REFRACTOMETRIC FIBER OPTIC SENSOR FOR IN-SITU MONITORING THE STATE-OF-CHARGE OF LEAD ACID BATTERY
REFRACTOMETRIC FIBER OPTIC SENSOR FOR IN-SITU MONITORING THE STATE-OF-CHARGE OF LEAD ACID BATTERY
© 2014 г. Supriya S. Patil*; V. P. Labade**; N. M. Kulkarni** and A. D. Shaligram***
*Dept. of Electronic Sc. Abasaheb Garware College, Pune 411 004, India **Dept of Electronic Sc., Fergusson college, Pune 411 004, India ***Dept. Electronic Sc. University of Pune, Pune 411 007, India
Е-mail: *patilss121@gmail.com; **vplabade@rediffmail.com; **nmkulkarni123@gmail.com; ***adshaligram@gmail.com
In-situ monitoring of the state of charge (SOC) of lead acid battery is important to understand the residual electrical energy. Usage of battery reduces the charge content of the active electrolyte which in turn changes its refractive index. This paper reports refract metric fiber optic sensor developed for on-line monitoring of SOC. The sensor is designed in such a way that it can be easily fitted in any cell of lead acid battery. The SOC of battery is estimated from sensor output. The battery performance parameters of the battery such as terminal voltage, discharge current, ampere-hour, battery temperature, SOC and Depth of discharge using fiber optic sensor output of battery are monitored for given electric load. The data is recorded continuously by data acquisition card USB 6009 using Lab VIEW Platform. Concurrently specific gravity of active electrolyte is measured using suck type of acidic hydrometer. The set of experiments are carried out for different discharge current by varying electrical load and SOC of the battery is monitored. After comparing the results for SOC of the battery with the conventional methods such as coulomb counting, terminal voltage and acidic hydrometer, it is concluded that developed sensor output is direct measure of SOC of battery.
Key words: fiber optic sensor, state of chаrge of battery, lead acid battery, refractometric sensor.
Received by Editorial office 12.09.2013.
Introduction
Electric vehicles, robotic systems, wheel chairs and submarine devices are typical examples where knowing the SOC (state of charge) is critical in order to optimize energy management and increase battery life. The SOC represents the fuel gauge of battery. Various techniques used to determine SOC of battery either needs manual intervention or sophisticated equipments. Optical sensors are also used for monitoring SOC because of its numerous advantages such as small size, light weight, immunity to EMI and RFI and ease of installation at the sight. There are few approaches of monitoring SOC based on optical sensors. Among those techniques some of them are measurement of specific gravity based on the principle of variation of the critical angle of reflection due to change in the density
of the surrounding sensor electrolyte [1–3] and monitoring the change of absorption of electrolyte due to variation of its chemical properties [4]. These optical sensors need special arrangements and are not convenient for in-situ monitoring.
This paper proposes refractometric fiber optic sensor probe for in-situ monitoring of SOC of the lead acid battery. Various battery parameters such as terminal voltage, discharge current, ampere-hour (AH), battery temperature, SOC and Depth of discharge (DOD) are also measured using designed hardware and data acquisition system USB 6009 for different electric loads. The measured battery performance parameters are continuously monitored using Lab VIEW tool. The recorded value of SOC using fiber optic sensor is compared with SOC value calculated using conventional terminal voltage and coulomb counting method.
“Оптический журнал”, 81, 3, 2014
61
Theory
The Lorentz–Lorenz equation gives the
relationship among density, refractive index
and polarizability for a classical, one component
fluid consisting of isotropic and non interacting
molecules –
R
º
1 ρ
n2 n2
-1 +2
=
4πAα 3M
.
(1)
In the equation (1), R is defined as specific
refraction, ρ is the mass density, n is the
refractive index, A is Avogadro’s number, α is the
molecular polarizability, and M is the molecular
weight. If the wavelength of the light is far from
any resonances in the molecules comprising the
fluid, the polarizability and hence the refractive
index depend only weakly on the wavelength.
The important point is that the polarizability is
presumed independent of the fluid density due to
the assumptions molecules do not interact. It is
further assumed that internal degrees of freedom
do not influence the polarizability; hence α is
independent of temperature. Thus under the
assumptions implicit in Lorentz-Lorenz equation,
at a given wavelength, the specific refraction is a
constant, and a measure of refractive index gives
the density directly, regardless of the temperature
or pressure. The Lorentz-Lorenz equation may be
generalized to non interacting molecules by the
use of molar refraction Rmix ≡ MR
Rmix
º
Mmix ρmix
nm2 ix nm2 ix
-1 . +2
(2)
The equation (2) states that if the molar refractions of the pure components of a mixture are known, then the molar refraction of the mixture may be calculated. Once Rmix is known a measurement of refractive index can be used to determine the density. Given the large number of assumptions the Lorentz-Lorenz equation is good approximation for highly associated fluids like water and for concentrated electrolyte solutions in water. That is these fluids exhibit a specific refraction that remains nearly constant over a wide range of temperature and density. Thus the specific refraction of a solution may be determined by measuring the refractive index and density [1].
In lead acid battery the SOC or energy content is linearly related to the concentration of the electrolyte i.e. sulfuric acid solution. As the battery is charged or discharged the concentration of the sulfuric acid changes depending upon the
type of battery. In the sulfuric acid solution the refractive index of the solution varies linearly with the concentration of acid in the solution. Therefore the battery charge level or discharge level is proportional to the acid concentration in the solution and this can be directly derived from the relative change in refractive index of the battery electrolyte.
Operating Principle
The proposed fiber optic based sensor works on the principle of refractometry. It is useful in detection of change in the refractive index of liquids [5]. The developed fiber optic sensor consists of two parallel fibers with a reflector at a distance. The light is launched into one of the fibers called transmitting fiber. The cone of emission from this fiber depends on numerical aperture (NA) of the fiber given by equation
θNA
=
sin-1
NA n1
.
(3)
Where θNA is a cone of emission, NA is the numerical aperture, n1 is the refractive index of the medium. For a fixed distance z between
the fiber end face and the reflector, amount of
light reflected from the reflector depends on the
refractive index of the medium between the fiber
sensor probe and the reflector. The reflected light
is collected by the receiving fiber. The output of
the receiving fiber varies in accordance with the
refractive index n1 of the medium. Operating principle of refractometric fiber
optic sensor is as shown in Fig 1. The region
between the fiber end faces and the reflector
is filled with liquid having refractive indices
n1 and n2 where n1 > n2. Using equation (1), θNA (n1) < θNA (n2). With medium having refractive index n1, emitted light energy density is more than that for the medium having refractive in-
dex n2. Thus received light intensity after reflection is more with medium having refractive
index n1 than for the medium having refractive index n2. Hence received light intensity is directly proportional to the change in refractive
index of medium.
Sensor Structure
FOS-SOC sensor is developed for on line detection of State-of-Charge of lead acid battery. It consists of two parallel fibers having
62 “Оптический журнал”, 81, 3, 2014
T Fiber
R Fiber
T
R Fiber
Z Z
θ1
Liguic (n1)
θ2
Liguic (n2) Z
Image of T Fiber
Image of T Fiber
Figure 1. Basic Principle of Refractometric Fiber optic sensor.
Figure 3. Photograph of FOS-SOC sensor.
LED
Transmitting Fiber
Photodetector
Receiving fiber
Glass Tube
Reflector
Cylindrical Glass Сavity
Figure 2. Schematic of FOS-SOC sensor.
specifications viz. fiber diameter (fd) – 2.2 mm, NA = 0.47, core diameter – 1 mm and cladding thickness – 0.612 mm. Fibers are encapsulated in the T-shaped glass tube as shown in Fig 2. In the T-shape structure the vertical part encapsulates two parallel fibers while the lower cylindrical part has reflector fitted at the base. Sensor is fabricated in glass to avoid the corrosive effect of the sulphuric acid, the electrolyte inside the battery. Cylindrical glass cavity is fused with the glass tube for placing reflector at a fixed distance of 6mm. The structure is immersed in the lead acid battery consisting of electrolyte which is the mixture of sulphuric acid and water. Light is launched in one of fibers called transmitting fiber using high bright RED LED with wavelength 673 nm.
Cone of light emitted by transmitting fiber depends on the refractive index of electrolyte of lead acid battery. Light reflected from mirror is collected by the receiving fiber. The received light is converted in electrical signal using photo detector L14G3. The amount of light received
is a function of refractive index of electrolyte. As discussed earlier refractive index is linear with specific gravity which is in turn related to the state of charge of lead acid battery. Figure 3 shows photograph of the developed fiber optic sensor for measuring state of charge of lead acid battery.
Experimental Setup
The light from high bright red light emitting diode was launched into one of the optical fiber i.e. transmitting fiber. The light reflected from the mirror is collected by another optical fiber i.e. receiving fiber. Collected light is converted to electrical form by using the photo detector (L14G3). The assembly is enclosed in the glass structure so that there will be no corrosion effects on the optical fibers. The local made lead acid battery having specifications as 12V/35Ah is used for experimentation. First battery is fully charged using charger 12V with 5A charging current for period of 7 hours. After the rest period of 1 hour, battery is connected to different electrical load for discharging. Considering the specific case for the load of 50Watt (halogen lamp) measurements are taken. During this, FOSSOC sensor is immersed into one of the cells of the battery. Temperature sensor LM 35 is used to sense the battery temperature during discharging of the battery. The discharging current is also measured using the developed hardware for coulomb counting to display ampere-hour consumption of the battery. The battery data i.e. current, temperature, sensor output and terminal voltage are acquired using data acquisition card of National Instrument USB 6009 of having sampling rate of 20 k samples/s. The Lead acid
“Оптический журнал”, 81, 3, 2014
63
Rechargeable Lead Acid Battery
Battery Terminal Voltage
Temperature FOS-SOC
Sensor
Sensor
Signal conditioning
Circuir 1
Signal conditioning
Circuir 2
Data Acquisition Card
Figure 4. Block Diagram of DAS with FOS-SOC.
Figure 5. Experimental Setup with DAS.
Figure 6. Sensor Mounting.
Figure 7. Screen shot for monitoring the SOC of battery. 64
battery 12v/35AH battery (Murphy Company) is used for experimentation purpose. Figure 4 shows experimental setup for on line monitoring of the SOC of lead acid battery. Figures 5 and 6 show actual photographs of DAS system and sensor mounting on the battery for on line monitoring.
The data acquisition module of NI USB 6009 card is used to read sensor output, terminal voltage, discharging current and temperature of the battery. The acquired data is processed and analyzed. Front panel developed in LABVIEW shows performance parameters of lead acid battery viz. terminal voltage, current, temperature, Ampere/hour, backup time, SOC and DOD. Current up to 50 A can be measured using the developed hardware. This current is integrated over the fixed period of time and ampere hours are calculated and displayed. Backup time of battery is calculated from the ampere hour and displayed.
State of charge (SOC) and depth of discharge (DOD) is estimated from the output of FOS-SOC sensor. Other parameters such as temperature, discharge current, DOD, battery terminal voltage are also monitored by using the LABVIEW as shown in Fig. 7.
Results and Discussion
The SOC of battery can be determined by measuring optical property of the electrolyte. When the load is connected to the charges in the battery get consumed. This changes the concentration of electrolyte which in turn changes the refractive index of it. Thus number of charges remained in a battery i.e. SOC is determined by concentration of charges left in the battery electrolyte. Equation (4) shows direct proportionality of concentration of charges with the refractive index. Therefore SOC can be determined by change in the refractive index of the electrolyte.
The lead acid battery uses lead dioxide (PbO2) as the active material in the positive electrode, and metallic lead (Pb) of a very porous structure, as the active material in the negative electrode. The electrolyte is formed by sulphuric acid (H2SO4) diluted in water (H2O), with concentrations between 8% and 40% depending upon state of charge and the type of the battery. The electrochemical reactions which occur during the process of charging and discharging are described in equations (4) to (9) [6].
“Оптический журнал”, 81, 3, 2014
During the charge: Anode (+):
PbSO4 + 2H2O → PbO2 + H2SO4 + 2H+ + 2e–, (4)
Cathode (–):
PbSO4 + 2e– → SO42– +Pb,
(5)
Global reaction:
2 PbSO4 + 2H2O → Pb + PbO2 + 2H2SO4. (6)
During the discharge:
Anode (+): Pb + SO42– → PbSO4 + 2e–, (7)
Cathode (–):
PbO2 + H2SO4 +2H+ +2e– → PbSO4 + 2H2O, (8)
Global reaction:
Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O. (9)
According to equation (7) to (9) during the discharging process both electrodes transform the active material into lead sulphate (PbSO4), with consequent consumption of H2SO4 and the release of water to electrolyte. As a result the concentration of 2e– in the electrolyte is decreased. This in turn reduces the refractive index of the electrolyte. In the charging process, equations (4) to (6), the opposite reaction occurs since H2SO4 is released and water is consumed thereby increasing the refractive index of the electrolyte.
Developed FOS-SOC sensor is refractometric retro-reflective fiber optic sensor. It is immersed into one of the cell of the battery for measuring the refractive index of the active electrolyte which changes during the charge and discharge process of battery. SOC of the battery should be monitored when battery is connected to the load i.e. during the discharge. Figure 8 shows that FOS-SOC sensor output is direct measure of SOC of battery as when battery is connected to the load of 100 Watt for the period of 5 hours with discharging current of 5 A/hour.
Similar readings are taken by connecting different loads to the battery viz. 50 Watt, 86 Watt, 100 Watt. Figure 9 shows the variation in the sensor output voltage during discharge for different loads taken over the time period of 1hour. It is observed that as the load value is decreased from 100 Watt to 50 Watt the sensor output voltage more for the same time interval. This is because for smaller values of load, the charge remained in the electrolyte is more
Sensor output in Volts
3 2,5
2 1,5
1 0,5
0 0 20 40 60 80 %SOC
Figure 8. Sensor output with SOC.
100
Sensor output in Volts
1,2 1
0,8
0,6 0,4 Load = 100 W 0,2
0
0 20
40
Time in minutes
60
Figure 9. Sensor output voltage during discharge of battery.
compared to that for the larger values of load. Thus this sensor is used to directly measure the battery SOC.
Conclusion
This paper mainly describes a sensor based on the refractometry principle for measuring the SOC of the lead acid battery. Most of the present day methods available for measuring the SOC of battery are either offline or indirect. Few on line measurement methods are reported but it requires modification in the structure of battery itself. Such modification in the battery structure is not required by the FOS-SOC sensor. It is a direct and online method of measurement of SOC of lead acid battery. Experiments are carried out with FOS-SOC sensor by discharging battery with different loads. The sensor shows good sensitivity for this. Thus the sensor is useful for online determination of the SOC of the lead acid battery.
Acknowlegment
One of the authors SSP wishes to acknowledge Head of the Department of Electronic Science, University of Pune for allowing the use of
“Оптический журнал”, 81, 3, 2014
65
laboratories for the work. She is also thankful to the Principal, Fergusson College for providing the necessary laboratory facilities. Author also expresses sincere thanks to UGC, New Delhi
for the sanction of teacher fellowship and The Principal, Dr. S. G. Gupta and authorities of Abasaheb Garware College, Pune, for granting the study leave.
* * * * *
REFERENCES
1. Anderson G.K. An Optical technique for in situ density determination in electrolyte solutions under hydrothermal conditions // International Journal of Thermo physics. 1997. V. 18. No. 3. P. 699–717.
2. Hancke G.P. A fiber-optic density sensor for monitoring the SOC of lead acid battery // IEEE Trans. 1990. P. 247–250.
3. Cortazar O.D. and Feliu V. A simple and robust fiber optics system for measuring the lead acid battery stateof-charge // Journal of power sources. 2006. V. 159. P. 728–733.
4. Jonathan D.W. Fiber optic refractive index monitor. Patent US № 6356675 B1. 2002.
5. Choudhari A.L., Shaligram A.D. Multiwavelength optical fiber liquid refractometry based on intensity modulation // Sensors and Actuators A: Physical. 2002. V. 100. P. 160–164.
6. Handbook of batteries. Edition 5.
66 “Оптический журнал”, 81, 3, 2014
© 2014 г. Supriya S. Patil*; V. P. Labade**; N. M. Kulkarni** and A. D. Shaligram***
*Dept. of Electronic Sc. Abasaheb Garware College, Pune 411 004, India **Dept of Electronic Sc., Fergusson college, Pune 411 004, India ***Dept. Electronic Sc. University of Pune, Pune 411 007, India
Е-mail: *patilss121@gmail.com; **vplabade@rediffmail.com; **nmkulkarni123@gmail.com; ***adshaligram@gmail.com
In-situ monitoring of the state of charge (SOC) of lead acid battery is important to understand the residual electrical energy. Usage of battery reduces the charge content of the active electrolyte which in turn changes its refractive index. This paper reports refract metric fiber optic sensor developed for on-line monitoring of SOC. The sensor is designed in such a way that it can be easily fitted in any cell of lead acid battery. The SOC of battery is estimated from sensor output. The battery performance parameters of the battery such as terminal voltage, discharge current, ampere-hour, battery temperature, SOC and Depth of discharge using fiber optic sensor output of battery are monitored for given electric load. The data is recorded continuously by data acquisition card USB 6009 using Lab VIEW Platform. Concurrently specific gravity of active electrolyte is measured using suck type of acidic hydrometer. The set of experiments are carried out for different discharge current by varying electrical load and SOC of the battery is monitored. After comparing the results for SOC of the battery with the conventional methods such as coulomb counting, terminal voltage and acidic hydrometer, it is concluded that developed sensor output is direct measure of SOC of battery.
Key words: fiber optic sensor, state of chаrge of battery, lead acid battery, refractometric sensor.
Received by Editorial office 12.09.2013.
Introduction
Electric vehicles, robotic systems, wheel chairs and submarine devices are typical examples where knowing the SOC (state of charge) is critical in order to optimize energy management and increase battery life. The SOC represents the fuel gauge of battery. Various techniques used to determine SOC of battery either needs manual intervention or sophisticated equipments. Optical sensors are also used for monitoring SOC because of its numerous advantages such as small size, light weight, immunity to EMI and RFI and ease of installation at the sight. There are few approaches of monitoring SOC based on optical sensors. Among those techniques some of them are measurement of specific gravity based on the principle of variation of the critical angle of reflection due to change in the density
of the surrounding sensor electrolyte [1–3] and monitoring the change of absorption of electrolyte due to variation of its chemical properties [4]. These optical sensors need special arrangements and are not convenient for in-situ monitoring.
This paper proposes refractometric fiber optic sensor probe for in-situ monitoring of SOC of the lead acid battery. Various battery parameters such as terminal voltage, discharge current, ampere-hour (AH), battery temperature, SOC and Depth of discharge (DOD) are also measured using designed hardware and data acquisition system USB 6009 for different electric loads. The measured battery performance parameters are continuously monitored using Lab VIEW tool. The recorded value of SOC using fiber optic sensor is compared with SOC value calculated using conventional terminal voltage and coulomb counting method.
“Оптический журнал”, 81, 3, 2014
61
Theory
The Lorentz–Lorenz equation gives the
relationship among density, refractive index
and polarizability for a classical, one component
fluid consisting of isotropic and non interacting
molecules –
R
º
1 ρ
n2 n2
-1 +2
=
4πAα 3M
.
(1)
In the equation (1), R is defined as specific
refraction, ρ is the mass density, n is the
refractive index, A is Avogadro’s number, α is the
molecular polarizability, and M is the molecular
weight. If the wavelength of the light is far from
any resonances in the molecules comprising the
fluid, the polarizability and hence the refractive
index depend only weakly on the wavelength.
The important point is that the polarizability is
presumed independent of the fluid density due to
the assumptions molecules do not interact. It is
further assumed that internal degrees of freedom
do not influence the polarizability; hence α is
independent of temperature. Thus under the
assumptions implicit in Lorentz-Lorenz equation,
at a given wavelength, the specific refraction is a
constant, and a measure of refractive index gives
the density directly, regardless of the temperature
or pressure. The Lorentz-Lorenz equation may be
generalized to non interacting molecules by the
use of molar refraction Rmix ≡ MR
Rmix
º
Mmix ρmix
nm2 ix nm2 ix
-1 . +2
(2)
The equation (2) states that if the molar refractions of the pure components of a mixture are known, then the molar refraction of the mixture may be calculated. Once Rmix is known a measurement of refractive index can be used to determine the density. Given the large number of assumptions the Lorentz-Lorenz equation is good approximation for highly associated fluids like water and for concentrated electrolyte solutions in water. That is these fluids exhibit a specific refraction that remains nearly constant over a wide range of temperature and density. Thus the specific refraction of a solution may be determined by measuring the refractive index and density [1].
In lead acid battery the SOC or energy content is linearly related to the concentration of the electrolyte i.e. sulfuric acid solution. As the battery is charged or discharged the concentration of the sulfuric acid changes depending upon the
type of battery. In the sulfuric acid solution the refractive index of the solution varies linearly with the concentration of acid in the solution. Therefore the battery charge level or discharge level is proportional to the acid concentration in the solution and this can be directly derived from the relative change in refractive index of the battery electrolyte.
Operating Principle
The proposed fiber optic based sensor works on the principle of refractometry. It is useful in detection of change in the refractive index of liquids [5]. The developed fiber optic sensor consists of two parallel fibers with a reflector at a distance. The light is launched into one of the fibers called transmitting fiber. The cone of emission from this fiber depends on numerical aperture (NA) of the fiber given by equation
θNA
=
sin-1
NA n1
.
(3)
Where θNA is a cone of emission, NA is the numerical aperture, n1 is the refractive index of the medium. For a fixed distance z between
the fiber end face and the reflector, amount of
light reflected from the reflector depends on the
refractive index of the medium between the fiber
sensor probe and the reflector. The reflected light
is collected by the receiving fiber. The output of
the receiving fiber varies in accordance with the
refractive index n1 of the medium. Operating principle of refractometric fiber
optic sensor is as shown in Fig 1. The region
between the fiber end faces and the reflector
is filled with liquid having refractive indices
n1 and n2 where n1 > n2. Using equation (1), θNA (n1) < θNA (n2). With medium having refractive index n1, emitted light energy density is more than that for the medium having refractive in-
dex n2. Thus received light intensity after reflection is more with medium having refractive
index n1 than for the medium having refractive index n2. Hence received light intensity is directly proportional to the change in refractive
index of medium.
Sensor Structure
FOS-SOC sensor is developed for on line detection of State-of-Charge of lead acid battery. It consists of two parallel fibers having
62 “Оптический журнал”, 81, 3, 2014
T Fiber
R Fiber
T
R Fiber
Z Z
θ1
Liguic (n1)
θ2
Liguic (n2) Z
Image of T Fiber
Image of T Fiber
Figure 1. Basic Principle of Refractometric Fiber optic sensor.
Figure 3. Photograph of FOS-SOC sensor.
LED
Transmitting Fiber
Photodetector
Receiving fiber
Glass Tube
Reflector
Cylindrical Glass Сavity
Figure 2. Schematic of FOS-SOC sensor.
specifications viz. fiber diameter (fd) – 2.2 mm, NA = 0.47, core diameter – 1 mm and cladding thickness – 0.612 mm. Fibers are encapsulated in the T-shaped glass tube as shown in Fig 2. In the T-shape structure the vertical part encapsulates two parallel fibers while the lower cylindrical part has reflector fitted at the base. Sensor is fabricated in glass to avoid the corrosive effect of the sulphuric acid, the electrolyte inside the battery. Cylindrical glass cavity is fused with the glass tube for placing reflector at a fixed distance of 6mm. The structure is immersed in the lead acid battery consisting of electrolyte which is the mixture of sulphuric acid and water. Light is launched in one of fibers called transmitting fiber using high bright RED LED with wavelength 673 nm.
Cone of light emitted by transmitting fiber depends on the refractive index of electrolyte of lead acid battery. Light reflected from mirror is collected by the receiving fiber. The received light is converted in electrical signal using photo detector L14G3. The amount of light received
is a function of refractive index of electrolyte. As discussed earlier refractive index is linear with specific gravity which is in turn related to the state of charge of lead acid battery. Figure 3 shows photograph of the developed fiber optic sensor for measuring state of charge of lead acid battery.
Experimental Setup
The light from high bright red light emitting diode was launched into one of the optical fiber i.e. transmitting fiber. The light reflected from the mirror is collected by another optical fiber i.e. receiving fiber. Collected light is converted to electrical form by using the photo detector (L14G3). The assembly is enclosed in the glass structure so that there will be no corrosion effects on the optical fibers. The local made lead acid battery having specifications as 12V/35Ah is used for experimentation. First battery is fully charged using charger 12V with 5A charging current for period of 7 hours. After the rest period of 1 hour, battery is connected to different electrical load for discharging. Considering the specific case for the load of 50Watt (halogen lamp) measurements are taken. During this, FOSSOC sensor is immersed into one of the cells of the battery. Temperature sensor LM 35 is used to sense the battery temperature during discharging of the battery. The discharging current is also measured using the developed hardware for coulomb counting to display ampere-hour consumption of the battery. The battery data i.e. current, temperature, sensor output and terminal voltage are acquired using data acquisition card of National Instrument USB 6009 of having sampling rate of 20 k samples/s. The Lead acid
“Оптический журнал”, 81, 3, 2014
63
Rechargeable Lead Acid Battery
Battery Terminal Voltage
Temperature FOS-SOC
Sensor
Sensor
Signal conditioning
Circuir 1
Signal conditioning
Circuir 2
Data Acquisition Card
Figure 4. Block Diagram of DAS with FOS-SOC.
Figure 5. Experimental Setup with DAS.
Figure 6. Sensor Mounting.
Figure 7. Screen shot for monitoring the SOC of battery. 64
battery 12v/35AH battery (Murphy Company) is used for experimentation purpose. Figure 4 shows experimental setup for on line monitoring of the SOC of lead acid battery. Figures 5 and 6 show actual photographs of DAS system and sensor mounting on the battery for on line monitoring.
The data acquisition module of NI USB 6009 card is used to read sensor output, terminal voltage, discharging current and temperature of the battery. The acquired data is processed and analyzed. Front panel developed in LABVIEW shows performance parameters of lead acid battery viz. terminal voltage, current, temperature, Ampere/hour, backup time, SOC and DOD. Current up to 50 A can be measured using the developed hardware. This current is integrated over the fixed period of time and ampere hours are calculated and displayed. Backup time of battery is calculated from the ampere hour and displayed.
State of charge (SOC) and depth of discharge (DOD) is estimated from the output of FOS-SOC sensor. Other parameters such as temperature, discharge current, DOD, battery terminal voltage are also monitored by using the LABVIEW as shown in Fig. 7.
Results and Discussion
The SOC of battery can be determined by measuring optical property of the electrolyte. When the load is connected to the charges in the battery get consumed. This changes the concentration of electrolyte which in turn changes the refractive index of it. Thus number of charges remained in a battery i.e. SOC is determined by concentration of charges left in the battery electrolyte. Equation (4) shows direct proportionality of concentration of charges with the refractive index. Therefore SOC can be determined by change in the refractive index of the electrolyte.
The lead acid battery uses lead dioxide (PbO2) as the active material in the positive electrode, and metallic lead (Pb) of a very porous structure, as the active material in the negative electrode. The electrolyte is formed by sulphuric acid (H2SO4) diluted in water (H2O), with concentrations between 8% and 40% depending upon state of charge and the type of the battery. The electrochemical reactions which occur during the process of charging and discharging are described in equations (4) to (9) [6].
“Оптический журнал”, 81, 3, 2014
During the charge: Anode (+):
PbSO4 + 2H2O → PbO2 + H2SO4 + 2H+ + 2e–, (4)
Cathode (–):
PbSO4 + 2e– → SO42– +Pb,
(5)
Global reaction:
2 PbSO4 + 2H2O → Pb + PbO2 + 2H2SO4. (6)
During the discharge:
Anode (+): Pb + SO42– → PbSO4 + 2e–, (7)
Cathode (–):
PbO2 + H2SO4 +2H+ +2e– → PbSO4 + 2H2O, (8)
Global reaction:
Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O. (9)
According to equation (7) to (9) during the discharging process both electrodes transform the active material into lead sulphate (PbSO4), with consequent consumption of H2SO4 and the release of water to electrolyte. As a result the concentration of 2e– in the electrolyte is decreased. This in turn reduces the refractive index of the electrolyte. In the charging process, equations (4) to (6), the opposite reaction occurs since H2SO4 is released and water is consumed thereby increasing the refractive index of the electrolyte.
Developed FOS-SOC sensor is refractometric retro-reflective fiber optic sensor. It is immersed into one of the cell of the battery for measuring the refractive index of the active electrolyte which changes during the charge and discharge process of battery. SOC of the battery should be monitored when battery is connected to the load i.e. during the discharge. Figure 8 shows that FOS-SOC sensor output is direct measure of SOC of battery as when battery is connected to the load of 100 Watt for the period of 5 hours with discharging current of 5 A/hour.
Similar readings are taken by connecting different loads to the battery viz. 50 Watt, 86 Watt, 100 Watt. Figure 9 shows the variation in the sensor output voltage during discharge for different loads taken over the time period of 1hour. It is observed that as the load value is decreased from 100 Watt to 50 Watt the sensor output voltage more for the same time interval. This is because for smaller values of load, the charge remained in the electrolyte is more
Sensor output in Volts
3 2,5
2 1,5
1 0,5
0 0 20 40 60 80 %SOC
Figure 8. Sensor output with SOC.
100
Sensor output in Volts
1,2 1
0,8
0,6 0,4 Load = 100 W 0,2
0
0 20
40
Time in minutes
60
Figure 9. Sensor output voltage during discharge of battery.
compared to that for the larger values of load. Thus this sensor is used to directly measure the battery SOC.
Conclusion
This paper mainly describes a sensor based on the refractometry principle for measuring the SOC of the lead acid battery. Most of the present day methods available for measuring the SOC of battery are either offline or indirect. Few on line measurement methods are reported but it requires modification in the structure of battery itself. Such modification in the battery structure is not required by the FOS-SOC sensor. It is a direct and online method of measurement of SOC of lead acid battery. Experiments are carried out with FOS-SOC sensor by discharging battery with different loads. The sensor shows good sensitivity for this. Thus the sensor is useful for online determination of the SOC of the lead acid battery.
Acknowlegment
One of the authors SSP wishes to acknowledge Head of the Department of Electronic Science, University of Pune for allowing the use of
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laboratories for the work. She is also thankful to the Principal, Fergusson College for providing the necessary laboratory facilities. Author also expresses sincere thanks to UGC, New Delhi
for the sanction of teacher fellowship and The Principal, Dr. S. G. Gupta and authorities of Abasaheb Garware College, Pune, for granting the study leave.
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