UNIVERSITY OF AKRONENGINEERING FACULTY

ELECTRICAL AND COMPUTER DEPARTMENT

Sensors and Instrumentation

Dr. Nathan IdaCourse 4400: 693

Report For course Projecton

Battery Management System

Student Name: Sarita BhandariStudent Number: 2353480Due Date: December 10, 2010

ABSTRACT

This report deals with the design and implementation of Battery Management system as a course

project for Sensors and Instrumentation. This project focused on the implementation of different

sensors to determine different parameters of the battery. This system is responsible for monitoring

temperature, pressure, voltage and current of the battery. By performing analytical modelling on the

sensor’s data, this system generates control signals in order to control the battery from excessive

temperature, pressure, voltage and current. Furthermore, this system is also responsible for displaying

the current status of the battery.

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Table of Contents ABSTRACT ...................................................................................................................................................... 1

I. INTRODUCTION ..................................................................................................................................... 4

I.1 OVERVIEW OF THE PROJECT ................................................................................................................ 4

I.1.1 Requirements of the System ......................................................................................................... 4

I.1.2 General Block Diagram and Components Requirement ............................................................... 4

II. DISCUSSION AND IMPLEMENTATION ..................................................................................................... 6

II.1 CONTROL UNIT .................................................................................................................................... 6

II.2 TEMPERATURE SENSOR ...................................................................................................................... 7

II.2.1 Implementation of Temperature Sensor ..................................................................................... 8

II.2.2 Implementation Issue of Temperature Sensor ............................................................................ 9

II.3 AUTOMATIC CHARGE DISCHARGE CIRCUIT WITH A VOLTAGE AND CURRENT DETECTOR .............. 10

II.4 FORCE SENSOR RESISTOR ................................................................................................................. 11

II.4.1 Calibration of Force Sensor Resistor .......................................................................................... 11

II.4.2 Implementation of Force Sensor Resistor ................................................................................. 13

II.4.3 Practical Issues with Force Sensor Resistor ............................................................................... 15

III. CONCLUSION AND POSSIBLE EXTENSION ....................................................................................... 16

IV. REFERENCES .................................................................................................................................... 17

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Table of Figures

Figure i: General Block Diagram of Battery Management System .............................................................5

Figure ii: Circuit Diagram of the Battery Management System ..................................................................6

Figure iiii: Block diagram of Temperature Sensor........................................................................................7

Figure iv: Circuit implementation of temperature sensor...........................................................................8

Figure v: Temperature verses voltage.........................................................................................................9

Figure vi: Circuit Diagram for Current and Voltage Detection and Automatic Charge Discharge .............10

Figure vii: Conductance verses voltage......................................................................................................12

Figure viii: Block diagram of FSR implementation......................................................................................13

Figure ix: Interfacing circuit for FSR to explorer 16....................................................................................14

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I. INTRODUCTION

A battery management system is a system that automatically charges and discharges a battery without

harming the battery. In order to protect the battery from excessive pressure, temperature, current and

voltage, the battery can be continuously monitored and appropriate decision can be made by the

system. For example, if the battery pressure changes to a high value, then the battery management

system will stop the operation of the battery in order to prevent it from being damaged. This project

deals with the design of the battery management system that can control the battery from over-

charging and discharging, over-heating and high pressures. The primary objective of this project is to

prolong battery life by detecting different parameters of the battery using sensors and generating

control signals from those detected parameters which are fed back to the battery control circuit.

I.1 OVERVIEW OF THE PROJECT

In this project, the requirements of the battery management system are identified and presented.

Various sensors are applied in the design of the battery management system and the performance and

implementation process of every individual sensors are briefly discussed. This project mainly focuses on

the use of sensors that are use to detect important parameters of battery in order to control the

battery. Hence, this project discusses all types of sensors that are implemented in this project along with

their practical implementation issues. The general block diagram of the battery management systems is

shown in figure below.

I.1.1 Requirements of the System

The basic requirements of the system are

The system should be able to charge and discharge the battery

Stop the charge discharge process when necessary

Detect the operating temperature of the battery

Detect the pressure of the battery

Display all the parameters (voltage, current, pressure, temperature) in computer or any display

unit

Switch the battery to charging state or discharging state depending on the charge status of the

battery

I.1.2 General Block Diagram and Components Requirement

The general block diagram of the system is shown in figure below.

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List of the components used in the system are

1) Control Unit

2) Display Unit

3) Temperature Sensor

4) Voltage and Current Sensor

5) Automatic charge discharge circuit

Control Unit

(PIC 24)

Battery

Automatic Charge

Discharge Circuit with

power supply

Display Unit

(Computer)

Analog to Digital

conversion unit

(ADC)

Temperature

Sensor

Pressure

Sensor

Voltage

Sensor

Current

Sensor

Digital Output

Analog input

Fed back and

Control signal

Figure i: General Block Diagram of Battery Management System

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II. DISCUSSION AND IMPLEMENTATION

This chapter focuses on the discussion and implementation of the above mention parts of the project.

The successful implementation of each and every part is most in order to obtain the objective of the

Battery Management System. Figure 2 shows the circuit diagram of entire system.

Figure ii: Circuit Diagram of the Battery Management System

II.1 CONTROL UNIT

To realize a Battery Management System with the above mentioned parts and objectives, interfacing

and co-working between all these components is mandatory. Therefore PIC24FJ128 Microcontroller is

chosen to process the data, and to generate the control signals. The objective of this processor is to

keep track of all the input data namely temperature, pressure, voltage, current. In addition, depending

upon the fed back data, this processor is designed to make the responsible decision to control the

battery from being harmed.

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Analog to Digital Converter Unit

All the sensors output data are analog in nature. Therefore analog to digital conversion of these analog

data need to be done before fed back these data to the processor. The PIC24FJ128 (1) is best processor

for this project because of the availability of the inbuilt analog to digital converter unit. Some pins of the

PIC24 (microcontroller) can be used as analog pin, by configuring these pins as analog pin. Proper

configuration bit has been calculated for these analog pin, as described in microcontroller datasheet (2),

to choose the reference voltage level and bit resolution.

II.2 TEMPERATURE SENSOR

Variety of temperature sensors are available in the market which focused on different parameters like

cost, measurement accuracy, durability and temperature sensing range. In general, there are three

types of temperature sensors and they are (i) Thermocouple, (ii) Thermistor and (iii) Integrated circuit.

Thermocouple is junction like apparatus that uses two dissimilar metals in its junction. In

thermocouples, voltage potential changes between two contact junctions with the changes in

temperature as these two terminals are made from dissimilar metals. Thermocouple works on the

principal of resistance temperature detectors (RTD). These RTD’s uses elements that are responsive

toward temperature. The resistance of these resistive elements changes with changing temperature.

Therefore, RTD’s use this property of the elements to detect the temperature. Some examples of such

resistive elements used in market are: Platinum, Nickel, and Copper. Integrated circuit temperature

sensors are the sensors that make uses of transistor and diode to detect the temperature.

LM35 (3) Integrated circuit is used in this Battery management project to detect operating temperature

of a battery. This Integrated circuit is easy to use, as there is a linear relation between the temperature

and a pressure. Another important feature of this Integrated circuit is that this LM35 is calibrated by the

manufacturing company. Therefore, when the power is applied to LM35, it produces the voltage as an

output that is proportional to the temperature in degree centigrade. The LM35 temperature sensor is an

Analog IC, therefore an analog to digital conversion is required to read the output voltage of the LM35.

In explorer 16, inbuilt analog to digital conversion is available therefore any analog pin can be used to

read the output voltage of temperature sensor. Furthermore, there is flexibility to choose internal or

external reference voltages for analog to digital conversion as per requirement. The general block

diagram for the implementation of temperature sensor is shown in figure below.

(LM35 )

temperature

Sensor

ADC

PIC 24 Vref (reference

voltage)

Figure i: Block diagram of Temperature sensor

Analog pin with inbuilt

analog to digital

conversion

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The specification of above mention temperature sensor is given as:

a) Directly Calibrated in degree centigrade

b) The output voltage is linear to the temperature with 10.0 mV/°C scale factor

c) Range of operation is :-55° C to +150° C

d) Operates from 4 Volts to 30 Volts

e) 0.5 °C accuracy is guaranteed

f) Nonlinearity of ± ¼ ° C

Base on datasheet of LM35, the relation between the output voltage and temperature is given by

(deg ) *10( / deg )outT C V mv C

II.2.1 Implementation of Temperature Sensor

A circuit designed to interface the LM35 temperature sensor with explorer 16 and the battery

management box is shown in Figure ii. Here, 104nF capacitor and 407 Ω resistor is used to control the

instability of the output voltage due to the self heating of the LM35 IC.

Output voltage Vout of Figure ii is fed to an analog pin of PIC24 (explorer 16) which is internally

configured (programmed) to read an analog voltage range from 0 Volt to +5 Volts. PIC24 provides 10 bit

resolution to the analog to digital conversion. The expression used to calculate the temperature by the

analog pin of PIC24 is given by

(deg ) 100* outT C V ... ... .. .... .. .... ........ ...... ..... ...(a)

where,

Analog Reading *( ) /1024out ref ref refV V V V

In spite of the specification provided by manufacturing company about the relation of temperature and

pressure given by equation (a), in practice slightly modification in this relation has been done to reduce

(LM35) +5V Vout (PIC

input)

100nF

470 Ω

Figure ii: Circuit implementation of temperature sensor

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the approximation error in temperature. After several experiment the relation between temperature

and pressure is slightly modified to reduce the error of approximation, and the modified expression is

(deg ) *100( / deg ) 2T C Vout mV C

Figure iii shows linear relation between temperature and pressure.

Figure iii Temperature verses voltage

II.2.2 Implementation Issue of Temperature Sensor

Problem faced during the implementation of the temperature sensor are listed below

Less stable temperature sensor output due to the present of offset voltage

Temperature rises than actual value due to the self heating of the IC

More than ±1°C of error in reading

In temperature sensor, which is factory calibrated, some level of instability is reduce by adding some

circuit complexity and taking the temperature reading for multiple times and averaging it value.

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II.3 AUTOMATIC CHARGE DISCHARGE CIRCUIT WITH A VOLTAGE AND CURRENT

DETECTOR

A circuit has been designed to provide constant current for charging and discharging the battery. As

these current and voltage are analog in nature, so an analog PIN of the microcontroller is used with

proper Analog to Digital conversion configuration. The current and voltage of the battery is monitored

continuously at fixed interval of time in order to keep track of the status of the battery. Simple analytical

modeling has been performed in the monitored data of battery. Base on this modeling, the processor

decides to either charge, discharge or rests the battery. Furthermore, the processor sends all these data

in the display unit i.e computer.

Figure vi: Circuit Diagram for Current and Voltage Detection and Automatic Charge Discharge

In addition, the circuit shown in figure above is also responsible for providing the constant current for

charging and discharging the battery. User has flexibility to change the charging current of the battery

by simply changing the resistance of a variable resistor (R26). Two Digital Pin of microcontroller are used

in this circuit in order to control the charging and discharging cycle. Full and detail design and

description of the automatic charge discharge circuit are beyond the gasp of this subject.

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II.4 FORCE SENSOR RESISTOR

Force Sensing Resistor (3) is thin polymer film device that can detect physical pressure, squeezing force

and weight. Force Sensing Resistor is simply a variable resistor whose resistance value changes with

changing the pressure or squeezing force or weight. (FSR) Force Sensing Resistor is ideal for measuring

the force on the static object, though it can also be used to measure the force of static as well as

dynamic object. FSR is made of four different layers of different material and they are

A thin layer of electrically insulating plastic

An active area with the conductors pattern which is connected to the leads on the tail to be

charged with an electrical voltage

A plastic adhesive spacer, which includes an opening aligned with the active area as well as an

air vent through the tail

A flexible substrate coated with a thick polymer conductive film, aligned with the active area.

Though the FSR comprises of four different layers, the thickness of FSR (four layers) is just 0.02 inch.

When there is no pressure, the sensor looks like an infinite resistor and as the pressure increases, the

resistance decreases. When external force is applied to the sensor, the resistive element present in the

active area is deformed against the substrate. Then air from the spacer opening is pushed through the

air vent in the tail, which causes the conductive material of the active area on the substrate to come to

contact. Therefore, the more the active area (conductive elements) touched by the load, the lower will

be the resistance.

Specification of the FSR mentions that it can measure the force range from 0 lb to 20 lb (0 Newton to

1000 Newton). The resistance of the FSR is in the range of Mega Ohm for no pressure, 100 kΩ for low

load and 100 Ω for high load condition. But the relation between the force and the resistance is not

linear in nature. From datasheets, it has been observed that it is hard to approximate the force by

simply looking at the change in resistance of the FSR.

II.4.1 Calibration of Force Sensor Resistor

In order to calibrate the Force Sensor Resistor one need to apply a known force and record its

corresponding resistance. If the corresponding resistance from a pool of known weight is tested, then

one can use interpolation formula to determine the relation between the resistance and weight for the

entire range i.e from no load to maximum load.

In order to properly calibrate the Force sensor Resistor, the upper area of the sensor is covered by the

rubber. Rubber is flexible and helps to equally distribute an applied force in the entire active surface

area of the Force Sensor Resistor, so it is chosen to cover the Force Sensor Resistor. This rubber also

helps to enhance the contact between the load and the sensor.

Highlighting points for the calibration of Forcer Sensor Resistor are

Ensure proper contact between the sensor and load

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even load distribution in the sensor’s active area

Avoid loading the sensor near the saturation level

Run experiment at same temperature

For calibration, resistance of the Force Sensor Resistor is measured for a series of different known Load.

Observed resistance value of the Force Sensor Resistor for every corresponding load is listed in Table 1.

No. of observation

Force in Lb Resistance

1 0 10 M

2 10 5K

3 20 2.5K

4 20 1.6K

5 40 1.4K

6 50 1.2K

7 60 1K

Table 1: Load verses Resistance

Another important fact about the Force Sensor Resistor is that though the relation between the resistance and Force is not linear, conductance of the Force Sensor Resistor has a linear relation with the load. Thus, to establish a relation between the conductance and the load, matlab simulation is performed on the above collected data. After performing simulation on the experimental data, linear relation between the force and the conductance is established .The figure and the equation (b) shows the relation between the force and the conductance for the experimental as well as simulated value.

Figure vii: Conductance verses Force

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Thus from the linear interpolation,

1* ( ) 1a F Force b …. ……. …. …. ..(b)

Coefficients are:

1 0.014182

1 0.10985

a

b

II.4.2 Implementation of Force Sensor Resistor

The main issue that needs to be considered during the implementation of Force Sensor Resistor is its

sensitivity requirement. Therefore, depending upon the system requirement of sensitivity and accuracy,

one can choose the appropriate reference voltage and power supply for the FSR. The sensitivity of the

sensor to some extent can be adjusted by changing the reference voltage, and the drive voltage. A high

reference voltage may result in high sensitivity of the Force Sensor Resistor. The general block diagram

of the implementation of the Force Sensor Resistor is shown in figure below. As the output value of the

FSR is just a voltage that ranges from 0 Volt to +3.3 Volts, thus an analog PIN of PIC24 is used. The

analog PIN is so configured that convert the analog value from the range of 0 volt to + 3.3 Volt with 10

bits bit resolution.

In general, Force Sensor Resistor can be symbolized as the variable resistor that range from several

ohms to mega ohms. The circuit used to interface the Force sensor resistor with the PIC24 is shown in

figure below. Here, a capacitor and a resistor were added to overcome the sensitivity issue of the Force

Sensor Resistor. When there is no load on the Force Sensor Resistor, the output voltage (Vout) is zero.

And when there is some load in the Force Sensor Resistor, the output voltage is not zero and can be

measured by the PIC24 analog PIN.

(FSR)

Force Sensor

Resistor

ADC

PIC 24 Vref (reference

voltage)

Figure iv: Block diagram of FSR implementation

Analog pin with inbuilt

analog to digital

conversion

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By measuring the output voltage via the analog PIN of this interfacing circuit for any loaded condition, one can calculate the Force of that particular load by using the simulated interpolation formula in following manner.

(Analog Reading)*( )*3.3 /1024out refV Vref Vref V Volt

From voltage divided rule,

3.3 *10 / (10 )outV Volt k k FSR

Therefore,

*10 / 3.3 10outFSR V k Volt K

Then the conductance is given by

1/ FSR

Now, the Force can be calculated as

( 1) / 1Force b a

Where,

1 0.014182

1 0.10985

a

b

Hence in this manner the force is calculated for any observed resistance.

(F

SR)

+3.3 V

Vout

407 Ω

10 KΩ

100nF

Figure ix: Interfacing circuit for FSR to explorer 16

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II.4.3 Practical Issues with Force Sensor Resistor

As the Force Sensor Resistor is not factory calibrated, the calibration part is difficult and time

consuming. Following are the list of problems that occurred while working with Force Sensor Resistor.

Difficult to establish the relation between the conductance and Force for interpolation

The accuracy offered by the interpolation formula is about ±5% to ±15%

The Force Sensor Resistor is unable to measure the lower Force value

The sensitivity level of Force Sensor Resistor is low, as it can not exactly detect the 0.5 Lb of

force change.

If the loading object surface area is wider than the active surface area of Force Sensor Resistor,

the accuracy level decreases than specified

The force sensor can properly sense the force that are perpendicular to the sensor, therefore

the Force sensor resistor is better to measure pressure or squeezing force.

The sensor connecting tips are very fragile

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III. CONCLUSION AND POSSIBLE EXTENSION

In conclusion, the design and implementation of the Battery Management System as a course project

was implemented successfully. In the first phase, interfacing circuit for the sensors and microcontroller

is designed. Before implementation of this circuit, simulation and calibration of these sensors and circuit

is done, in order to properly understand the operation and minimize the inaccuracy level. During the

implementation phase, sensitivity of the sensors was the major problem faced on the sensors. The issue

of sensitivity is handled by adding stability circuit and reading the sensor’s value for multiple times and

averaging it.

For future work, various other components can be added in the system, to make the system more

robust. Gas emission sensor is one very important factor that can be incorporated to detect the

emission of harmful gases, which helps to avoid battery explosion.

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IV. REFERENCES

1. Programming 16-Bit PIC Microcontrollers in C: Learning to Fly the PIC 24 (Embedded Technology) . [book auth.]

Lucio Di Jasio.

2. PIC24FJ128GA010 Family Data Sheet. MICROCHIP. [Online]

http://ww1.microchip.com/downloads/en/devicedoc/39747D.pdf.

3. Force Sensor Resistor. Forums. [Online] http://www.ladyada.net/learn/sensors/fsr.html.

4. LM35 PRECISE TEMPERATURE SENSOR. [Online] National Semiconductor.

http://www.national.com/ds/LM/LM34.pdf.

5. Explorer 16 Starter Kit. Microchip. [Online]

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en027853.