Final Report (Buet) - Ifec 2007

International Future Energy Challenge 2007

Final Report

Universal Adapting Battery

Charger

Submitted by

Bangladesh University of Engineering and Technology

Undergraduate Student Team

Faculty Advisor

Dr. A.B.M. Harun-Ur-Rashid

Development of a Universal Adapting Battery Charger

IFEC TEAM, BUET

Abstract

The objective of 2007 Future Energy Challenge is to develop a low cost universal battery

charger for Li-ion, SLA, Ni-Cd and Ni-MH batteries. The battery charger must be energy efficient and

comply with requirements regarding power factor, power loss and module size and shape. In order to

fulfill the objectives as specified by the IEEE committee, a number of schemes were considered. After

careful consideration, the best scheme was chosen and the project has been implemented according

to that scheme. This report discusses the developed circuits of various parts of the proposed scheme

and also the algorithm which is used to control the battery charging process. A hardware prototype

consisting several inputs, outputs and feedbacks is developed, which is able to supply a variable dc

voltage ranging from 2V to 26V, with a maximum charging current of 2A. The algorithm developed so

far is programmed into a micro-controller which automatically detects the connected battery chemistry

and configuration and controls the hardware to charge the given battery. This prototype has

succeeded to charge all kinds of batteries as per requirements of IFEC 2007.

IFEC 2007 Final Report Bangladesh University of Engineering and Technology

Page 1

CONTENTS

Pages

1. Introduction………………………………………………………………………………………… 05

2. Battery Information………………………………………………………………………………… 06

2.1. Sealed Lead Acid (SLA) battery……………………………………………………………… 06

2.1.1. Charging………………………………………………………………………………… 06

2.1.2. Discharging……………………………………………………………………………… 08

2.2. Lithium-ion (Li-ion) battery……………………………………………………………………… 08

2.2.1. Charging…………………………………………………………………………………… 08

2.2.2. Discharging……………………………………………………………………………… 09

2.3. Nickel Cadmium (NiCd) Battery………………………………………………………………. 10

2.3.1. Charging………………………………………………………………………………… 10

2.3.2. Discharging……………………………………………………………………………… 11

2.4. Nickel Metal Hydride (NiMH) Battery……………………………………………………… 11

2.4.1. Charging……………………………………………………………………………………… 11

2.4.2. Discharging…………………………………………………………………………………… 12

3. Scheme…………………………………………………………………………………………… 12

4. Switch Mode Power Supply………………………………………………………………………… 13

4.1. Component design………………………………………………………………………………… 17

4.1.1. Transformer Design………………………………………………………………………… 17

4.1.2. MOSFET Selection……………………………………………………………………… 18

4.1.3. Snubber Design……………………………………………………………………………… 18

4.1.4. Feedback Circuit………………………………………………………………………… 18

4.1.5. Microcontroller Selection………………………………………………………………… 19

4.2. Performance Analysis…………………………………………………………………………… 20

4.2.1. Losses………………………………………………………………………………………… 20

4.2.2. No Load Power…………………………………………………………………………… 20

4.3. Previous Approaches……………………………………………………………………………… 20

4.3.1. Performance Comparison Between Approaches………………………………………… 21

5. Charger Circuit…………………………………………………………………………………… 22

5.1. Circuit Description………………………………………………………………………… 23

5.1.1. Buck Converter………………………………………………………………………… 23

5.1.1.1. MOSFET Switching Driver in Buck Converter………………………………. 24

5.1.2. Battery Presence Detection and Polarity Sensing Circuit………………………… 25

5.1.3. H-bridge…………………………………………………………………………… 26

5.1.4. Discharger………………………………………………………………………… 27

5.1.5. Logic Control……………………………………………………………………… 28

5.2. Performance Analysis……………………………………………………………… 29

5.2.1. Proper Charging……………………………………………………………… … 29


Page 2

Pages

5.2.2. Power Calculation……………………………………………………………… ….. 32

6. Efficiency………………………………………………………………………………………… 32

7. Method of Battery detection…………………………………………………………………… … 33

7.1. Constant current Charging at 500mA for Approximately 10 Minutes………………… … 33

7.2. Discharging the Battery…………………………………………………………………… 34

8. Working Algorithm……………………………………………………………………………… 39

9. Previous Approaches……………………………………………………………………………. 41

9.1. Chemistry Detection………………………………………………………………………… 41

9.1.1. From β of a Battery………………………………………………………………… 41

9.1.2. SoC Method………………………………………………………………………… 42

9.2. Capacity Measurement…………………………………………………………………… 43

10. Key Innovations………………………………………………………………………………… 43

10.1. Data Logging Hardware…………………………………………………………………… 44

10.2. Data logging Software……………………………………………………………………… 44

11. Cost Analysis……………………………………………………………………………………… 45

12. Summary of Work Done………………………………………………………………………… 46

13. Project Timeline………………………………………………………………………………… 47

14. Project Budget…………………………………………………………………………………… 47

15. Conclusion…………………………………………………………………………………………. 47

16. Team Information………………………………………………………………………………….. 48

17. References………………………………………………………………………………………… 48


Page 3

LIST OF FIGURES

Pages

Figure 2.1: Typical charging profile of a Sealed Lead Acid battery……………………………… 07

Figure 2.2: Typical charging profile of a lithium-ion battery……………………………………… 09

Figure 2.3: Typical charging profile of Nickel-Cadmium batteries………………………… 10

Figure 2.4: Typical charging profile of Nickel-Metal Hydride batteries…………………… ............. 11

Figure 3.1: Functional block diagram of the developed scheme for the charger…………………… 12

Figure 4.1: (a) Functional block diagram and (b) Photograph of Switch Mode Power Supply……… 14

Figure 4.2: Circuit diagram of the Switch Mode Power Supply……………………………………… 16

Figure 4.3: Circuit diagram of SMPS built using NCP1651……………………………………… 21

Figure 5.1: (a) Functional block diagram and (b) Photograph of the charger circuit………………… 22

Figure 5.2: Buck converter with driver circuit………………………………………………………… 24

Figure 5.3: Functional block diagram of battery presence detection and polarity

sensing circuit with H- bridge……………………………………………………………… 25

Figure 5.4: Circuit diagram of battery presence detection and polarity sensing

circuit with H-bridge………………………………………………………………………… 26

Figure 5.5: Circuit diagram of the discharger circuit…………………………………………………… 27

Figure 5.6: Functional diagram of the microcontroller section………………………………………… . 28

Figure 5.7: Charging profile of a 3200mAh, 6V Sealed Lead Acid battery………………………… 30

Figure 5.8: Charging profile of a 700mAh 3.7V Li-ion battery……………………………………… 30

Figure 5.9: Charging profile of 600mAh 4.8V NiCd battery………………………………………… 31

Figure 5.10: Charging profile of 2300mAh 4.8V NiMH battery……………………………………… 31

Figure 6.1: Plot of efficiency of the charger circuit and estimated efficiency of the device

at 1A constant output current…………………………………………………………… 32

Figure 7.1: Discharging Characteristics of a 650mAh Li-ion single cell battery………………….. 35

Figure 7.1: Discharging characteristics of a 900mAh Li-ion single cell battery…………………….. 35

Figure 7.2: Discharge characteristics of a 700mAh NiCd two-cell battery…………………………… 36

Figure 7.3: Discharge characteristics of a 4600mAh NiMH two-cell battery……………………… . 36

Figure 7.4: Discharge characteristics of a 3200mAh SLA 3-cell battery………………………… 37

Figure 7.6: Discharge characteristics of a 4500mAh SLA 3-cell battery…………………………… 37

Figure 9.1: beta of the batteries of different chemistries……………………………………………… 41

Figure 7.2: Characteristic curve of different batteries in SOC method……………………………… 42

Figure 10.1: Block Diagram of Data logging hardware………………………………………………… 44

Figure 10.2: Screenshot of BUET IFEC 2007 TEAM of data logging software…………………… 44

Figure 13.1. Project timeline……………………………………………………………………………… 47


Page 4

LIST OF TABLES

Pages

Table 4.1: Table of estimated losses in SMPS………………………………………………………… 19

Table 4.2: Measured no load power in SMPS……………………………………………………… 19

Table 5.1. Battery charging process of SLA and Li-ion……………………………………………….. 29

Table 5.2. Battery charging process of NiMH and NiCd…………………………………………… 29

Table 5.3: Power loss in charger circuit when supplying 1A at 22V…………………………………. 32

Table 7.5: Table of tested batteries…………………………………………………………………… 33

Table 7.2: Table of decision in battery chemistry detection…………………………………………… 38

Figure 11.1: Cost estimation of the device……………………………………………………………… 45

Table 12.1: Summary of the achievements…………………………………………………………… 46


Page 5

1 Introduction

The portable battery is still considered one of the most important and reliable source for

portable energies. Though the nature of battery applications has changed over the years, new

technologies have ensured that batteries remain a reliable and efficient source for general purposes.

The rechargeable battery is one such development that allows the user to make use of a single

battery over and over again by simply recharging it when the battery charge runs out. These batteries

have saved the user from the hassle of acquiring a stock of batteries for continuous use, since a

single battery may be used repeatedly by simply recharging it after its capacity is finished. This has

decreased the necessity to use the one-time use batteries which are generally discarded after one

single use.

The mass usage of these rechargeable batteries in different applications means that there are

batteries of different types and capacities with varying recharging techniques. It is therefore

convenient for the user if there was one single charger to recharge every kind of battery regardless of

their capacity and type. Hence, the idea of the universal battery charger has come into the fore. This

report discusses the development of a universal adaptive battery charger that will be able to recharge

four different types of batteries of different quantities and capacities. The IFEC Challenge 2007

specifies the universal battery charger that will be able to recharge SLA, Li-ion, Ni-MH and Ni-Cd

batteries up to 24 Voltage and a maximum charging current of 2A. The universal battery charger

developed must also conform to several other stipulations regarding power consumption, size and

economic feasibility. All these requirements were taken into consideration when designing the battery

charger.

The universal battery charger designed by the BUET IFEC 2007 team meets most of the

primary requirements set by the IFEC Committee and fulfills a few of the secondary features. It takes

power from an A.C. supply which may vary from 95-270V rms. It converts it into a 35V fixed DC with a

flyback converter which was implemented by a microcontroller. The maximum output current is

regulated to 2A. This fixed DC voltage is controlled by another microcontroller to adapt to different

voltages of batteries. Some key aspects of the battery charger are mentioned below:

Auto-delectability of connected battery chemistry and configuration.

Charging range from 2-26V battery voltages.


Page 6

Supports up to 2A charging current.

Cost efficient and portable sized design.

We have been able to add a few extra features of our own:

This circuit may be used as a 0-25V variable DC voltage source with 2A current limiting

capability.

A keypad has been attached by which advanced users may provide battery information for

quicker and more efficient recharging.

Advanced control is enabled by implementation of an LCD-keypad.

2 Battery Information

Several types of rechargeable batteries has been developed till today. These batteries work

on different chemistries and so they show different characteristics. Some batteries can provide very

high load current, whereas some cannot take the strain of high discharge. Batteries like nickel-based

ones show almost constant voltage profile regardless of their state of charge, where Lithium based

batteries have varying voltage according to their state of charge.

The IFEC committee has selected four types of rechargeable battery chemistry for this

project. They are:

1. Sealed Lead Acid

2. Lithium-ion

3. Nickel-Cadmium

4. Nickel-Metal-Hydride

These batteries require different charging procedures and also show different discharging

profiles. Brief descriptions of them are given below:

2.1 Sealed Lead Acid (SLA) battery

2.1.1 Charging

The charging procedure of SLA contains two main stages and an additional third stage. The

stages are as follows:


Page 7

i. The first stage applies a ‘constant current charge’, raising the cell voltage to a preset

voltage. After this stage, the battery is charged to about 70%.

ii. The second charge is ‘topping charge’ state. In this state the current is reduced gradually

by applying the preset ‘constant voltage charge’ as the cell is being saturated. Full charge

is attained the current has dropped to 3% of the rated current or has leveled off.

iii. The additional third stage, the ‘float charge’ state, is applied to compensate for the self

discharge. Correct settings of the voltage limits are critical and range from 2.30V to 2.45V.

The voltage and current profile of a typical SLA battery charging are given in figure 2.1.

Some important factors regarding this charging should be taken in consideration:

A high voltage limit (above 2.40V per cell) produces good battery performance but shortens

the service life due to permanent grid corrosion on the positive plate.

A low voltage limit (below 2.40V per cell) is safe if charged at a higher temperature but the

cell is subject to sulfation on the negative plate.

The charging current should be set between 10% and 30% of the rated capacity.

Figure 2.1: Typical charging profile of a Sealed Lead Acid battery.

Voltage/cell

Charge/current

0.4

1.6

1.2

0.8

Stage 2 Constant voltage. Constant 2.4V charge

Stage 1 Constant Current

Stage 3 Float Charge (2.25 V)

2.5

2.0

1.5

1.0

0.5

3 6 9 12

Cu

rren

t (A

)

Vo

ltag

e (

V)

Time (hrs)

2


Page 8

2.1.2 Discharging

Most batteries are rated at 5-hour discharge or 0.2C; some may be even rated at slow 20-

hour discharge.

Deep discharging is discouraged as it shortens battery life.

Performs well on high pulse currents when discharge rates well in excess of 1C can be

drawn.

The battery can be discharged to a minimum 1.75V per cell, but it is discouraged to push the

voltage below 2.1V.

2.2 Lithium-ion (Li-ion) Battery

2.2.1 Charging

Like SLAs, lithium-ion batteries require a charging method containing three stages- two main,

one additional.

Stage 1 of the charging uses a constant current until 4.2V per cell is achieved. The charge

level at this point is about 70%.

Stage 2 maintains a constant voltage while the charging current is gradually reduced. Full

charge is attained after the voltage has reached the threshold and the current has dropped to

3% of the rated current or has leveled off.

Occasionally a topping charge is used, but in most cases it is omitted to guard against

overcharging.

The voltage and current profile of a typical li-ion battery charging are given in figure 2.2.

Lithium-ion batteries require special attentions in some factors:

Overcharging is disastrous in case of Li ion batteries. If charged above 4.30V, the cell causes

plating of metallic lithium on the anode; the cathode material becomes an oxidizing agent,

loses stability and releases oxygen. Eventually the cell is heated up, and if left unattended,

the cell could vent with flame.


Page 9

A standard li-ion battery pack contains a protection circuit built in it. This limits the peak

voltage of each cell during charge and prevents the cell voltage from dropping too low on

discharge.

The battery should be partially charged during storage.

Most cells are charged to 4.20 volts with a tolerance of 0.05V per cell. Charging only to

4.10V reduces the capacity by 10% but provides a longer service life.

The maximum charge current on most packs are is limited to between 1C and 2C.

2.2.2 Discharging

Maximum discharging current in most cells is limited to 1C or 2C.

Should not be discharged below 2.5V per cell.

If the cells have dwelled at 1.5V per cell and lower for a few days, recharge should be

avoided.

Time (hrs)

Figure 2.2: Typical charging profile of a lithium-ion battery.

Charge current

Voltage/cell

2 1 3

Stage 1 Max. charge current is applied until the cell voltage limit is reached.

Stage 2 Max. cell voltage is reached. Charge current starts to drop as full charge is approached.

0.25/1

0.5/2

0.75/3

1.25/5

1.00/4

Cu

rren

t/V

olt

ag

e

(A/V

)

Terminate charge when current < 3% of rated current

.…….…

.…….…


Page 10

2.3 Nickel-Cadmium (NiCd) Battery

2.3.1 Charging

Nickel-Cadmium requires constant current charging unlike SLA and Li-ion batteries. The

voltage profile of a typical NiCd battery charging are given in figure 2.3.

Factors to be considered are:

Prefers fast charge to slow charge (as opposed to other chemistries) and pulse charge to DC

charge.

Overall charge efficiency is about 90% if fast charged at 1C. On a 0.1C overnight charge, the

efficiency drops to about 70%.

Interspersing discharge pulses between charge pulses (commonly referred to as burp or

reverse load charging) improves the charge acceptance.

Full-charge can be detected observing a voltage drop at full charge (negative delta V) after

reaching a peak voltage ranged between 1.35V and 1.55V per cell.

Figure 2.3: Typical charging profile of Nickel-Cadmium batteries.

0 50 100

1.30

1.34

1.38

1.42

1.46

1.50

State of Charge (%)

Vo

ltag

e (

V)


Page 11

2.3.2 Discharging

Allows deep discharging to the contrary of other battery chemistries.

Suffers from memory effect; if not periodically full discharged (once a month), large crystals

form on the cell plates reducing performance.

Relatively high self discharge.

2.4 Nickel-Metal Hydride (NiMH) Battery

2.4.1 Charging

Like NiCd batteries, NiMH batteries require constant current charging. They also show almost

same charging voltage profile as NiCd batteries. The voltage profile of a typical NiCd battery charging

are given in figure 2.4.

Factors to keep in mind are:

NiMH battery prefers fast charge to slow charge.

At a C-rate of 0.1-0.3C, the voltage profile fail to exhibit defined characteristics to measure the

full charge state accurately and full charge detection becomes difficult if not impossible.

Figure 2.4: Typical charging profile of Nickel-Metal Hydride batteries.

0 50 100

1.30

1.34

1.38

1.42

1.46

1.50

State of Charge (%)

Vo

ltag

e (

V)


Page 12

Trickle charge settings are critical to avoid overcharge.

Produces a very small voltage drop (8mV-16mV) at full charge.

Making the charger too sensitive may terminate the fast charge halfway through the charge

due to voltage fluctuations and electrical noise. Full charge detection is possible by observing

negative delta V after reaching a peak voltage ranging from 1.35-1.5V per cell.

The battery is less prone to memory.

NiMH battery cannot absorb overcharge.

2.4.2 Discharging

Cycling under heavy load reduces battery life.

NiMH battery requires full discharge once every three months to prevent memory effect.

3 Scheme

The scheme for “Universal Adapting Battery Charger” consists of both hardware and software

portions. The functional block diagram for the hardware potion of the charger is given in figure 3.1.

EMI

FILTER

MICROCONTROLLER

ATMega32

~

SWITCH

MODE

POWER

SUPPLY

(WITH

FLYBACK

CONVERTER)

PULSE WIDTH

MODULATED

BUCK

REGULATOR

AND

ASSOCIATED

DRIVE

CIRCUITRY

H-BRIDGE INVERTER

WITH CHARGER-

DISCHARGER SELECT

SWITCH

DISCHARGER

Charger-Discharger

Select

Command

AC Power

Supply

Isolated-Ground

35V Dc

Battery to be charged

Polarity

Setup

Command

Presence

and

Polarity

FeedbackPWM

Voltage

Feedback

Current

Feedback

Discharger

Pulse

+ -

Figure 3.1: Functional block diagram of the developed scheme for the charger device.


Page 13

The device mainly comprises of two part:

a. Switch Mode Power Supply (SMPS),

b. Charger circuit.

The high voltage AC is given input to the SMPS through an EMI filter and rectified to provide

a pulsating DC voltage for the flyback converter to generate a regulated 35V DC for the charger. This

converter has another purpose. It is used for power factor control. The converter has a over voltage,

under voltage and over current protection by means of the feedback circuit, which also has an error

amplifier for regulating the DC output voltage.

The 35V DC output from the Flyback converter is used to drive the charger circuit. This is

done by the DC-DC Buck converter.

The microcontroller controls the output voltage of the Buck converter and varies the duty

cycle according to the measured voltage and current of the battery as per requirement of the battery

chemistry. The Analog to Digital Converters (ADCs) integrated in the microcontroller senses the

voltage and charging current.

A discharger circuit is built in order to allow pulse discharge of the battery to generate a

discharging profile. Also, the pulse discharge between charge pulses is good for NiCd batteries and

not harmful for other chemistries.

An algorithm based on both charging and discharging characteristics to detect the battery

chemistry, no. of cells and capacity is programmed into the microcontroller. The algorithm uses any

one of the above profiles which comes first to give a certain decision about the battery.

Initially the charger starts to charge all the batteries in constant current (CC) mode, which is

common for all battery types using 500mA charging current. Once the microcontroller identifies the

battery properly, it is charged accordingly.

4 Switch Mode Power Supply

The SMPS converts the ac line voltage into a dc voltage which is to be used by the charger.

Th SMPS serves a dual purpose- generation of a regulated low voltage DC from the high voltage AC

input .


Page 14

As the SMPS based on the NCP1651 did not satisfy one of the primary requirements- support

for 48Hz to 440Hz ac line frequency, the controller IC was replaced by a microcontoller ATmega8 by

Atmel, with startup circuitry. This new design is slightly modified version of the converter based on

NCP1651 described in previous report. Almost all the circuit required for the NCP1651 was removed,

except the ac line voltage divider and the current sensing resistance. Every other things like- high

frequency transformer, feedback circuits, snubber circuits remain same, as they would be required in

any controller. The functional block diagram of the SMPS is given in figure 4.1.

(a)

Figure 4.1: (a) Functional block diagram and (b) Photogrph of Switch Mode Power Supply

EMI

FILTER

BRIDGE

RECTIFIER

FLYBACK

CONVERTER

STARTUP ATMega8

RECTIFIER

AND FILTER

OPTOCOUPLER

AUXILIARY

~VOLATAGE

SENSE

PWM

FEEDBACK CIRCUIT WITH

OV, UV AND OC

PROTECTION WITH

ERROR AMPLIFIER

MAIN (35VDC)

TO

CHARGER

PR

IMA

RY SE

CO

ND

AR

Y

AU

X 1

AU

X 2

HIGH

VOLTAGE

SIDE

LOW

VOLTAGE

SIDE

LEGEND

OV = Overvoltage

UV = Undervoltage

OC = Overcurrent


Page 15

The AC line voltage is passed through the EMI filter and the bridge rectifier converts the AC

voltage into a pulsating DC voltage. This voltage is converted to a lower voltage by a voltage divider

and one of the ADC channels is used to measure this voltage. The duty cycle of the converter is

changed in such a way that when the pulsed DC voltage peaks, the duty cycle is minimum and when

the pulsed DC voltage is minimum the duty cycle is maximum. In this way a constant voltage is

achieved at the output. However this voltage sensing is not enough to produce a regulated DC

voltage at the converter output. To obtain a regulated DC output a feedback circuit with over voltage,

under voltage and over current protection is used. There is also an error amplifier in this feedback

circuit, which is used in regulating the output voltage. And optocoupler is used for interface the two

isolated circuits.

In the high voltage side, there is also a startup circuit, which is used to provide the

microcontroller power during a brief period. After the microcontroller has started it can power itself

through the auxiliary winding. The circuit is designed in such a way that the startup circuit will be

inactive after the microcontroller gets power from the auxiliary winding.

For the microcontroller and other ICs in the low voltage side another auxilliary windng is used

to generate a lower voltage, which is then regulated by linear regulators.

The complete circuit diagram of the converter is given in figure 4.2.


Page 16

C19

1n

12

CHOKE

L

1324

- +

D1

BRIDGE2

1

3

4

U4

7805/TOVIN

1V

OU

T3

GND2

J2

1A FUSE

12

U5

7812/TOVIN

1V

OU

T3

GND2

D12

1N4748

J4

XF

12345

109876

J1

AC

12

C1

.68u

1 2

C3

.68u

1 2

R10

10k

2

1

R1

56k

21

R2

56k

21

R3

1.8k

21

R4

56k

21

R5

56k

21

D2

12V

Q1

BUT11AF

D11

MUR160

C4

100u

C15

100u

D3

22V

D13

1N4748

C5

100u

C6

470u

C18

470u

C16

100u

C17

470u

C7

470p

12

R6

82k

21

R7

82k

21

D5

MUR160

M1

2SK1358

R8

.1

21

D4

1N4734

C10

18p1

2

C11

1n

1

2

C9

18p1

2

PROGRAMMER

J3

1 2 3 4 5 6

X1

16MEG

12

J5

OUT

123

D6

MUR160

C2

.68u

1 2R9

1k

2

1

C8

1n

1 2

U2

ATMega8

1234567891011121314

19 20 21 22 23 24 25 26 27 2815 16 17 18

D7

FCU10B60

C12

1500u

U3A

LM324/ON

+3

-2

V+4

V-11

OU

T1

U3B

LM324/ON

+5

-6

V+4

V-11

OU

T7

U3C

LM324/ON

+10

-9

V+4

V-11

OU

T8

U3D

LM324/ON

+12

-13

V+4

V-11

OU

T14

R11

10k

2 1

D8

D1N4148

D9

D1N4148

R12

10k

2 1

R13

220

2 1

R14

220

2 1

R15

1k

2 1

R20

5.6k

2

1

D10

D1N4148

R21

3.9k

2

1

C13

1u

1

2

U6

4N35

R16

2.2k

2 1

R17

0.1

2

1

R18

220

2

1

R22

3.3k

2

1

C14

100n

1

2

R23

10k

2 1

R24

3.3k

2 1

U1

78XX/TOVIN

1V

OU

T3

GND2

Figure 4.2: Circuit diagram of the Switch Mode Power Supply


Page 17

4.1 Component Design

4.1.1 Transformer Design

For the transformer, a core which is widely available in local market was selected. The turns

were calculated in the following way-

V = Maximum primary voltage = 375V

f= Switching frequency = 100kHz

a= Core cross sectional area = .11in2

B= Flux density in the core = 1Kgauss

N= Number of turns

For safety, turns ratio is increased to 125.

Using the following formula secondary turns were calculated-

Np = Primary turns = 125

Ns = Secondary turns

Vp = Primary minimum voltage = 134.3

Vs = Secondary voltage = 35

For safety, secondary turns were increased to 35.

Similarly auxiliary windings were calculated to have 20 turns. But as they are far from the

center of the core, they were also increased to 35 turns to provide the necessary voltage.

The wires were selected as follows-

Primary - 28 SWG

Secondary - 26 SWG

Auxiliary – 34 SWG


Page 18

4.1.2 MOSFET Selection

Vpeak= Maximum voltage across the MOSFET

V0 = Output voltage = 35V

n = transformers turns ratio = 3.6:1

Vspike = Voltage spike due to transformer leakage inductance = 100V (assumed)

For safety, 2SK1358 was selected, which has a maximum drain to source voltage of 800V.

Besides as a microcontroller is used to drive the MOSFET, the threshold voltage of the MOSFET

must be less than 5V. 2SK1358 also fulfills this requirement.

4.1.3 Snubber Design

For snubber circuit design, the following parameteres are required-

The snubber circuit consists of a RC network. The C is calculated-

And R is calculated by

For safery, C was selected 470pF and R was selected two 82k resistance in parallel.

4.1.4 Feedback circuit:

The feedback circuit consists of an optocoupler and four op-amps. The optocoupler is used

for isolation. The LED of the optocoupler is connected to the output of the op-amps and the transistor

is connected in series with a resistance in such a way that the voltage across the transistor is

inversely proportional to the current through the LED. And in the microcontroller it is programmed that

the duty cycle is proportional to the voltage across the transistor. So finally the duty cycle is inversely

proportional to the current through the LED.


Page 19

The first op-amp is used as a under voltage comparator. Until the output voltage is above

34V, output voltage of this comparator is low; current through the LED is zero, causing the duty cycle

to be maximum.

The second op-amp is used as an error amplifier. It is basically an inverting integrator. The

output voltage of this op-amp depends upon how much the output voltage deviates from 35V. If the

voltage is above 35V then the voltage at the output of this op-amp increases, causing less current to

flow through the optocoupler diode and the duty cycle to decrease, which in turn reduces the output

voltage. If the output voltage is below 35V, then the output of this op-amp decreases, causing more

current to flow through the optocoupler diode and the duty cycle increases, generating more voltage.

The third op-amp is used as an over voltage comparator. If the output voltage is over 36V,

then the output of this op-amp becomes high, decreasing the current through the optocoupler diode to

zero. This causes the duty cycle to be minimum and the output voltage is decreased.

The fourth op-amp is used as an differential amplifier. If the output current becomes than 2.5A

then the output of this op-amp goes high, causing an artificial overvoltage situation by increasing the

voltage at the under voltage comparator‟s non-inverting terminals. This would cause the duty cycle to

be minimum.

By using this circuit, it becomes possible to maintain a regulated 35V DC at the output of the

flyback converter.

4.1.5 Microcontroller selection:

The microcontroller selected fo this converter is Atmega8. This microcontroller is selected for

its low price, smaller size, ADCs and high speed. To use its maximum potential a 16MHz crystal is

used. The 16 bit timer was used for generating the gate pulses for the MOSFET.


Page 20

4.2 Performance Analysis

4.2.1 Losses:

The estimated losses in loaded condition in the SMPS portion of the device are given in table

4.1.

Input rectifier 1W

MOSFET 4.8W

Output Rectifier .8W

Transformer 6W (estimated)

Snubber .4W

Miscellaneous 1W

Total 14W

Table 4.1: Table of estimated losses in SMPS

Calculated Efficiency = 60/(60+14) = 81%

4.2.2 No load power:

The measured no load power of the SMPS based on ATMega8 is given in table 4.2.

AC line Voltage No load power

95V 0.3W

110V 0.4W

220V 0.8W

Table 4.2: Measured no load power in SMPS

4.3 Previous approaches:

The SMPS was initially built using the flyback converter with power factor control IC NCP1651

by On Semiconductors. The converter gave a stable 31V with a maximum current of 2.5A. The circuit

diagram is given in figure 4.3.


Page 21

The output voltage of the SMPS was later increased to 35V due to the reason that the charger

prototype needs about 32V to give a output charging voltage of 25.2V, which is required for charging

6 Li-ion cells.

Though the cirucit was quite satisfactory in performance, this circuit does not fulfill the

frequency range support requirement (48-440Hz).

4.3.1 Performance Comparison between the approaches:

As the SMPS was burnt due to an accidental short circuit during the last test with the charger

circuit, real data cannot be provided except the no load power. Efficiency and power factor could not

be measured. However, from the simulations and calculations it may be said that the SMPS based on

the microcontroller would be about 80% efficient, but power factor may not be controlled much. Where

as, the circuit based on NCP1651 was more efficient and also it had very good power factor. So it can

be said that, based on the application, where higher frequency AC source (>60Hz) is required, the

ATmega8 based SMPS should be used and where normal AC source (50/60Hz) is present, NCP1651

based SMPS should be used.

Figure 4.3: Circuit diagram of SMPS built using NCP1651


Page 22

5 Chager Circuit

The functional block diagram of the prototype charger is given in Figure 5.1.

35V DC

OUTPUT

DC-DC BUCK

CONVERTER

DRIVER CIRCUIT FOR

MOSFET IN BUCK

CONVERTER

H-BRIDGE

INVERTER

BATTERY

MICROCONTROLLER

KEYPAD LCD UART

DISCHARGER

CURRENT

FEEDBACKVOLTAGE

FEEDBACK

BATTERY

PRESENCE

DETECTION AND

POLARITY

SENSING

SELECTOR

SWITCH

PO

LA

RIT

Y

SE

TU

P

DIS

CH

AR

GE

R

PU

LS

E

PW

M

(a)

(b)

Figure 5.1: (a) Functional block diagram and (b) Photograph of the charger circuit


Page 23

The flyback converter supplies a stable 35V DC to the charger circuit which is used to charge

SLA, Li ion, NiCd and NiMH batteries as per chemistry.

The charger is in standby mode initially. If no battery is connected and the prototype is turned

on, a menu appears on the LCD screen to ask for whether the prototype will be used as a power

supply or a battery charger. If a battery is connected and the prototype is turned on, the prototype wait

for 30 seconds and if no key is pressed, it automatically goes to the adaptive-charging mode.

When a battery is connected, the battery presence detection and polarity sensing circuit

automatically detects the presence of the battery and senses its polarity and then sends data signal to

the microcontroller. Based on these signal data, microcontroller configures the h-bridge so that the

positive terminal of the battery is connected to the positive output of the Buck converter.

In the adaptive-charging mode, the charger starts the sequential process of constant current

charging at 500mA of 10mmins and discharging. In this process, the algorithm programmed in the

microcontroller observes some specific parameters in the charging and discharging voltage

characteristics via the ADCs built in the microcontroller and decides the battery chemistry and

according number of cells. After that, the charger charges the batteries constant current mode at

500mA or constant voltage mode as required for the detected chemistry.

5.1 Circuit description

5.1.1 Buck Converter

The Buck DC-DC voltage converter used in the charger is a simple asynchronous Buck DC-

DC converter. The input voltage is 35V. The output voltage is ranged 2-26V. The filter capacitor and

inductor are calculated for 1mV p-p ripple at the output. The maximum current is 2A. The circuit

diagram of this converter is shown in figure 5.2, which includes the drive circuit.


Page 24

M1 IRF540N

L1

3.9mH

1 2

Q2

BD244Q3

BD244

+C7

1.5m

12GND

BUCK_OUTPUT

D2D1N4148

D3

D1N4148

Q1

Q2N3904

D4

MBR2045CT

R11k

R210k

R310

R4

1k

R51k

R610.

+C6

4.7u

12

+12 VDC

MAIN

PWM C5

100n

1

2

Figure 5.2: Buck converter with driver circuit.

The MOSFET is driven by the driver circuitry. Switching frequency used is 20 kHz. This

frequency was selected specifically so that it would not generate any sound from the mechanical

vibration of the inductor, which was previously observed using lower frequency.

5.1.1.1 MOSFET switching driver in Buck converter

The switching device used here is an N-channel MOSFET, IRF540. It is selected for its low

cost, low on time resistance (<77 milliohms), high current capability (max 23A) and high durability.

The PWM signal from the microcontroller is a digital logic signal of 0-5V which is not sufficient to drive

the MOSFET as its source pin is not in the circuit ground. So a drive circuit is used which takes the

logic level signal as input and produces another signal with higher voltage level of about 0-40V,

without changing the duty cycle of the signal. The driver circuit with the Buck converter is given in

figure 5.2.

In the circuit the signal from the microcontroller is given to a buffer IC. The output of the buffer

IC is connected to the base of the switching transistor Q1. In the positive portion of the PWM signal,

Q1 is turned on and the capacitor C1 is charged to 35V through the diode D1. As D1 is forward

biased, transistor Q2 is turned off. So MOSFET M1 is turned off as there is no voltage at its gate. In

the negative portion of the signal, Q1 is off and the voltage across C1 is 42V resulting the D1 reverse

biased and Q2 turned on. This 42V is passed to the gate of M1 through diode D2 and 10 ohm

resistance and the MOSFET is turned on.


Page 25

5.1.2 Battery presence detection and polarity sensing

To detect the presence of the battery and change the H-bridge to appropriate connections a

battery presence detection and polarity sensing circuit is used. Functional block diagram as well as

the circuit diagram of the battery presence detection and polarity sensing circuit along with the H-

bridge is given in figures 5.3 and 5.4 respectively. It consists of two optocouplers. The H-bridge is

connected in such a way that the default polarity is BATT1 positive and BATT2 negative. If the battery

is connected in this way, then the presence of the battery is detected by the high voltage generated at

the output of the optocoupler 1. When the battery is connected in the other way, then the presence of

the battery is detected by the high voltage generated at the output of the optocoupler 2. The outputs

of both optocouplers are logically Ored and connected to the active low reset circuit of the

microcontroller. So whenever a battery is connected to the charger, the battery presence is detected

and the microcontroller is turned on, regardless of the polarity.

BUCK OUTPUT

BATTERY

PRESENCE AND

POLARITY

SENSE

CHARGER DISCHARGER

RELAY 3

RELAY 1

RELAY 2

PULSE LOAD

BY MOSFET

SWITCHING

+ -

Figure 5.3: functional block diagram of battery presence detection and polarity sensing circuit

with H-bridge.


Page 26

D6

D1N4148

D7

D1N4148

M2

IRF540ND8

D1N4148

OPTO_1

OPTO_2

R111k

R121k

BATT_1

RELAY_3GND

GND

BUCK_OUTPUT D5MBR2045CT

R140.1

I_SENSE

R13

100k

GND

U84N35

CHARGER_ENABLE

RELAY_2

GND

RELAY_1

R10

100k

U74N35

BATT_2

+5 V_H-BRIDGE_&_LOGIC

DISCHARGER_IN

GND

C8

100n12

Q4

Q2N3904

Q5

Q2N3904

Q6

Q2N3904

R7

1k

R8

1k

R9

1k

U4

RELAY_SPDT_3

354

12

U5RELAY_SPDT_1

35

412

U6

RELAY_SPDT_2

35

412

Figure 5.4: Circuit diagram of battery presence detection and polarity sensing circuit

with H-bridge.

As the output of the optocoupler 2 is connected to the H-bridge, when the battery is

connected in the opposite polarity, the H-bridge connections are altered. Thus the charger becomes

polarity insensitive.

5.1.3 H-Bridge

One of the primary requirements of this charger is to be polarity insensitive. For this purpose

an H-Bridge IC L298 was used at first. The IC was selected for high current rating (4 A), low

saturation voltage, over temperature protection and high noise immunity. This IC can also handle the

back EMF of motors, which is similar to battery charging application. The IC contains two individual

modules, which were used in parallel to reduce power loss. The battery to be charged was connected

to the output terminals of the H-bridge.

But it was found that this IC consumes about 6 watts power at all output voltages which

degrades the power efficiency of the prototype greatly. So, after considering some alternatives, it was

decided to construct the H-bridge with two SPDT relays. The block diagram of this H-bridge is shown

in Figure 5.3 along with the battery presence detection and polarity sensing circuit.


Page 27

The relay 1 acts as the source and the relay 2 acts as the sink. The common terminal of the

relay 1 is connected to the output of the Buck converter, the normally closed (NC) terminal to the

BATT1 and the normally open (NO) terminal to BATT2. The common terminal of the relay 2 is

connected to the drain of the Charge Enable MOSFET, NC terminal to the BATT2 and NO terminal to

the BATT1.

The assumed default polarity of the connected battery is BATT1 positive and BATT2

negative. If the battery is connected inversely, then the detection circuit would give a high voltage to

the base of the relay driving transistors causing the relays to be tripped. In this way the positive

terminal of the charger is always connected to the positive terminal of the battery and the negative

terminal of the battery is always connected to the negative terminal of the charger.

5.1.4 Discharger:

In order to detect the battery chemistry, number of cell and capacity, sometimes a discharging

profile is needed. So, a discharger is integrated in the charger prototype which uses discharge pulse

to discharge the batteries at 400mA.

The functional block diagram of the discharger integrated with the charger prototype is shown

in figure 5.3.

I nitially Relay 3 is has its NC contact connected to the presence and polarity of battery

detection circuit. If the algorithm finds it necessary, Relay 3 is tripped so that NO its contact becomes

closed, the charging ciruit is disconnected and discharger is turned on.

R17

1k

Q7

Q2N3904

GND

DISCHARGER_PULSEDISCHARGER_IN

R15

1.5

M3IRF540N

R16

1k

Figure 5.5: Circuit diagram of the discharger circuit

As shown in figure 5.5, with the discharge in is high, the microcontroller sends discharge

pulses. When the pulse is high, the MOSFET is turned on and the battery starts to discharge through


Page 28

1.5 ohm resistance. As the battery starts to discharge in a higher rate, the base voltage of the BJT

increases, causing it to lower its collector to emitter voltage, which is also the gate voltage of the

MOSFET. As a result of the lowered gate voltage, the current through the MOSFET decreases,

keeping it in the same value as before. The circuit thus uses a negative feedback to act as a current

sink.

5.1.5 Logic Control

The microcontroller chosen to control the charging process and utilize the algorithms

developed is ATMega32L by ATMEL. The reason for choosing this microcontroller is for its higher

amount of RAM, lower price, availability, speed and above all, free and user friendly programming

hardware and software.

A 4x3 keypad and a 20X4 LCD display are connected for manual control and user friendly

interface. The 16 bit timer in microcontroller is used to produce the PWM signal used in the Buck

converter. The UART (Universal Asynchronous Receiver Transmitter) interface of the microcontroller

is connected to a computer to log the charging voltage, open circuit voltage and charging current in

real time. This system helps to realize the performance of the charger.

A T

M e

g a

3 2

L

Battery Presence

and Polarity

Sense

Manual Selection

from Keypad

Batt 1 Terminal

Voltage Feedback

Batt 2 Terminal

Voltage Feedback

Current Feedback

UART

LCD

Charger Enable

Discharger Pulse

Charger-

Discharger Select

PWM

Polarity Setup

Port B

Pins 1-7

Port D

Pins 16 & 17

Port C

Pins 20 & 21

Port D

Pin 18

Port D

Pin 19

Port C

Pin 28

Port C

Pin 29

Port C

Pin 22-27

Port D

Pin15

Port A

Pins 36-38

(ADCs 4-2)

Port A

Pins 33-35

(ADCs 7-5)

Port A

Pin 39

(ADC 1)

Figure 5.6. Functional diagram of the microcontroller section


Page 29

To measure the voltages of the battery, voltage dividers are used so that the voltages to be

measured are converted to low values. The maximum voltage to be measured is 35V. As there are 8

ADCs to be used, 6 of them are used to measure the voltages of the battery terminals. Of the ADCs

for each terminal one ADC measures a voltage range of 0-5V, another measures 5-15V and the last

ADC measures 15-35V. One of the remaining two ADCs, one is used to measure the charging

current. Some capacitors are also connected to the ADC input terminals. These capacitors acts as

filters to suppress the voltage ripples generated from the Buck converter switching at the ADC input

terminals. The values of the capacitors are calculated to be 2uF, for 20 kHz higher cutoff frequency.

5.2 Performance analysis

5.2.1 Proper charging

The charger device is able to perform proper charging of batteries of all the four chemistry as

required by the IFEC2007 committee, which are given in tables 5.1 and 5.2.

Battery Starting

mode

Starting mode

up to

Ending

mode Termination

SLA CC 2.3V/cell CV Ich<Ith

Li-ion CC 4.2V/cell CV Ich<Ith

* CC = Constant Current CV = Constant Voltage Ich = Charging Current Ith = 3% Current

of CC mode

Table 5.3. Battery charging process of SLA and Li-ion

Battery Charging mode Termination

NiMH CC dv/dt<0

NiCd CC dv/dt=0

* CC = Constant Current CV = Constant Voltage

Table 5.4. Battery charging process of NiMH and NiCd

Some sample charging profiles of batteries of different chemistries are given in figures 5.6

through 5.9.


Page 30

Figure 5.7: Charging profile of a 3200mAh, 6V Sealed Lead Acid battery

Figure 5.8: Charging profile of a 700mAh 3.7V Li-ion battery


Page 31

Figure 5.9: Charging profile of 600mAh 4.8V NiCd battery

Figure 5.10: Charging profile of 2300mAh 4.8V NiMH battery


Page 32

5.2.2 Power calculation

A calculation of power loss in the charger circuit is given in table 5.3.

Components Loss

Buck MOSFET 0.4W

Reverse Blocking Diode 0.4W

Charger Enable MOSFET 0.4W

Relays 1W

Others 0.1W

Total 2.3W

Table 5.3: Power loss in charger circuit when supplying 1A at 22V.

6 Efficiency

A plot of efficiency of the charger circuit and estimated efficiency of the device is given in

figure 6.1.

Figure 6.1: Plot of efficiency of the charger circuit and estimated efficiency of the

device at 1A constant output current


Page 33

7 Method of Battery Detection

As per the project requirements, it is needed to detect 4 types of battery chemistries. They

are: NiMH, NiCd, Li-ion and Sealed Lead Acid (SLA). It has been observed that there is not much

difference in the charging technique for NiMH and NiCd batteries. So they can be treated as same

type. Thus, we have now three major battery types to differentiate: Li, Ni and SLA.

By extensive study of battery charging and discharging characteristics, a technique has been

found to identify the three battery types. The key point in this regard is that, all the experiments and

tests are conducted with the available batteries in local market. A list of mostly locally available local

batteries is given in table 7.1.

Batteries Manufacturers Size

Li-ion Anik, Nokia, Samsung Cell phone batteries

SLA CoolPower, Unicol, Free Tat Holdings _

NiMH Sony, Sanyo, Panasonic AA, AAA

NiCd Sanyo, Cadnica AA, AAA

Table 7.1: Table of tested batteries

Our implemented method for battery detection comprises two steps.

1. Constant current charging at 500mA for approximately 10 minutes.

2. Discharging the battery

7.1 Constant current charging at 500mA for approximately 10 minutes

During Charging for a short span of time (approximately 10 min), there is a possibility of

detecting NiMH or NiCd batteries. This can only be possible when almost full charged battery is given

and after 10 min charging the battery has reached end of charge. At that moment, Open Circuit

voltage of the battery fall below peak voltage by at least 15 mV, we can say it to be Nickel battery. In

this case if the fall of voltage is quite greater than 15mV we need to charge it again using Ni()

subroutine where appropriate termination condition is used to charge Ni batteries.


Page 34

One more thing that should be noted is overcharging protection of Li batteries should be

provided. For this purpose, continuous checking is done using the voltage range of charged Li

batteries. If it falls in the specified range, the battery may be detected as Li and charging must be

stopped. But using this method does not ensure that it is Li, because Ni battery of different cell

numbers can show the same range of voltage as Li batteries when they are discharged. For this

reason, we have introduced discharging during battery detection process.

7.2 Discharging the battery:

After a constant current charging of 10 min if battery type is undetermined, the battery is

allowed to discharge at 400mA discharging current. The main philosophy behind this discharging

method is that every battery type has distinctive discharging characteristics. If we are able to define

suitable parameters which have different values for different types of batteries, we can easily

distinguish different cell chemistry.

Three parameters are defined namely „N‟, „L‟ and „DV2‟ which would formulate the procedure

for battery detection.

N = MA(DV) × 400 ÷ Vpeak

L = MA(dV ÷ Vpeak) × 5

DV2= MA (DV) × 1000

Here,

DV = Vpeak(t-1) – Vpeak(t)

dV = Vpeak(t) – Vtrough(t)

MA = Moving Average


Page 35

At first the discharge characteristics of Li-ion is considered.

From the above discharge curves of two Li-ion batteries having different capacities it is

observed that the relative difference between Vpeak (open circuit voltage) and Vtrough (short circuit

voltage, battery delivering a specific load) remains almost constant.

Figure 7.1: Discharging Characteristics of a 650mAh Li-ion single cell battery.

Figure 7.2: Discharging characteristics of a 900mAh Li-ion single cell battery


Page 36

Now the discharge characteristics of Nickel batteries are considered.

Figure 7.3: Discharge characteristics of a 700mAh NiCd two-cell battery

Figure 7.4: Discharge characteristics of a 4600mAh NiMH two-cell battery


Page 37

From the above curves, it is clear that both NiCd and NiMH shows almost the same discharge

characteristics. In these cases, the relative difference between Vpeak and Vtrough rises sharply at

discharge point. This phenomenon can be used for both discharge point detection and for detecting

Ni+ battery.

At last, two discharge characteristics of SLA batteries are considered.

Figure 7.5: Discharge characteristics of a 3200mAh SLA 3-cell battery

Figure 7.6: Discharge characteristics of a 4500mAh SLA 3-cell battery


Page 38

From the six curves presented earlier, threshold values for N, L and DV2 can easily be

chosen to differentiate the three types of batteries. The ranges of these parameters for different types

of batteries are given in table 7.2.

Condition Decision

N > 1 AND L > 1 Ni

N < 1 AND L > 1 AND DV2 > 2.75 Li-ion

N < 1 AND L < 1 AND DV2 > 2.75 SLA

Table 7.2: Table of decision in battery chemistry detection

The ranges presented in the table are totally exclusive and there is apparently no chance of

overlapping if some initial data are discarded. The only assumption of this approach towards battery

detection is that the battery should consist some charge initially.

This assumption can be bypassed if the battery given for charging is trickle charged at a

constant current for a specific period of time without detecting its chemistry. After a certain time of

charging, we would discharge the battery for a pre-determined time and thus detect its chemistry. If

the battery could not be detected in this time, the battery would be trickle charged for a longer time

than the previous time interval. Then again an attempt would be taken to detect the battery by

discharging.


Page 39

8 Working Algorithm

Automatic Charging Subroutine

1) Start.

2) Set Cell_Type = 0.

3) Go to Constant_Current_10min() subroutine.

4) If Cell_Type = 0, then Battery type is undecided yet. Go to Discharge() subroutine.

5) If Cell_Type = 4, then the battery is Nickel and it is fully charged. Go to step 11.

6) If Cell_Type = 6, then 10 min more charging is needed without checking that if it is Li and fully

charged. Go to Constant_Current_10min() again.

7) If Cell_Type = 5, then Battery may be BAD and could not be charged. Go to step 9.

8) Else If Cell_Type = 3, then Battery is Ni and some more charging is needed. Go to Ni()

subroutine.

9) Else if Cell_Type = 1, then battery is SLA. Go first to Constant_Current() subroutine and then to

constant_voltage() subroutine.

10) Else If Cell_Type = 2, then battery is Li-ion. Go first to Constant_Current() subroutine and then to

Constant_Voltage() subroutine.

11) how to the LCD "Charging is Finished.Press Any Key to continue."

12) Wait for any key to be pressed.

13) If any key is pressed, jump to main().

14) End.

Constant_Current_10min() Subroutine

1) Start.

2) Go to Charge Mode.

3) Set Initial DUTY according to the charging current of 500mA.

4) Start PWM Buck to make a constant current charging.

5) Check if Battery runtime voltage rises to integer multiple of 4.2V.

6) If yes break the charging.

7) Check if battery open circuit voltage falls below peak voltage by at least 15mV.

8) If Yes then assign Cell_Type = 3 and calculate N_cell = Vpeak/1.4


Page 40

9) Check if battery open circuit voltage falls below by at least 8mV per cell.

10) If Yes, assign Cell_Type = 4 and break.

11) Stop PWM Buck

12) End

Discharge() Subroutine

1) Start.

2) Go to Discharge Mode. Set Discharge_current to 400 mA.

3) Record Open Circuit Voltage and Voltage with load alternatingly.

4) Using that values, calculate DV = Vopen_final-Vopen_previous, and dv = Vopen-Vload.

5) Adjust these values by taking moving average, scaling and normalizing by dividing with Vopen.

6) Calculate three deciding parameters - N, L, DV2.

7) If N>1 or L>1 and no. of iteration do not exceed 18 assign Cell_Type=6 and break the loop.

8) If N<1 and L<1 and DV2>2.75, then Cell_Type=1(SLA) and end of Discharge is reached.

9) If N>1 and L>1, then Cell_Type=3(Ni) and end of discharge is reached.

10) If L>1 and N<1 and DV2>2.75, then Cell_Type=2(Li) and end of discharge is reached.

11) End.

Constant_Current() Suroutine

1) Calculate n = ceil (Vtrm/Standard Voltage at discharge point)

2) Set Charging Current to 500mA.

3) Charge the battery in CC mode until runtime voltage reaches Vtrm = n*Standard_Cell_Voltage

(3.7 for Li and for SLA)

4) End

Constant_Voltage() Subroutine

1) Charge in CV mode until charging current falls to 6 percent of the current at CC mode.

2) End

Ni() Subroutine

1) Calculate n= ceil(Vtrm/Standard Voltage of Ni at discharge point)

2) Charge in CC mode and monitor the battery terminal voltage Vtrm

3) Detect the peak Vpeak of Vtrm

4) If Vtrm< Vpeak-0.008*n go to step 5, else go to step 1


Page 41

5) End

9 Previous Approaches

Some other approaches for the detection of battery chemistries and capacity have also been

tried. But extensive testing of different batteries has proven these methods failed. Short descriptions

of these other approaches are given below.

9.1 Chemistry detection

9.1.1 From β of a battery

Initially, a detection technique was developed on the quantity „β‟. β stands for the charge

acceptibilty of a battery. it is defined as

where,

Vcharge = Measured voltage across a cell during charging

Vbatt = Open circuit voltage of a cell

Icharge = Charging current

Figure 9.1: beta of the batteries of different chemistries

From different charging profiles plotted in different times, it has been seen that the range of

values of β for different chemistry during almost the whole charging period highly overlaps. This


Page 42

eventually leads to faulty detection of chemistry and hence number of cells. A barchart of range of

values of beta of different chemistries is given in figure 9.1.

9.1.2 SoC Method:

As an alternative method of battery detection, the State of Charge (SoC) method was

formulated. In this method, it is assumed that the discharge current of a battery is proportional to the

SoC of the battery and that different capacity battery would have the same characteristic curves if

open circuit voltage is plotted against discharge current. Moreover, different types of batteries should

show distinguishably different Open circuit voltage versus Discharge current plots. A combined

hypothetical plot of three different types of batteries is shown below:

Figure 7.2: Characteristic curve of different batteries in SoC method

It was first assumed that in the final circuit, the battery would be discharged for a certain

period. The acquired data would be matched with the pre-mapped battery characteristic plots and

thus the battery chemistry would be detected by their identical set of curves.

.Experiments on this method carried out. The data from several batteries were mapped and

the feasibility of the method was tested. Finally, this approach did not result in a good manner for

detecting battery types.


Page 43

9.2 Capacity Measurement:

Several approaches were undertaken to measure the battery capacity. First, it was assumed

that the battery having a high capacity would take proportionately longer time to be charged. It implies

that the dv / dt of high capacity batteries would be lower than batteries of lower capacity.

Here

dt = a small time interval

dv = change in battery open circuit voltage in time „dt‟

From this inverse relation an estimate of battery capacity can be found.

Next, another approach was initiated. In this approach a battery would be first trickle charged

for 20 minutes. Then the battery would be discharged with constant discharge current. Now, if the

battery is of high capacity, it would discharge quickly than a battery with lower capacity. That is, a high

capacity battery would posses a high dv / dt ratio where

dt = a small time interval

dv = change in battery open circuit voltage in the time interval dt

Again from this relations, an estimate of battery capacity could be found.

A similar approach was formulated by changing the definition of „dv‟. It is defined as the

difference of two battery open circuit voltages such that one is the current Vpeak and another is

immediate previous sampled value of Vpeak. That is

dv = Vpeak (t) – Vpeak (t-1)

This case would also result in a high dv / dt for higher capacity batteries which are discharged

after a short period of charging.

10 Key Innovations

A data logging equipment is of prime importance when it comes to obseving voltges and

current for a long period. As in this project, plenty of battery data had to be recorded fo analysis, a

unique data logging equipment was built to record, store and display the data.

The equipment consists of two parts:

1. Hardware

2. Software


Page 44

10.1 Data logging hardware

The basic block diagram of the data logging hardware is shown in figure 10.1.

Figure 10.1: Block Diagram of Data logging hardware

First the data from the microcontroller is fed to Universal Adapting Receiver Transmitter

(UART, in this case MAX 232 chip), which transfers data to the computer using serial port. Any of the

serial ports (COMPort 1 or 2) can be used for data transfer.

10.2 Data logging software

Figure 10.2: Screenshot of BUET IFEC 2007 TEAM of data logging software

The software is created using Visual Basic language. It takes reading of battery open circuit

voltage, runtime voltage and charging/discharging current and generates a Microsoft Office Excel

Sheet where these data are stored. In order to get a runtime view of the current and voltage profile, a

graphical interface is also attached.


Page 45

Before the charger is turned on, the software is provided with battery data such as battery

type, no. of cell and capacity. Thus data for a certain type of battery can be stored and analyzed. The

software may also be defined to select which serial port will be taken as source.

A screenshot of the data logging software is given in figure 10.2.

11 Cost Analysis:

The costs of the components are calculated from the local wholesale price. The cost of the

whole project is given block wise in table 11.1.

SMPS Charger

Block Cost Block Cost

EMI Filter and Bridge

rectifier .75$

Buck converter with

driver 1.2$

Startup .4$ H-bridge 1$

Microcontroller 1.4$ Discharger .6$

MOSFET and Snubber 1$ Microcontroller 2.2$

Transformer .5$ PCB .6$

Feedback .55$ Others 1$

PCB .4$ Total 6.6

Others 1$

Total 6$

Grand Total 12.6$

Figure 11.1: Cost estimation of the device

As all the electrical components are imported in our country, manufacturing cost in high

volume will be significantly lower than given here. If the manufacturing cost in high volume is 80% of

this cost, then the cost is 9.76$. So it may be said that the cost requirement is fulfilled.


Page 46

12 Summary of the work done

Requirements Status Comments

AC power supply Partially Achieved Different SMPS for different requirement

Charger Achieved With optional requirements

Charging current Achieved With optional requirements

Appropriate charging Achieved

External indications Achieved LCD panel is used, LED can be used for lower cost

Polarity insensitivity Achieved

SC and OC protection Achieved Implemented in SMPS

No load power Not achieved Slightly greater than the requirement

Power drawn after

charge finished Not Applicable No float charge is implemented

Power drawn during

charge Partially Achieved At low output power efficiency is low

Power quality Partially Achieved Microcontroller based SMPS does not fulfill but

NCP1651 based SMPS fulfills this requirement

Manufacturer‟s

recommendation Achieved

Hot pluggable Achieved

Connector Achieved

Display Achieved Through the LCD

Power supply Achieved 12V and Variable voltage with CV and CC mode

Safe indoor use Achieved Battery terminals are isolated from the mains

Cost Achieved 12.2$ in our country, which will be less in others

Table 12.1: Summary of the achievements


Page 47

13 Project Timeline

The team submitted their proposal on July 2006. after being selected, they started working

hard for achieving the required goals right away. Except for the occasional breaks such as termfinal

exams, the team worked continuously.

Figure 13.1. Project timeline

A graphical representation of the project time line is given in Figure 13.1.

14 Project Budget

The estimated budget of the project was USD 1000 (70,000 BDT). However, we were able to

complete the total project within a budget of USD 850 (almost 60,000 BDT). A local power

management company Enercon Systems International Limited has provided sponsor for the total

project. BUET has provided the IFEC team with the necessary support and facilities.

15 Conclusion

The BUET IFEC Team has worked tirelessly to complete this project. This has been a

thoroughly challenging and exhilarating experience for all of the students involved in the project. The

team is especially grateful to our faculties and lab coordinators for providing us with the necessary

support and aid. The proposed design fulfills most of the primary requirements specified by the IFEC

Committee as well as some extra features that were developed.


Page 48

During the work, the major problem faced was regarding the flyback converter. Due to

unavailability of the NCP 1651 chip hinderd the development of the SMPS. An alternative plan was

adapted which also met the frequency specifications required by IFEC. The scheme using

microcontoller showed encouraging result, but due to an accidental short circuit, final tests could not

be run and it was not possible to submit the SMPS in the final event. But other than this, the project

can be considered as an successful one.

16 Team Information

The IFEC 2007 team from Bangladesh University of Engineering and Technology (BUET)

consists of 16 undergraduate students at present. They are-

Sarkar Rahat Md. Anwar

Ahmed Tashrif Kamal

Md. Shahriar Jahan

Fahmida Shaheen

Samia Nawar Rahman

Raina Rahman

Hasan Md. Faraby

Abdullah Al Helal

A.T.M. Golam Sarwar

Md. Naimul Hasan

Ahmed Zubair

Mahmudur Rahman Siddiqui

Hafiz Imtiaz

Md. Ryyan Khan

Md. Raisul Islam

Yasin Sumon

Faculty advisor Dr. A.B.M. Harun-Ur-Rashid is in constant interaction with the IFEC team

providing valuable suggestions.

17 References

1) www.atmel.com

2) www.onsemi.com

3) www.wikipedia.org

4) www.greenbatteries.com

5) www.smps.us

6) www.batteryuniversity.com

http://www.atmel.com/

http://www.onsemi.com/

http://www.wikipedia.org/

http://www.greenbatteries.com/

http://www.smps.us/

http://www.batteryuniversity.com/

Documents

Final Report (Buet) - Ifec 2007