistance will take the highest

current. Therefore, for high power supplies, the parallel sense resistor concept may not be

practical. Further, a high-power resistor in a TO-126 case that can be attached to a heat

sink that can handle 15W was sourced. At 100C the de-rating is 40% so it can handle


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6W in a worst-case scenario. Although the resistor has leads and the associated

inductance, its reliability is of primary importance [38].

PR_sense = Iin_RMS2

* Rsense = 5.0 A2

* 0.09 = 2.25 W (21)

System Efficiency. The losses in the power components are summed in

equation (22).

Plosses = Pd_br + PMOSFET + Pdiode + PL_boost + PR_sense (22)= 9.5 W + 9.52 W + 1.97 W + 1.025 W + 2.25 W = 24.26 W

Total power and efficiency are calculated in (23) and (24).

Ptotal = Plosses + Pcharger = 24.26 W + 574.7 W = 598.97 W (23)

= Pcharger/ Ptotal = 574.7 W / 598.97 W = 95.9% (24)

Losses due to control power requirements, battery losses, and boost-capacitor

losses can account for another 1%.

Electromagnetic Interference Control. The best means of reducing the noise

emitted by a device is to eliminate unintentional antenna structures in the circuitry and

provide filters for the power lines that must connect the various components. If the cable

runs are kept as short as possible, their utility as an antenna is greatly reduced.

To combat noise produced by the power supply, there is a ground plane to

eliminate high-frequency current loops in the control system. Circuit-board traces have

been made as short as possible in the power side of the circuit board. Both sides of the

circuit board have been used to allow components to overlap, further reducing trace

length.

Alternating current input power is filtered for EMI and the charger is directly

connected to the battery management system, both of which are shielded in aluminum


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enclosures. The battery management system will not allow the motor controller to operate

while the charger is being used. This is an important safety mechanism and also an

effective means of reducing EMI because the wire between the battery pack and the

motor controller is disconnected during charging.

Thermal Management. Several of the chargers components produce enough

heat to cause internal damage. These components include the bridge rectifier, switching

MOSFET, diode, current sense resistor, and the solid-state relay used to control power for

the controller. All of these components are connected to a common heat sink with a

water-cooling system. A temperature-protection circuit has been provided in case of a

failure with the cooling system. To monitor temperature, a negative temperature

coefficient (NTC) resistor is paired with a standard resistor to make a voltage divider.

When the temperature reaches 80C, the voltage divider output will be 1.6V. This signal

is sent to a voltage monitor IC U3 and on overtemperature the control line for the 3V

reference voltage will be pulled down, forcing the charger into sleep mode. This will

allow a cooling-off period to protect the power components from damage.

A parallel temperature-monitoring system for the charger is provided for the

battery management system. The two systems provide redundancy for safety, and the

battery management system will display an error message.

Pulse Hystresis Finish Charge

Using the battery management system for executive control provides the

opportunity to implement pulse charging for the battery finish charge. The battery

management system has access to all information about the individual cells, battery-pack

status, and battery-charger status. This provides the opportunity for the battery


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management system to directly control energy coming from the charger based on battery

status. As discussed in the safety systems section, the BMS microcontroller can turn off

and on controls in the charging system through optically isolated digital control lines.

To turn off the chargers output, the ICE2PSCS01 is forced into sleep mode,

which is preferable to turning it off because the charger will not have to go through an

internal re-initialization every time it is turned back on. The charger is forced into sleep

mode by overriding the chargers feedback control. As discussed previously in the

section on voltage control, the voltage feedback was replaced by a voltage regulator that

supplies a constant 3V. To force the ICE2PSCS01 into sleep mode, the shutdown pin of

the voltage regulator is pulled to ground. When the voltage regulator is shut down, it

produces 0V, which is below the ICE2PSCS01s 0.6V sleep threshold.

The battery management system microcontroller implements the pulse

charging algorithm. When the pack voltage reaches a set point corresponding to the

individual cells full state, the charger is turned off and the pack voltage settles to a lower

steady level. To determine settling, voltage drop versus time is monitored.

When the battery pack settling rate reaches a predetermined slope and the

voltage is below the pre-set charged state, the feedback voltage is switched back to 3V

by the battery management system. This initiates the next pulse. If instead the settled

voltage reaches the pre-set charged voltage, a flag is set in the battery management

system. The charged flag causes the software to turn off control power to the

ICE2PSCS01, forcing the charging system to shut down.


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Schematic

The schematic for the Charger design is shown in Figure 4. On the left side of

the design are passive safety circuits. The boost converter is on the upper right and the

SMPS controller is on the lower right.

Hardware

The PCB layout for the schematic shown above is in Figure 5. The board has

been divided into two ground planes to separate the low voltage control circuits from the

high voltage power components. All power loops are as short as possible and power

components are placed on both top and bottom sides of the board to reduce trace length.

The complete charger is shown in Figure 6. The board was soldered using the

reflow method in a toaster oven.

The Charging system and battery management system are designed to fit in

the enclosure shown in Figure 7. This enclosure incorporates the Charger board (shown

above), and the BMSs isolation board, as well as the BMSs CPU. Power input is

controlled with a GFI breaker; and battery power output is controlled with the main

contactor.

Figure 8 shows the assembly of the Charger, Isolation Board (middle), and

CPU (bottom).

Charging System Testing

The charger is a complicated and extremely dynamic component, which can

respond to feedback faster than a human can react. To add to the testing difficulty, the

voltages produced by the charger are lethal and can easily damage measuring equipment.


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Fig. 4. Schematic of charger circuit.


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Fig. 5. Circuit layout for charger circuit board.


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Fig. 6. Complete charger circuit board.

For high-voltage testing, special care and procedures must be exercised, such

as using isolation transformers for test equipment. For these reasons, a testing program of

ensuring core functionality of shut down and safety subsystems has been implemented.

Once those systems are operational, the charger will be tested as a high voltage, low

current power supply. The complete system will only be combined when the battery

management system and charger are fully operational.

Safety Device Actuation

The charging system has three passive subsystems to ensure that the chargers

output will not harm the battery pack or the charger will not overheat. It also has three

controls from the Battery Management System. These active systems add another layer of

safety to the system. These systems were tested before testing the charger, as their proper

operation would ensure safe testing and their absence would allow no output.


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Fig. 7. Solid model of charger and BMS control.


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Fig. 8. Charger and BMS control circuit assembly.

Charger Power Control

The 15V power supply for the SMPS controller and the 3V regulator can be

turned off in the case of a fully charged battery pack. Initial testing of the SMPS control

board will use this control turned off to reduce DC output voltages. As discussed

previously, all controls are set up so a signal must be present for operation. Table 3

details the control states.

Charger Sleep Control

The 3V feedback voltage produced by VR1 can be turned off by the BMS to

force the SMPS controller into sleep mode. This feature is used to control pulse charging


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TABLE 3

TESTED STATES OF CHARGERPOWERCONTROL

Connector1, pin1 Voltage at VR1 pin 2

0V 0V4.99V 15.05V

and is also used by the charger control board in the case of a charger overtemperature

condition. During initial testing this control will be off to reduce DC output voltages.

Table 4 details the control states.

Table 4

TESTED STATES OF SLEEP CONTROL

Connector 1 pin 5 Voltage at VR1 pin 4

0V 0V

4.98 3.01V

Temperature Sensor. This is a passive control used to detect an overheating

charger that is activated by a NTC resistor and a voltage monitor. When the NTC resistor

exceeds 70C, the voltage monitor will pull down a control line forcing VR1 into standby

mode. Tests were conducted using a Greenlee CM4000 DMM and a Traceable 4354CC

Digital Temperature Probe. Table 5 shows the temperature response of the control

system.

TABLE 5

TESTED STATES OF TEMPERATURE SENSOR

Sensor Temperature Voltage VR1 pin 4

21.3 C 4.98V 3.02V

64 C 1.62V 0V


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Under Voltage Sensor. A resistor network is combined with a voltage monitor to

provide low voltage protection. This is primarily to protect the charger from large inrush

currents. The system de-activates a triac, forcing incoming AC current to flow through an

inrush current limiting NTC device. The resistor network is designed to read bus voltage and

provide feedback voltages for the voltage monitors and the SMPS controller. Part of the

network is distributed in the battery modules, and the nominal voltage supplied at pin 6 of

connector 1 is between 3V and 7V. To test this function an adjustable voltage was connected

and adjusted until the triac activated. For this test, the AC line voltage was connected to the

charger. Since previous tests showed that the SMPS controller should not operate with the

voltage at connector 1 pin 1 at 0V, the voltage measured across the output capacitor should

only be 169VDC. The voltage across inrush protector was measured to verify operation, as

shown in Table 6.

TABLE 6TESTED STATES OF INRUSH PROTECTION

Connector 1 pin 6 Inrush triac

3.9V off

4.1V on

Over Vo

Documents

Kirk and Friedrich