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School of Electronics and Computer Science

Faculty of Engineering, Science and Mathematics

University of Southampton

Andrew J Benn

7th May 2008

Battery Sizing and design of a user friendly

monitor/charger for a bicycle light

Project supervisor: Dr Tim Forcer

Second examiner: Dr Paul Lewin

A project report submitted for the award of

Electromechanical Engineering MEng

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Abstract

With the increasing utilization of high power density batteries coupled with the

technological advances in efficiency, smart power management is becoming an

increasingly desirable asset. Most of the uses for this technology do not have a source of

constant power instead the power supply and demand are usually in constant flux,

requiring advanced power flow control. This control is desirable not only to system

developers but also to consumers of electronic and electrical goods. The hybrid electric

vehicle is one application for a power management system, where energy can be

variably generated, stored or used depending on the state of the car. No pre-fabricated

consumer microprocessor currently available can handle all the required tasks. This

report intends to show that by using commercially available microprocessors as building

blocks, an entire power management system can be built.

The issue of DC power management and storage will be introduced with a review of

background literature. A design for a bicycle light power management system using

commercially available semiconductor devices will be described with models and

practical measurements. A reasoned suggestion of further work is given as well as a

review of budgeting and project timescale for this project.

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Contents

Abstract .......................................................................................................................................... 2 Contents .......................................................................................................................................... 3 Acknowledgements ........................................................................................................................ 4 1 Introduction ........................................................................................................................... 5

1.1 What is Power Management ................................................................................ 6 1.2 Applications of the Technology .......................................................................... 6

1.2.1 Vibration Energy Harvesting (VIBES) ........................................................ 6 1.2.2 Hybrid Electric Vehicle ................................................................................ 7 1.2.3 Small Scale Renewable Energy Generation ................................................. 8

1.3 Types of Energy Storage ..................................................................................... 9

1.3.1 Kinetic .......................................................................................................... 9 1.3.2 Super-capacitor............................................................................................. 9 1.3.3 Secondary Cells ............................................................................................ 9

2 Design Process ..................................................................................................................... 12 2.1 The System Management Bus (SMBus) ........................................................... 13 2.2 What is a Smart Battery ..................................................................................... 14

2.3 System Design ................................................................................................... 14 2.3.1 System Redundancy ................................................................................... 16

2.3.2 Overall design ............................................................................................ 18 2.3.3 Component Value Calculations .................................................................. 19

3 Modelling, Tests and Measurements ................................................................................... 22 3.1 Testing and Measurement .................................................................................. 24

3.1.1 SMBus Communication ............................................................................. 24 3.1.2 Current and Voltage Limit ......................................................................... 26

3.1.3 Charging the Li-Po Battery ........................................................................ 26 3.1.4 Effect of Loading on the Charger ............................................................... 27

3.1.5 System Stage Three Testing ....................................................................... 28 3.1.6 Transients ................................................................................................... 28 3.1.7 LED Driver General Operation .................................................................. 29

3.1.8 Host Programming ..................................................................................... 29 3.2 Evaluation of Testing ........................................................................................ 30

4 Future Work ......................................................................................................................... 31 5 Conclusion ........................................................................................................................... 32 6 Project organisation ............................................................................................................. 33 References .................................................................................................................................... 34 Appendix A. General Supporting Documentation ................. Error! Bookmark not defined. Appendix B. Graphs and Measurements ............................... Error! Bookmark not defined. Appendix C. Project Costing and Timescale ......................... Error! Bookmark not defined.

Appendix CD. Code and Detailed measurement ……………………………………...45

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Acknowledgements

I would like to thank my supervisor Dr Tim Forcer for all his help and encouragement

over the past year, and my housemates Jill Hazelton and Kunal Nirmal for stopping me

procrastinating these last few weeks.

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1 Introduction

A Bicycle light is a good example of a device that has to tackle the issue of portable

power storage and efficient use of available power. This project is concerned with

establishing the common building blocks that can be used in an application of this type

and tries to establish itself within a commercial framework; to do this there is a short

exploration of the possible market and range of applications. The overall purpose of the

project is to show that a single microprocessor can implement a complete power

management topology but it must also demonstrate that it is a commercially attractive

product.

Bicycle lights initially consisted of a simple battery and bulb arrangement; the design

was limited by battery power density and the inefficiency of the incandescent bulb. To

solve the problems of bulky and limited life batteries some designs started to use

dynamos to create power from the mechanical movement of the bike. A series of

improvements in battery technology, as well as light and dynamo efficiencies has

produced better designs; the main advantages of these design optimisations are longer

life, increased brightness and reduced weight. However there are still some sectors of the

cycling community that believe the bicycle light design needs further improvements to

extend its range of useful applications (see Appendix A).

Current good designs use modern high power density batteries, as well as super bright

LEDs. This project attempts to show how a better design may come from the use of a

power management system; controlling the flow of power so any excess generated

energy is stored, and the stored energy can then be used to reduce the peak load on the

dynamo. Using a system like this it may be possible to reduce the dynamo size while

increasing the life of the product.

This kind of advanced power flow technology has applications in other devices that use

a similar configuration, such as the hybrid electric car. In this kind of system the

generation could be provided by regenerative breaking or fuel cells, while the energy

storage would consist of batteries or a super capacitors [1].

In my opinion there is a large commercial market for compact, cheep and pre-

manufactured power management systems. However despite there being a number of

devices that offer some power management functionality there is no one device that can

deal with the full complexity of the problem. Instead it is left to the developer to patch

together several devices in order to get the desired effect; the initial research found that

these devices often have to be sourced from separate manufactures and often have

different operational constraints.

This report will show that by using commercially available microprocessors as basic

building blocks it is possible to create a power management device. Demonstrating that

it would be possible to create a single microprocessor that contains all the necessary

functionality to implement power flow control.

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As part of the power management system the report also investigates the possibility of

using energy storage to vary the load on the power source such that the power flow in

the system is optimised for its application. The system design is mainly orientated

around the consumer requirements of bicycle users, but demonstrates a clear

methodology for other applications.

1.1 What is Power Management

Power management system can be described as the implementation of a control structure

that balances the power requirements of an electrical system. To do this it must make a

series of measurements to detect power sources and load current, from this it decides on

the best way to handle the power flow within the system.

Advanced Power Management (APM) [2] is a control structure the Intel Corporation

developed to handle power management applications in computing. APM uses power

scaling methods to alter load characteristics to suit the system status; it does this by

changing the clock frequency or CPU core voltage, known as CPU throttling.

Triggering events form the remainder of the system activate a change in power scaling

status; a trigger event may be user activated or come from system measurements such as

AC power source detection or battery alarm warnings.

APM is an important part of modern portable computing in enabling longer operation

and size minimisation. For example when not connected to a power source the APM

may reduce the power consumption so to increase the battery life, the system may also

limit input current from an AC adapter so the adapter rating can be reduced saving

money and size.

Computer power management is already a well researched area, it can be argued that

most power management devices are created for this type of market. This report

considers a fundamentally more generic load type but with a dynamic power supply,

where the power is not a constant.

1.2 Applications of the Technology

So far the use of this technology has been discussed in relation to a bicycle light and the

implementation in computing. In order to put this report in a commercial context it will

now conceptualise other possible application areas for this research.

1.2.1 Vibration Energy Harvesting (VIBES)

Mechanical energy harvesters use an inertia generator to extract energy from vibrations

in their environment (Fig 1). These devices are being designed to replace battery

dependent wireless equipment; the small power generation is limited to very specific

applications for embedded sensors. An electromechanical vibration energy harvester

developed by the University of Southampton has received interest from the US Navy

and the Oil Industry as potential customers. [3]

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The generation capacity of a vibration energy harvester is usually in the scale of 1mW or

less, for this to be useful it is usually stored over a period of time and wireless equipment

is used in bursts. If the magnitude of vibration decreases then the available power will

decrease, to handle this the power will be stored over a longer period of time; this can

vary from seconds to hours. The energy storage device is usually a capacitor or super

capacitor.

These inertial generators experience a maximum power output at their resonant

frequency. For use in practical applications the harvesters must be carefully designed to

match the natural frequency of the environment; consideration of the harvesters

bandwidth is also needed, as narrower bandwidth generators generally give greater

power output at the resonant frequency. A process of tuning the generator using

electrical loading can be used; with this a narrow bandwidth can be maintained while

resonant frequency can be adjusted to suit the system. [4]

From this description of vibration energy harvesting it is clear that an embedded

compact power management system would be well suited to the load management and

power conditioning needs. However due to the extreme low or high voltages created,

depending on the harvesters transduction mechanism, there needs to be greater

consideration of the semiconductor devices than is within the scope of this report.

1.2.2 Hybrid Electric Vehicle

Hybrid electric vehicles are classified according to their use of energy sources; the

categories include series and parallel drive chains. Despite the differences in each type

the basic concept is the same; a generator is driven from some form of engine, energy

storage occurs and the vehicle is driven by an electric motor. [2] The whole system is

linked by a DC power train, which must be controlled as the requirements of the system

change. The energy storage device supplies energy during peak loads such as

acceleration, and stores energy when there is a low or negative load, i.e. breaking.

Fig 1. Model of inertia generator [3]

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When referring to electric hybrid vehicles it is common to think of cars however, this

concept of an electrical power train also applies to marine transport. A report on the use

of the Integrated Power System (IPS) [3] in naval prolusion and electrical applications

shows a modular design for a power management. The system modules consist of

generation, distribution, propulsion, storage and control modules; with this kind of

structure it is clear that IPS satisfies all the conditions to implement power management.

1.2.3 Small Scale Renewable Energy Generation

Small-scale renewable generation is not a new concept however, it has only recently

become a truly viable consumer option as an alternative source of power. The increase in

technology has increased the efficiency of the generation devices; the photovoltaic cell is

an excellent example of a generation method that has been hampered by cost and

efficiency.

Photovoltaic generation units are usually compared by initial investment and time until

there is a return on the investment. Because of this a large amount of research in the

industry is aimed at obtaining higher efficiency and lower cost. Osahon H Okunbo

discusses the application of “maximum power point tracking” in order to obtain higher

efficiency from solar systems [5]. This research looks at the dynamic control of the

system load in order to extract the maximum available power from the cell (Fig 2).

Again the ability for a power management device to control storage capacity and active

load could be an applicable technique to increase efficiency. With the suggested design

of a single chip management structure the cost may also be decreased making this type

of generation more attractive.

Fig 2. Graph of cell current (red line) and power (blue line) as a function of voltage [5]

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1.3 Types of Energy Storage

Although this project studied the integration of cells in a power management system it is

worth considering the use of other storage methods. In this section the report will

quickly review alternative options before talking in detail about cell chemistry and sizing

issues.

1.3.1 Kinetic

By applying rotational acceleration to a symmetrical mass such as a flywheel energy is

stored in the masses inertia. In standard systems the flywheel is made from a heavy mass

with enough strength to cope with the mechanical forces during operation; there is a

proportional linear relationship between the mass and the energy storage capacity.

Because of this relationship most flywheel systems are very heavy and sizable.

Recently an article in the IET power magazine [6] reported the use of lighter but faster

flywheels. With the relationship between speed and energy capacity being energy

squared, such that “doubling the speed quadruples the energy [6]” stored, much smaller

and lighter designs are possible.

It can be argued that this has no real application in portable systems and its usage is

fairly limited however, research in 2007 showed it was possible to create a Micro

Electro-Mechanical System (MEMS) flywheel storage device. The device, sized 100 m

x 100 m x 50 m, was capable of 51,000rpm storing 337J. [7]

1.3.2 Super-capacitor

Super-capacitors offer an alternative for short term power storage, useful for dealing

with power fluctuations as described by Abby and Joos [8]. They perform well

compared to different storage mediums such as lead acid batteries with 83% against 63%

efficiency. The high efficiency is mainly due to the low electrical series resistance

(ESR); this will also allow for very high charge and discharge currents. Comparatively

super-capacitors are also preferable in terms of power density. [8] [9]

The main disadvantages to the technology are cost and high self-discharge. The cost is

significantly greater per what hour than other storage methods, while the self-discharge

rate makes these super-capacitors only useful in short term storage.

1.3.3 Secondary Cells

Secondary cells are reversible electro-chemical reactions, thus they are capable of both

supplying and storing energy; conversely a primary cell is a non-reversible reaction and

forms the common disposable battery. Secondary cells come in various different

arrangements of chemical makeup each with different characteristics that need to be

considered for the application.

Lead Acid cells are a relatively inexpensive option in that they have the lowest cost per

watt-hour, however the unit weight and unit size per watt-hour are considerably less

impressive; seriously restricting its use in portable application. A Valve Regulated Lead

Acid (VRLA) battery is a totally self-contained unit and requires only a small amount of

maintenance.

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Despite having no memory effect the use of lead acid cells in charging and discharging

cycles must be very carefully controlled; deep discharges significantly reduce cyclic life

of the battery. The optimum temperature is 25°C, the cell voltage can vary considerably

either side of this value and even damage the capacity; because of this they usually

require a temperature controlled environment for best performance. This type of

chemistry will usually be used in large-scale industrial applications where cost takes

president. [10]

Nickel Metal Hydride (NiMH) does not suffer the same drawbacks that the lead acid

cells do; much higher power per unit weight values are achievable, a typical value is 50

W/kg. These cells are less sensitive to temperature than the lead acid type, although if

stored at high temperatures its performance is compromised. Through normal operation

a crystalline build-up occurs on the electrodes creating a „memory‟ effect; to avoid this

the battery requires a full discharge/charge cycle. The relatively low cyclic life of a few

hundred cycles and higher price prevents the use of this type of cell as an imbedded

power source; for maintenance it must accessed relatively regularly. [10] [11]

Lithium Polymer (Li-Po) cells have a high energy density, the energy density of Lithium

Ion cells is actually higher but Li-Po cells can give better energy per unit weight. This is

because they have no need for a metal case; values well in excess of 100 W/kg are

available. This battery chemistry provides better cyclic life than NiMH, it is also

maintenance free and does not suffer from any memory affect. The only problematic

limitation of Li-Po chemistry is it high manufacturing cost. [11]

Fig 3. Graph of a battery in a charging cycle, showing charge current and voltage/cell [12]

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It is important to prevent excessive charge and discharge; both of which are easy to

provide as protection. When charging there are three key stages, in the first stage a

constant current charge is applied. Usually the current is set at 1C or less, however in

some fast chargers higher charge currents are used to rush through this stage. The

transition to the second stage occurs when the charger voltage reaches the pre-set battery

voltage, now a constant voltage charge is applied. The transition to the third stage

generally occurs when the charge current falls below 0.03C depending on the charger,

the battery is now fully charged. In this third stage a small charge current is supplied in

order to counter any self-discharge; this final stage should not be used on all batteries. It

is very important that the battery chemistry and design limits are considered before

applying a charge cycle. Fig 3 is taken from Batteries in a Portable World online

resource page [12]; this shows a trace of cell voltage and current in the three stages of

charging.

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2 Design Process

The primary aim of this project was to demonstrate a working power management

system for a bicycle light; the minimum requirement of this was to achieve charging of a

suitable battery as well as discharging into a load. There was a requirement to identify

the best cell chemistry for this application and size the battery pack. It also proved

desirable to conduct research into the use of load sharing between the battery and power

source; reducing the load on the battery during operation of the bicycle light.

The system is in the context of a bicycle light so the first step in designing the system

was to identify the consumer requirements and existing designs. A simple questionnaire

was used to identify key design aspects consumers found important; bearing in mind this

project is only concerned with the electronics aspects. A copy of the data gathered can

be found in Appendix A.

While considering this data it is important to note there are several types of cyclist,

commuters, touring, racing, and off-road. The different genres of cyclist have different

requirements; in particular commuters have less demanding requirements because of the

conditions they usually ride in, such as a relatively light environment and short journey

times. Also racing cyclists may never actually need a bicycle light so they were discounted

from the survey.

In general cyclists wanted a better luminescent intensity and greater battery life for a single

cycle. After this the results became varied; off-road cyclist classed better weather resistance

highly while touring cyclists rated a reduction in weight and the supplementary dynamo

option. Cost was a consideration but not as important as the above options.

Having searched the UK Intellectual Property Office database there were a few results that I

considered good design ideas; one of these was the patent Rechargeable battery unit for

bicycle illumination [13]. In this patent the user can switch between a dynamo and a battery pack

and includes “indicator lamps” to show the charge status. Although this design publication is old

it addresses the issue of light operation during standstill. More current patents such as

“Electronic rear lamp for bicycle” [14] use capacitors to store energy so during standstill they

can be discharged keeping the lamp illuminated.

The patent “Bicycle lamp housing nickel cadmium battery” [15] involved using a dynamo to

charge a rechargeable battery which then powered a lamp for a couple of hours. In this design

the battery was totally enclosed providing excellent weather proofing, however a fairly crude

electrical circuit limited the controllability of the design.

As well as using patent searches, commercial solutions were considered; two leading

manufactures of bicycle lights are lupine and cateye. Lupine [16] and cateye [17] offer products

that make use of „Ultra Bright‟ LED technology and offer very high lumens, they use

rechargeable battery packs containing Li-Po cells. However neither manufacturer allows for any

kind of supplementary supply and as such the battery packs are of a relatively large capacity and

weight.

In this report the research into load sharing between the battery and power source is an

innovative design that should show a flexibility and optimisation that the above designs

do not have.

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2.1 The System Management Bus (SMBus)

The SMBus [18] is an existing two-wire serial bus for communication between system

components; this bus is based on the I2C Bus. SMBus and I2C Bus devices are in

general interchangeable, however slight changes in architecture can raise some issues if

the two are mixed. [19]

The SMBus provides a control bus for system and power management related actions; a

system may use SMBus to send messages to and from devices instead using individual

control lines. The development of the SMBus was based on the need for a reduced pin

count and increased integration between power management devices; this utilized the

advantages of serial communication, with a relatively slow baud rate and reduction in

wiring a high flexibility was achieved.

This project is concerned with showing that a complete management system can be built

with standard components, it argues that because of this a single chip system can be

developed. In a single chip system only one CPU would be needed, effectively

eliminating the need for the SMBus; despite this the Bus makes up a large proportion of

the understanding and implementation of this project. Because of this a short technical

introduction to the SMBus and its usage is now given.

The Bus frequency must be 10 – 100 KHz, the clock and data lines are held high with a

current source of 100-350 A and a voltage of 3.3-5V. When a device wishes to writea

„0‟ to the bus it must be able to sink this current. Two types of device can access the bus,

master and slave devices; master devices must support slave mode and bus arbitration in

case there is another master device controlling the bus.

The master device must generate a Start condition before sending an address byte, the

data line must then be released so the slave can generate an Acknowledge pulse; the

master will generate the clock for the acknowledge pulse. Upon receiving an

acknowledge the master sends the control byte and then writes or reads commands from

the bus. After each byte the slave must generate an Acknowledge pulse, else an error

condition is entered. After communication the master must generate a Stop condition;

Fig 4 and Fig 5 shows the layout for communication on the SMBus.

Fig 4. SMBus communication format, including Start and Stop conditions [18]

Fig 5. SMBus write byte to slave protocol, grey area indicates slave in control of data line [18]

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2.2 What is a Smart Battery

A Smart Battery contains a microprocessor that retains information on the battery; it is

capable of communicating with the charge over the SMBus and as a minimum supplies

the charge with the correct charging algorithm. The Smart Battery Systems (SBS)

Implementers Forum [20] specifies that a smart battery must give information on the

battery state of charge as well as the charging algorithm. [11] [20]

Some Smart Battery implementations provide extra functionality such as cell balancing

or protection against over-current, short-circuit and deep discharge. Fig 6 shows the

usual layout of a Smart Battery system. [21]

2.3 System Design

The system components were selected from popular commercial manufactures including

ATmel, Microchip, Maxim and Linear Technology. This report will now briefly

introduce the selected devices before giving details of the overall electrical system

design.

Fig 6. One possible Smart Battery model, for use in notebooks, camcorders and other portable equipment [20]

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It was decided to use a Lithium Polymer battery pack, this seems to be the predominant

chemistry used in current portable devices; it has one of the highest energy capacities

available with next to no maintenance required. As different cell chemistries have

different cell voltages and charging tolerances a specific charging scheme must be used;

this type of cell has a charged cell voltage of 4.23V and a discharged cell voltage of

2.7V. If the cell experiences overcharging or deep discharge the cell temperature will

increase and chemical reactions will cause a metallic plating of lithium on the anode as

well as oxidization at the cathode. [12]

In order to prevent heating of the cell my system will implement voltage limits of 3–

4.1V. The battery selected for this project was not a Smart Battery due to cost restraints

and difficulty sourcing; if needed a laptop battery can be used to confirm Smart Battery

implementation. The battery sourced from „BRC hobbies‟, costing £18.44 (See

Appendix C), is an 11.1V 1.3Ah pack consisting of three cells in series; the

manufacturer recommends charging at 1C. [12] [22]

The Max8731A Smart Charger was sourced from Maxim, this will provide all the

charging functions necessary to charge the Li-Po battery pack. This charger is a level 2

SMBus charger because of this it needs a host device to communicate the charging

needs. The device has an 8-26V input rang with the logic voltage being supplied by an

internal regulator; this regulator is a good way to confirm power is being supplied.

Using two independent feedback loops the charger is able to monitor the charge current

to a maximum value of 8A, and an input current of 11A. The control implemented by

this device reduces the charge current linearly to zero as load current increases after a set

level; this attempts to reduce the maximum input current, reducing the AC adapter

rating. As well as this voltage regulation is supplied by a voltage sense feedback. The

charger is capable of charging one of two batteries at a time, this can be changed at any

time using the battery select pin. The charger uses a Buck topology and has efficiencies

in excess of 95%; the charging current over 2A gives the best efficiency values as shown

in figure 7. The Max8731A was sourced as part of an evaluation kit after attempts to

construct a charging circuit using the Max1647 were unsuccessful; the two devices

involve a similar circuit. [23] [24]

Fig 7. Efficiency Vs Charge Current for the Max8731A [23]

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The host controller is being supplied by Atmel, the ATMega406 like the Smart Charger

has a wide input voltage range and uses an internal regulator to supply the logic voltage.

This device is designed for integration with the smart battery providing the functions as

described previously; it is also capable of hosting the SMBus, efectivly performing the

roles of the host and Smart Battery shown in figure 6. To meet the minimum

requirements set by the SBS forum the Atmega406 gives information on the correct

charging scheme as well as stage of charge information, provided by a coulomb counter.

Protection control, cell balancing and temperature sensing are further functions that are

offered by this particular device. The device is programmable via the JTAG interface

which is supported by AVR studio 4. [21] [25]

Luxeon super bright LEDs were chosen for the system load, simply because of their high

efficiency light output. The white K2 emitter is capable of a luminous flux of 100

Lumens (lm = cd·sr, luminous flux = luminous intensity per solid angle) while only

consuming approximately 4 watts; giving a luminous efficacy of 25 lm/W. [26]

The K2 datasheet suggests the use of a constant current source to drive the emitters

ensuring a stable light intensity. The LT3474 [27] is a variable 1A constant current

supply, this is formed of a constant frequency step down converter and will need a

number of external components. This device has a maximum input voltage rating of

36V and a variable current output from 35-1000mA. To protect against open circuit

conditions the output will clamp at 14V and with the use of additional circuitry an

undervoltage lockout can be added. The LED current is controlled via an analogue

input, where 1.25V represents 1A and drops linearly to zero; the frequency can also be

altered from 200kHz to 2MHz by changing the value of the timing resistor (see

Appendix A). The majority of component values need to be calculated specifically for

the application from equations given in the datasheet. [27]

2.3.1 System Redundancy

The system components described in the previous section often have duplicated

functions, this is a cause of redundancy in the system increasing the overall footprint and

pin count. Table 1 highlights duplicated functions that may be cause of redundancy this

is then discussed. Yellow highlight indicates a direct redundancy, green semi-

redundancy.

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Max8731A Smart Charger

[23]

Atmega406 Smart Battery

and protection functionality

AVR [25]

LT3474 1A constant

current driver [27]

SMBus Host

SMBus Slave SMBus Slave

High Vcc of 25V High Vcc of 25V High Vcc of 36V

Internal Voltage regulator

for logic supply

Internal Voltage regulator

for logic supply

Detects presents of battery

charger

Cell Voltage measurements

Coulomb Counter

Charge current sense Charge / Discharge current

sense

Discharge Current Sense

Input current sense

Battery voltage sense

Internal Temperature Sense Internal Temperature Sense

controlled charge controlled charge

/discharge

controlled charge, including

trickle charge

controlled pre-charge

Cell Balancing

Deep under-voltage

protection

Under-voltage lock out

possible

Current limit control loop Charge/discharge over

current protection

Discharge current control

Short circuit Protection Open and closed circuit

protection

Sleep / Idle reduced power

modes

Idle mode reduced power

In system programmable

It is clear from this comparison of component functionality that the selected components

result in a larger than necessary footprint, excessive measurement and peripheral

components. The semi-redundant functions such as the SMBus and charger detect are

necessary in a modular power management system however, if all the functionality was

performed in a single chip the communication systems wouldn‟t be needed reducing pin

count and complexity.

The direct redundancies are created by modular functionality overlap, for example the

Max8731A and the LT3474 control the charge and discharge current individually; so

this function isn‟t needed in the ATmega406. The only functions specifically needed by

the ATmega406 for this design involve cell balancing, coulomb counting and SMBus

hosting.

Table 1. Showing the functionality of system components and possible areas of redundancy.

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2.3.2 Overall design

The overall modular design seen in figure 8 looks similar to that in figure 6; this section

will describe the operation of the modular design before talking in detail about the

selection of peripheral components.

In this system the Smart Charger will charge the battery pack according to a set scheme

dictated by the smart battery; the host in this system will also be implementing the smart

battery functionality. The main data flow on the SMBus between the host and the

charger will contain voltage and current information; device addresses and ID can be

obtained if needed.

Ideally the host would be in actual physical contact with the battery so its internal

temperature sensor can be used to detect any heating; this was not possible in this

prototype design but the battery pack will not operated near its limits to avoid heating.

The host will be performing cell balancing to avoid mismatched cells that could damage

the performance of the system. Charge and discharge current will be controlled by other

modules so are not implemented by the host; similarly the protection functionality of the

host is not needed as seen in table 1. Discharging of the battery pack should only occur

when a load is present which is to large for the power source to supply, the host will then

control the discharge of the battery pack in order to supplement the power source. The

only exception to this is the occasional calibration cycles required by the coulomb

counter to avoid the „electronic memory‟ effect. [11]

The constant current driver will ultimately control the load however, the control line

from the host allows pulse width modulation and shut down. The LED driver will be

primarily supplied by the power source; because of the buck topology used by the

charger this power supply will have to deliver a higher nominal voltage then the

maximum battery voltage. If the supply source is not able to deliver sufficient power a

voltage drop will be detected, as shown in figure 9, on detection of this condition the

charger should attempt to reduce the system load. Reduction of the system load can be

achieved firstly by reducing charge current to zero and secondly by discharging the

battery into the load; the diode will prevent back driving the power supply. Smoothing

capacitors are used on the power lines to reduce peak current demands and all ground

lines are common.

Control line

Power

Source

Smart

Charger

Host and Smart Battery

Implementation

Li-Po

Battery

LED

Driver

SMBus

Discharge Fet Power

Cell Balancing

Fig 8. Diagram showing modular design including power connections, control lines and SMBus

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The overall system should operate in three modes, charging with no load, charging with

load and discharging into load; these three states are studied in further detail in section 3.

2.3.3 Component Value Calculations

So the charger could implement the correct charging scheme the maximum battery

voltage needed to be calculated.

Vbat-max = #cells · Vcell-max

= 3 · 4.1 = 12.3V

Similarly the minimum allowable battery voltage must be calculated for discharge

protection; implemented by the LT3474.

Vbat-min = #cells · Vcell-min

= 3 · 3 = 9V

The battery manufacturer recommended the maximum charge current as 1C while

discharge current could be as large as 20C. For safety this system will operate below

these values.

Charge current = 1.3A

Discharge current =26A

Fig 9. A theoretical model of a non-ideal power supply, a common characteristic of a solar array or dynamo [28]

(1)

(2)

20

In Appendix A the general circuit layout for the LT3474 1A LED Driver can be seen,

the component values for this design must be calculated for the application as described

in its datasheet. [27]

Firstly it is clear from the efficiency graph (see appendix A) that the step down converter

is significantly more efficient when driving two LEDs than when driving one; so the

load is chosen to be two K2 emitters.

To meet the requirement that the battery must not be discharged below 9V the following

undrevolatge lockout will be implemented.

Where;

Vth=9.5V

R1=12kΏ

. . R2=4.7kΏ

Because this is a fixed frequency DC converter the operating voltage is set by the duty

cycle of the system; so the selection of an appropriate switching frequency depends on

the voltage range desired. It is desirable to use the lowest possible frequency in order to

reduce component size.

To calculate the lower voltage limit the following equations are used, where Vout is

desired output voltage, Vf the forward voltage drop across the diodes used, Vsw the

voltage drop over the internal switch. DCmax is the maximum Duty cycle and can be

calculated by equations 5.

Fig 10. Implementation of undervoltage lockout using the SHDN pin [27]

(3)

(4)

(5)

21

Vout = ~7V (as assessed from the K2 emitter datasheet [26]

Vf = 0.38V (as shown in 1N5821 datasheet [29])

Vsw = 0.4V

toff = 200ηs

The maximum voltage limit is calculated in a similar way using DCmin instead; simply

given by 160ηs · f.

Inputting these equations into excel it was possible to optimise the values; the optimum

frequency is 600kHz.

From the log graph in appendix A its can be seen that a value of 60KΏ should be used

for the timing resistor in order to achieve the desired frequency.

A simple formula in the LT3474 datasheet suggested an inductor size of 12μH, however

larger inductor sizes increase maximum load current and reduce voltage ripple. The

final inductor value for L1 was 22 μH. The low DCR series resistance of this inductor,

60mΏ, allows for higher efficiency; the maximum DCR benchmark set in the datasheet

is significantly larger, 200mΏ.

The Smart charger and Host system will be provided by the Max8731A evaluation kit

and SMBuscon 2; these devices are supplied in a working condition for development

and testing purposes.

Table 2. Equations in excel allowed the manipulation of system values to obtain a suitable frequency. Frequency

should be labelled in MHz not kHz

22

3 Modelling, Tests and Measurements

In modelling the system three distinctive stages are considered, firstly charging with no

load.

To illustrate the operation of the system in the first state an ideal power supply is

considered. The charger will deliver the most power when approaching the voltage

regulated stage of charging; in this model the charging parameters are 12.2V at 500mA

giving a total power of 6.1W. If a charger efficiency of 90% is assumed then the total

power drawn from the power source is 6.77W; 14V at 0.48A. This model seems

straightforward and simple however so far it has been assumed that the power supply

can deliver the full amount of power. Figure 12 shows the ideal power supply

characteristic as the current limit is reduced from 0.5A and the corresponding power

drawn from the system.

As the current limit is reduced below 480mA the charger enters discontinuous

conduction, it can be seen that the voltage falls to ~12.2V reducing the power supplied.

While in discontinuous mode the power drawn by the load is not the maximum power

available; coupled with the inefficiency of discontinuous conduction the system does not

make good use of the power source.

If the system was able to detect the voltage drop from the power supply it would be

possible to reduce the system load; in this way it would be capable of maintaining the

charger in continuous conduction mode and obtain the maximum power from the supply.

This maximum power point tracking will be discussed more later in this section.

Fig 11. A simple steady state model of the charging state

Figure 12. graph showing the ideal power supply characteristic and the power drawn by the load

23

In the second state the user has activated the load; the system is now charging with load.

To start we consider the system when using a dependable supply and discuss the effect

and possible coping methods for a non-dependable power supply.

If the power supply can supply 14V at 0.5A, and the system host instructs the charger to

allow a maximum input current of 0.5A. As the load is increased this input current limit

will be reached, after this the charger reduces the charging current linearly to zero to

compensate for the load. This method allows for lower power source current ratings

however, it still relies on a dependable power source being available.

If the power supply was only able to deliver 0.4A then the input current limit would not

help the charger scale the system load; insufficient power would be available and both

DC DC converters would enter a discontinuous conduction mode.

Again a maximum power point tracking system would be needed to detect a voltage

drop and reduce the input current limit to give a real-time representation of what the

power supply can deliver.

The third stage deals with the controlled discharge of the battery into the load; this stage

occurs when the power supply is not able to deliver sufficient power to a load, and

charging has already been reduced to zero; possibly detected by the system by a voltage

drop to ~12V.

Fig 13. Simple representation of the system under load and charging.

Fig 14. Model of system in stage three, discharging into load

24

In figure 14 the load is drawing 7.5W, with a converter efficiency of about 90% this will

be perceived by the supply as a load of 8.3W. At its maximum power point the power

supply can only deliver 7W so a supplementary power supply is needed; this is achieved

by a battery discharge. The diode, as represented in figure 14 by an ideal diode in series

with a voltage source, prevents the battery back driving the supply.

They supply voltage should be represented by a current source where the current is

function of the voltage, this is due to the characteristic of the power supply as shown in

figure 12. With the ideal power supply current limited at 0.5A the power supply will

deliver 6W; the battery will supply the additional 2.3W with a discharge current of

0.19A, see appendix B for this simulation. The actual discharge current will depend on

the battery voltage, so it will differ depending on the batteries state of charge; because of

the circuit configuration the power supplied by the ideal power supply will also reduce

as battery voltage reduces.

The scope of this project does not cover the implementation of a maximum power point

tracking (MPPT) system however, the description of the system states above describe

the possible use of MPPT and discussed methods of load scaling. The use of an ideal

power supply in the prototyping of this project limits the study to steady state

experimentation, so MPPT was manually applied to the prototype via the host system

interface during testing. The implementation of a MPPT system is discussed further in

section 4 with suggestions for a suitable testing setup.

Other modelling included simulation of the LT3474 LED driver, using the demo circuit

simulator given by Linear Technology [30]; this was used to confirm the component

values calculated in section 2.3.3. From the simulation, shown in Appendix B, it can be

seen that the 1A output is achievable after a 0.5ms transient period. It is also worthy of

note that there is a large transient input current response in the first 20μs, it is important

that the system is able to support this transient load. Although transient conditions are

not studied in detail in this report the effect of a sudden large load on the system will be

tested for and commented on.

3.1 Testing and Measurement

3.1.1 SMBus Communication

As a step to building a power system host device a method of SMBus communication

needed to be developed, the following test established the successfulness of the

algorithms used in hosting the SMBus. Using the assembler programming language a

series of routines were implemented in a PIC16F88 in an attempt to access and

eventually host the SMBus.

With the system design the host would only have to communicate with a Smart charger

slave device; because of this the full implementation of the SMBus was not considered

necessary, it was sufficient to read and write to the charge. (code is supplied in the

appendix CD)

25

The implementation of „smBus control (1).asm‟ simply implements the in-circuit

programming ability of the PIC16F88, and ensures correct addressing of the clock and

data pins.

The first SMBus communication attempt was made by „smBus control (3).asm‟ this

routine repeatedly sent the charger address. By implementing this test it was possible to

check the start condition was working properly, the „Byte_out‟ routine operated

correctly and that the charger address was correct. Observation of the SMBus was made

using a Agilent digital Oscilloscope series 6000, the I2C function could be used to

trigger the oscilloscope of a start condition; allowing for easy observation of the serial

bus.

The above trace of the SMBus communication confirmed, the routine used was sending

the address byte correctly, the slave was generating an acknowledge pulse and that this

was detected by the host. This allowed conformation of the Smart Charger slave address

as 0001001X; the last bit was reserved for a read or write bit.

The routine for reading data from the slave was implemented in „smBus control(5)‟ this

confirmed the routine for requesting data from the slave and also confirmed that the host

was able to read data off the SMBus. A trace of this can be found on the appendix CD.

After some initial problems with loops caused by a misplaced goto command the full

implementation of write and read routines was possible with „smBus control(8).asm‟.

Implementing the full charging function of the slave device was attempted using this

read and write routine however, the device failed and no longer responded to the SMBus

even when using a earlier proven communication routine. The failure mode was such

that the clock line was held at a semi-high potential of 2.4V. Another characteristic of

the failure was a potential of zero on the output from the internal linear regulator (VL)

but a correct potential of 3.9v on the internal reference output (REF).

Fig 12. Oscilloscope CSV traces showing a START, address send, ACK generated by „smBus control (3).asm‟

26

These VL and REF values should not have been possible as REF seems to depend on VL

according to the datasheet. After email consultation with Maxim, and the building of a

new test circuit a second device failed in the same way. After this it was decided to be

in the best interest of the project to use the development kit supplied by Maxim as

described in previous sections.

3.1.2 Current and Voltage Limit

To ensure safe controlled charging could take place using the Max 8371A evaluation kit,

it was important to check the voltage and current loops worked. To do this the host

system and charger were set up and accessed using the computer interface tool provided

with the smbuscon2; a supply of 14V and sufficiently high current limit was used.

Current and voltage measurements were made by Agilent desktop digital multimeters,

while the load was adjusted by the use of the labs variable resistor that was able to

dissipate the energy safely. Using the computer interface a current limit of 1A and

voltage limit of 12.25V was set.

The graph clearly shows the current and voltage limit in effect with a sharp transition

between the two. The measured current limit and voltage limit were 1.077A and

12.235V respectively giving error values of 7.7% and -0.12%. It can be seen from the

current limit error and voltage limit error graphs in appendix A that these error values

are within the design tolerances of the device. The test clearly confirms the devices

ability to control voltage and current within the expected error margin. (test

measurements on appendix CD)

3.1.3 Charging the Li-Po Battery

With the previous test showing that accurate control of charging voltage and current is

possible this test looks at the charging characteristic of the Li-Po battery in a 1A ,

12.25V charging regime.

Figure 13. graph showing the results of a varying load in terms of I and V. The current and voltage limits are clear.

27

Measurements were taken simultaneously every 60 seconds by three Agilent desktop

digital multimeters triggered by a signal generator. The result of this experiment clearly

shows a standard charging characteristic as can be seen if you compare this result to

figure 3 in section1. The battery voltage increases from 11.8V to 12.235V and the

charger goes through the voltage regulated and current regulated stages. It can also be

seen that this charger applies trickle charging when the charge current drops below a

minimum value, this should not be used for Li-Po batteries so it is necessary for the host

system to terminate charge at this point. (test results on appendix CD)

3.1.4 Effect of Loading on the Charger

By sensing the current drawn by both the charger and the load the smart charger can

attempt to limit the supply current demands; it does this by linearly reducing the charge

current after a maximum input current set point has been reached. To test this the

charger was set up with a 14V power supply and a charging scheme of 0.8A at 12.25V

was applied, it was told to limit input current to 1A. An external load was then applied

and increased incrementally to study the effect on the system power demand.

Figure 15 shows the results of the experiment; when no load is present the full charge

current is supplied, the charge current starts to decrease when the input current reaches

its limit. When the charger current reaches zero nothing more can be done to reduce

system load so the input current increased with the load. When no charge current is

present a small negative current is supplied by the battery, this is believed to be due to

surface mount LEDs indicating battery status in the charger circuit.

Fig 14. graph of charging characteristic of Li-Po battery , showing cell voltage, charge voltage and charge current.

28

3.1.5 System Stage Three Testing

As discussed in system modelling the load as seen by the power source can be further

reduced after the charge current has been reduced to zero by discharging the battery into

the load. In this test supply and battery currents were observed to confirm that this

system will work. As predicted the battery current increased as the power supply current

decreased, ensuring enough power was delivered to the load at all times. This test did

highlight the issue of a negative discharge current, for example if the load were suddenly

disconnected the current flows into the battery. An uncontrolled charge such as this is

dangerous so it is important to include a diode to prevent this. (See appendix CD for

stage three test measurements)

3.1.6 Transients

Fig 15. Graph showing load, charger and input currents and system loading is changed

Fig 16. Transient of charger activation Fig 17. Transient of LED Driver activation

29

The transient currents seen when the charger or LED Driver are first turned on must be

delivered by the supply. Figure 19 shows a large peak current in the first 10 s as

predicted by simulation earlier on.

3.1.7 LED Driver General Operation

In the general operation of the LED driver the device dissipates a significant amount of

heat, if the device is used for any significant period of time forced cooling is required; an

internal temperature sensor should disable the device if excessive heating occurs.

The undervoltage lockout disables the LED driver when the supply voltage drops below

approximately 9.3V. Measurements also show the device is able to supply the 1A

current required by the LEDs, this can be reduced but results in visible flashing at the

lower levels. During an open circuit test the output voltage was measured at 14.2V.

3.1.8 Host Programming

Several different programming interfaces were tested in an attempt to program the

atmega406, this device does not have a usual ISP programming interface used by most

AVRs so a different method had to be found. The super pro 580 U programmer supplied

in the lab claims to be able to program the AVR with the use of a 48pin QFP to DIL

adapter. This was attempted but a correct device signature could not be read.

The STK500 has the ability for high voltage parallel programming, according to the

AVR datasheet this is a suitable programming interface; however certain start up

conditions need to be met and this is not described well by the datasheet, nor is it easy to

implement. With the need of over 20 connections to the chip, high voltage parallel

programming was not practical.

The AVR Dragon, a programmer with the JTAG interface was tested using the AVR

studio 4 computer interface from atmel, this proved successful in reading and writing to

the device. Using the programming interface the device ID was correctly read as 0x1E

0x95 0x07 (JTAG ID0x05950703F).

After some trial and error it was found that Vfet of the target device must be connected

to a voltage source of 5.3V minimum for programming to be successful.

30

3.2 Evaluation of Testing

Experiments 3.1.3 to 3.1.5 clearly show the three main stages of operation of this

system, albeit with the majority of control manually implemented. These experiments

clearly demonstrated a systematic control of the power follow in the system, further to

this experiment 3.1.5 shows how controlled discharge of the battery can supplement the

supply voltage and maximise system efficiency. With this technique of load reducing by

using the battery to supplying part of the power the power delivered by the supply is

maximised, this reduces the load on the storage facility and so maximises the overall life

of the system. In terms of non-dependable supply systems this shows significant

advantages compared to the common power management implementations.

In experiments 3.1.2 and 3.1.7 charge and discharge systems show a suitable level of

control and protection, current accurately measured and voltage limits effective.

The transient responses shown in 3.1.5 were expected but could create some problems in

the implementation of the power management control. A transient study of the whole

system would reveal the effects more precisely however, the implementation in this

project was only able to look at steady state responses; this is a clear design limitation of

the implementation tested. Despite this reasonable suggestions can be made for a fully

implemented control structure which can be tested for transient response. For example

the LED driver transient suggest that when the load is activated, rather than have the

control structure cycle through state 2 before reaching state 3 it may prove better to go

straight for state 3. i.e. when the load is activated assume the supply cant deliver enough

power, stop charging and discharge the battery into the load, from this the load can be

scaled up if needed.

Experiment 3.1.5 also raised the issue of current feeding back into the battery via the

original circuit arrangement, as this is uncontrolled it could be dangerous. A simple

schotty diode would prevent this problem.

The use of an ideal voltage regulated power supply with current limit in these

experiments is inaccurate; a real power supply would not exhibit this kind of

characteristic. Because part of this project wished to look at the effect of a non-

dependable power supply on the system such as a dynamo the accuracy of some

experiments is not 100%. Despite this the use of the power supply voltage and current

limits create an adequate representation at this stage, in future work a suitable supply

model is discussed.

Finally in experiment 3.1.8 an appropriate programming tool was found after a number

of attempts, the method of connecting the programmer correctly is also described.

Although it was not possible to use this device as a host controller in this project this

information should save significant time for any future work on this device.

31

4 Future Work

As discussed in the evaluation of testing the use of an idea voltage supply limits type of

tests that can be performed with accuracy; although this report did consider basic

transient conditions it was only able to accurately show the system operation in a steady

state. Further experimentation into the transients of the control system are desirable, this

could be achieved by using an actual dynamo with some form of controllable

mechanical input. A far more reliable and controllable method would be to use a non-

ideal power supply model. One such model is being developed by D. Spencer to

simulate solar arrays, this is a programmable device that can simulate a range of

characteristics by changing values such as the shunt or series resistance

characteristics.[28]

During this project system hosting was provided by the smbuscon2, a computer interface

device. This allowed experimentation of the general operation of the device but couldn‟t

implement an overall active control structure, thus some of the control was dealt with

manually to study the system capabilities. With the information contained in this report

it should be possible to program the atmega406 in a suitable was to implement system

hosting and control. The first step would be to ensure the device can host the SMBus

basic operations and then to implement a form of control to deal with a non-ideal

variable power supply.

So far this project has shown a very basic method of load scaling using charging and

discharging, in an attempt increase operational life and make maximum use of available

power, it did not however maintain the maximum power delivered from the supply.

Okunbo discuses the process of maximum power point tracking (MPPT) [5] by varying

the load as seen by a solar array, in his report a circuit for maintaining the output voltage

at 76% is introduced. Research into the application of MPPT for this power

management system may be able increase the effectiveness of the load scaling

functionality shown it this project. Using the programmable nature of the atmega406 it

would be possible to develop a MPPT system that can be customised for any power

source characteristic.

This project and research has shown the capacity for using currently available

microprocessors to build an entire power management system, implying the possibility

of a single chip design. Future work may include trying to use a single programmable

microprocessor to implement power management functionality.

32

5 Conclusion

This report presented the argument that a complete power management system for a

bicycle light can be constructed using commercially available microprocessors,

demonstrating a single chip solution is not only possible but it is also commercially

attractive.

To establish a commercial basis for the report a review of wide ranging power

management applications in modern electronic and electrical systems was performed.

This demonstrated the extent to which this type of system is being used in modern

electronics, highlighting some of the approaches and solutions to power problems. The

applications ranged from MEMS devices dealing with µW to hybrid vehicles dealing

with kW, clearly showing that power management is an important part of modern

electrical and electronic design.

The first step in designing the system was the selection of a suitable battery chemistry

and capacity, again a review of current products was performed including the most

popular cell chemistries; lead-acid, nickel metal hydride and Lithium polymer. The

selection of lithium polymer as the projects battery chemistry came from the power

density and capacity requirements of the system; as well as the realisation that most

portable battery powered devices use this chemistry, making this chemistry perfect in

terms of commercial attractiveness. Sizing requirements were assessed from information

obtained in the literature review, partly driven by cost considerations.

In terms of the design it was required that a full management system could be

demonstrated, showing controlled charge and discharge as minimal functionality. From

the test applied it was shown that controlled charge and discharge was made possible

using the system implementation of host, charger and LED driver. Tests demonstrated

the control of charge current and voltage regulation while the LED driver was able to

control the discharge current. Extra protection functionality was provided to prevent

deep-discharge ensure the system operates safely. A graph generated from

measurements made while charging the battery agree with the expected charging

characteristic. Although the project failed to implement the atmega406 as system host it

was still able to fulfil the project design goals and provide suitable background

information for the continuation of this project.

The final requirement of the project was to research the application of load sharing

between the battery and power supply to make efficient use of the available power. In

the literature review MPPT was discussed as a method for extracting the maximum

power from solar arrays. The design tried to achieve a similar result by load scaling,

reduction of charge current and supplementing the power source with the battery. Not

only did the design show this technique works well but also discussed the effect of

transient behaviour on this system. Future work could improve this area further.

In general the report achieved its goals and shows potential for future progress and

optimisation of the design.

33

6 Project organisation

In the run-up to the interim report the majority of progress involved the review of

literature. This involved understanding how a power management system operates and

assessing information on current products. This resulted in the sourcing of the Li-Po

battery pack and charger using the Max1647 in early December. At the same time the

LT3474 was ordered as a sample from linear technology.

The first prototype charge was constructed in mid December and programming of the

16F88 began to host the system and confirm the charger address. A sample of the

atmega406 was ordered so building of a proper host system could begin.

Early January the Interim report was written with an updated gaunt chart (see appendix

C). Following this, testing continued on the Max1647 charger, rapid advances were

made in the 16F88 routines to allow read and write commands, however the charger

inexplicably stopped working and did not respond to proven communication routines.

Immediately new components were ordered to build a new charge circuit, the Max1647

sample came in a pack of four so a replacement was already available.

While waiting for components system modelling was achieved, and LT3474 components

were calculated and ordered. At this point I discovered the atmega406 sample was lost in

the post so ordered another sample. By mid February the second prototype was being

built, using a totally new circuit and new microprocessor the charge failed.

The failure mode was the same as before and after email consultation with maxim it was

decided to order the Max8731A development kit and sumbuscon2. During this time the

LT3474 was built and tested. The atmega406 sample was taking to long so I ordered the

device from RS.

The sample and ordered atmega406 ironically arrived at the same time, also the

development kit arrived at the start of the Easter vacation, experiments on voltage and

current limits, charging characteristic and input current limit were completed as seen in

this report.

The final part of the project involved the programming of the atmega406, initially it was

attempted using the usbasp developed by Dr Tim forcer but the microprocessor was not

ISP compatible. During the absence of the project supervisor in the last week of the

Easter vacation the super Pro 580U was used, this device claims it supports the

atmega406 but was not able to read a correct device ID. It was only by the end of April

after several other programming attempts that it was possible to use the AVR Dragon to

confirm the atmega406 signature, indicating a correct programming interface. During

this time in April other experiments on the system were conducted to evaluate such

things as transient response.

Details of the project budget and final gaunt chart can be found in appendix C.

34

References

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[14] H. Patrick, “Electronic rear lamp for bicycle - uses storage capacitor to provide current

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8303307, 18/06/1984

[16] (2008) the Lupine website. [Online], Available at

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[18] System Management Bus (SMBus) Specification, Version 2.0, SBS Implementers

Forum, (08/2000)

[19] Comparing the I2C Bus to the SMBus, Application Note 476, Maxim, (12/2000)

[20] Smart Battery Data Specification, Revision 1.0, Release A, Smart Battery System

Implementers Forum, (Feb 1995)

[21] Atmel‟s ATmega406 AVR Microcontroller Provides Full Smart Battery and Battery

Protection Functionality for 2 – 4 Li-ion Cells in a Single Chip, White Paper, Atmel

Corporation.(03/2005)

[22] (2008) BRC hobbies website Lithium Battery Users Guide. [Online], Available at

www.brchobbies.com (link checked 02/05/08)

35

[23] SMBus Level 2 Battery Charger with Remote Sense (MAX8731A), Maxim, revision 0,

(01/2007)

[24]MAX8731A Evaluation Kit/Evaluation System, Maxim, Revision 1, (08/2007)

[25]ATmega406 preliminary, ATmel, (08/2006)

[26]Luxeon K2 emitter Datasheet DS51,Technical Datasheet, Luxeon, (05/2006)

[27] LT3474/LT3474-1Step Down 1A LED Driver, Revision C, Linear Technology,(2005)

[28] D. Spencer, “Design, Build and Test a Solar Array Simulator”, School of Electronics

and Computer Science, Uni of Southampton, Southampton, UK, Jan 2008

[29]1N5820 1N5821 1N5822 Axial Lead Rectifiers, Revision 9, On Semiconductor,(June

2006) [order number 1N5820/D]

[30] LT3474 design and simulation tool and software can be downloaded from, LT website

[online], Available at www.linear.com (link checked 04/05/08)