Project 2 Thermal modeling of battery pack
Objective: To model and simulate 10 cell series Li-ion battery ,analyze the thermal effects and compare life cycle performance at various temperature,charge and discharge rates.
Theory:Li-ion batteries are widely used in hybrid and electric vehicle as energy source.The operating cycle depends upon oprating temperature and to improve these characteristics good thermal system should be utilized.
The temperature of Li-ion battery is important for performance,effeciency,safety,capacity and is influenced by environmental temperature and by charging and discharging process itsely.Therefore a thermal model is used to calculate and estimate cell temperature.
Li-ion battery pack with fault was simulated.A battery pack consists of series connected cells in an effecient manner.It also shows that how a fault can be introduced into one of the cells to see the impact on battery performance cell temperature.
For effeciency, identical series connected cells are not modelled by simply connecting cell models.Instead a single cell is used and terminal voltage is scaled by number of cells.The fault is represented by changing parameters for cell5 fault subsystem,reducing both capacity and open circuit voltage and increasing resistance values.
Construction of Model:
Cell 1 to 4 Subsystem:cell subsystem consists of Thermal model and lithium cell 1RC block which calculate temperature and thermal effect for cells and overall calculate for battery pack.
The block which are inside the cell subsystem are PS gain which multiplies input physical signal by a constant.The other blocks are voltage sensor and current voltage source.
The voltage sensor block represents an ideal voltage sensor,that converts voltage measured between any electrical connections into a physical signal proportional to the voltage.
Current voltage source represents an ideal voltage source that is powerfull enough to maintain specified voltage and its output regardless of the current passing through it.The output voltage V=Vs,where Vs is the numerical value presented at physical signal port.
Thermal Model subsystem:
A.Thermal Reference: This block represents the thermal reference point in a thermal network where temperature is equal to absolute zero.
B.Controlled heat flow rate source:This block represents an ideal energy source in a thermal network that can maintain a contolled heat flow regardless of temperature difference.The heat flow rate is set by signal port S.A positive heat flow rate flows from port A to port B.
C.Thermal Mass:This block models internal energy storage in a thermal network.The rate of temperature increase is proportional to heat flow rate into the material and inversely proportional to mass and specific heat of material.
D.Temperature sensor:This block measures temperature in a thermal network.There is no heat flow through the sensor.The physical signal port 'T' reports the temperature difference across the sensor.The measurement is positive when temperature at port A is greater than temperature at port B.
Lithium cell 1RC Subsystem:
This block consists of capacitance values which depends upon external physical signal inputs SOC and T.It is assumed that capacitance value is slowly varrying over time and hence the equation i=C*dv/dt holds.
Resistance value R table which depends upon external physical signal inputs SOC and T.
Voltage value (EM) which implements the cells main branch voltage source and dtermines value for capacity(C) and state of charge(SOC).The defining equations depends upon cell temperature T.
Thermal Behaviour and fault step:
Li-ion cell one RC branch equivalent circuit detail for 10 cells.
For Cell 01 to 04
No.of series connected cells are 4.
Initial temperature 299.1K.
For cell05 (fault cell)We have done some changes in cell ,so that we can observe the fault.
No.of series cells connected-1
Initial temperature is 299.1k.
Capacity (A*hr)=Capacity_LUT*0.95
Em open circuit voltage(volts):Em_LUT*0.9
R0 terminal resistance(ohms):R0_LUT*5
R1 cell resistance(ohms):R1_LUT*5
cell Capacitance(farads):C1_LUT*0.95
For cell 06 to 10
No.of series connected cells are 5
Initial temperature is 299.1k.
Here output of one cell is connected with SOC calculation whereas other output is connected to AC current source for cyclic charge/discharge profile.
The peak amplitude is set to 50A for cyclic charge and discharge .A basic parameter of current source is as
The ideal Ac current source maintains sinusoidal current through it,independent of voltage across the terminals.The output current is defined by I=IO*sin(2*pi*f*t+PHI),where IO is the peak amplitude,f is the frequency in Hz and PHI is the phase shift in radians.
Other blocks which are used in Li-ion battery pack are:
Temperature source:This block represents an ideal energy source in a terminal network that can maintain a constant output temperature at the port regardless of the heat flow rate.
This block is used for ambient temperature which i set as 299.1K or 25.95C.
Convective Heat transfer:This block models heat transfer in a thermal network by convection due to fluid motion.The rate of heat transfer is proportional to temperature difference,heat transfer coeffecient,surface area in contact with fluid.
Conductive heat transfer:This block models heat transfer in a thermal network by conduction through a layer of material.The rate of heat transfer is governed by fourier law and is proportional to temperature difference,material thermal conductivity,area normal to heat flow direction and inversely proportional to layer thickness.
The conduction is done for 2 set of blocks ,1 block is 01-04 and other is 06-10.
For SOC calculation Gain block,Integrator and sum block with PS simulink convertor was used.
Results:Simulation based on different cell temperature.
In this simulation ambient temperature was set to 299.1k or 25.95C and other cell temperature are as follows:
color Indication for cells:
Yellow color:cell 4
Purple color:cell 5(faulty cell)
Green color:cell 10
Battery pack Configuration:
Test 1: Equal Temperature
Scope results:
As we can see much fluctuation in cell 05 due to fault.
Calculated SOC:As we see that battery is fluctuating every 301.200 seconds which depend upon thermal behaviour of battery pack.
Conduction: It is basically Heat flow rate (Q) and temperature difference (T) vs Time.
Cyclic Charge/Discharge Profile:
Charge and discharge profiles are depending upon current(I) and Voltage(V) vs Time(T).
The cyclic charge and discharge were calculated at 50A peak Amplitude.
Test 2: Cell 5 minimum temperature.
As we can see that in cell 5 the temperature rise till 312 and it goes fluctuating from 313k to 307k and end at 311k.
Calculated SOC:As we see that the battery is fluctuating every 301.200 seconds due to thermal behaviour of battery pack.
Conduction:
Cyclic Charge/Discharge profiles:
Charge and discharge profiles are depending upon current(I) and Voltage(V) vs Time(T).
The cyclic charge and discharge were calculated at 50A peak Amplitude
Test 3:Low to High Temperature
As we can see that in cell5 the temperature quickly falls and it fluctuate for 5k and end at 307k.
Calculated SOC:As we see that the battery is fluctuating every 301.200 seconds due to thermal behaviour of battery pack.
Conduction:
Cyclic Charge/discharge profiles:
Charge and discharge profiles are depending upon current(I) and Voltage(V) vs Time(T).
The cyclic charge and discharge were calculated at 50A peak Amplitude
Test 4(High to Low Temperature):
As we see that in cell 5 the temperature quickly falls and it fluctuate for 5k and end at 309k.
Calculated SOC:As we see that the battery is fluctuating every 301.200 seconds due to thermal behaviour of battery pack.
Conduction:
Cyclic Charge/Discharge Profiles:
Charge and discharge profiles are depending upon current(I) and Voltage(V) vs Time(T).
The cyclic charge and discharge were calculated at 50A peak Amplitude.
Test 5(Cell-5 Maximum temperature)
As we can see that temperature quickly falls and fluctuates for 5k and end at 309k.
Calculated SOC:As we see that the battery is fluctuating every 301.200 seconds due to thermal behaviour of battery pack.
Conduction:
Cyclic Charge/Discharge Profiles:
Charge and discharge profiles are depending upon current(I) and Voltage(V) vs Time(T).
The cyclic charge and discharge were calculated at 50A peak Amplitude.
Conclusion:
1.The flow of heat is decreased at constant and minimum temperature that is 309k for different temperature based.
2.Based on different temperature we will see difference in conduction and convection.
3.There is slightly change in cyclic charge and discharge of cell which is fixed at 50A peak amplitude.
4.As we have set fault in cell 5 ,there is a lot of fluctuations and temperature rise in cell 5 as compared to other cells connected in series.
5.SOC for all different temperature are same.There is no change in SOC.