Project 2 Thermal modeling of battery pack

For a 10 cell series lithium ion battery model, simulate the thermal effects and compare life cycle performance at various temperatures, charge & discharge rates using MATLAB.

ANS:

Introduction:

Lithium-ion battery

A Li-ion battery or Lithium-ion battery is one of the type of rechargeable batteries. Li-ion batteries are commonly used for electric vehicles and portable electronics and are growing in popularity for military & applications related to aerospace . 

 

In battery, the lithium ions move from negative electrode to the positive electrode through an electrolyte during discharge, and back when in case of  charging. Lithium-ion batteries use an intercalated Lithium compound as the material at positive electrode and typically graphite at negative electrode. The batteries have low self discharge, no memory effect (other than LFP cells) and high energy density. However, Since they contain flammable electrolytes, they can be a safety hazard, and if they are incorrectly charged or damaged can lead to explosions and fires. Samsung was forced to recall Galaxy Note 7 handsets following lithium-ion fires, & there have been several incidents involving batteries on Boeing 787s.

Chemistry, cost, performance, and safety characteristics differ across types of lithium-ion batteries. Handheld electronics mostly use Li-polymer batteries (electrolyte being polymer gel), a graphite as anode, and a lithium cobalt oxide (LiCoO₂) as cathode material, which all together offer a high energy density. Lithium nickel manganese cobalt oxide (LiNiMnCoO₂ or NMC), Lithium manganese oxide (LiMn₂O₄ spinel, or Li₂MnO₃-based lithium-rich layered materials (LMR-NMC)) and Lithium iron phosphate (LiFePO₄), may offer longer lifecycle/lives and may have better capability rate. Such batteries are widely used for medical equipment, electric tools, and other applications. In electric vehicles NMC & its derivatives are widely used.

Research areas for Li-ion batteries include extending increasing energy density, lifetime, reducing cost, improving safety, and increasing charging speed, among others. Research has been underway in area of non-flammable electrolytes as a pathway to increased safety based on the flammability and volatility of the organic solvents used in typical electrolyte. Strategies include ceramic solid electrolytes, aqueous lithium-ion batteries, ionic liquids, polymer electrolytes, and heavily fluorinated systems.

 

Advantages

• Improved safety - more resistant to overcharge; less chance for electrolyte leakage
• Lightweight-gelled electrolytes enable simplified packaging by eliminating the metal shell.
• Flexible form factor - manufacturers are not bound by standard cell formats. With high volume, any reasonable size can be produced economically.
• Very low profile - batteries resembling the profile of a credit card are feasible.

   

Limitations

• Higher cost-to-energy ratio than lithium-ion
• No standard sizes. Most cells are produced for high-volume consumer markets.
• Expensive to manufacture.
• Lower energy density and decreased cycle count compared to lithium-ion.

 

 

PROCEDURE:

The following figure is the Simulink model of a 10 cell series Li-ion battery using an AC Current source providing cyclic discharge/charge profile.

 

The fundamental blocks used to built  this model are as follows

 

1. AC Current Source: The AC Current Source block represents an ideal current source that maintains the sinusoidal current through it, independent of the voltage across its terminals.

The output current is defined by the following equation:

`I=I0⋅sin(2π⋅f⋅t+ψ)`

This block has two electrical conserving ports associated with its terminals

 

2. Current Sensor:  The Current Sensor block represents an ideal current sensor, that is, a device that converts current measured in any electrical branch into a physical signal proportional to the current.

Connections '+' and '–' are electrical conserving ports through which the sensor is inserted into the circuit. The connection 'I' is a physical signal port that outputs the measurement result.

 

3. Lithium Cells: The Lithium cell block represents an ideal lithium-ion cell used in a battery pack with thermal ports.

Connections '+' and '–' are electrical conserving ports through which the cells are inserted into the circuit. The Thermal port used for further thermal network connections.

Here, the Cells pack 1-4 and 6-10 are normal and healthy cells, whereas Cell 5 is the faulty cell.

 

4. Temperature Sensor: The Temperature Sensor block represents an ideal temperature sensor, that is, a device that determines the temperature differential measured between two points without drawing any heat. Connections 'A' and 'B' are thermal conserving ports that connect to the two points where the temperature is being monitored. Port 'T' is a physical signal port that outputs the temperature differential value.

 

 

5. Conductive Heat Transfer: The Conductive heat transfer block models heat transfer in a thermal network by conduction through a layer of material. The rate of heat transfer is governed by Fourier's law and is proportional to the temperature difference, material thermal conductivity, area normal to the heat flow direction, and inversely proportional to the layer thickness.

 

 

6. Convective Heat Transfer: The Convective heat transfer block models heat transfer in a thermal network by convection due to fluid motion. The rate of heat transfer is proportional to the temperature difference, heat transfer coefficient, and surface area in contact with the fluid.

 

 

7. Temperature Source: The Temperature source block represents an ideal energy source in a thermal network that can maintain a constant absolute temperature at the port regardless of the heat flow rate.

 

8. Rate Transition: Rate Transition block transfers data from the output of a block operating at one rate to the input of another block operating at a different rate. The Rate Transition block's parameters allow you to specify options that trade data integrity and deterministic transfer for faster response and/or lower memory requirements. 

 

9. Discrete-Time Integrator: The Discrete-Time Integrator block implements discrete-time integration or accumulation of the input signal. The block can integrate or accumulate using the Forward Euler, Backward Euler, and Trapezoidal methods.

 

10. Mux: The Mux block combines inputs with the same data type and complexity into a vector output. 

 

11. Thermal Reference, Electrical Reference, Scope, Solver Configuration, Display, Constant, Gain, Add, PS-Simulink Converter, Goto tag, Simulink-PS Converter, and From are the miscellaneous blocks used in this model.

 

The three subsystems used to measure the temperature in this model for the respective cells packs are as below

 

 

They are all named 'T' (temperature) and the output of all these subsystems are given through Goto tags which will show the temperature variation through curves using a Scope.

 

MODEL PARAMETERS:

A) From Workspace:

 

B) AC Current Source:

 

C) Lithium Cells:

For Cells 1-4:

                                    

 

                              

 

For Cell 5:

                         

 

                          

To provide fault in 5th cell, all the electrical parameters are multiplied by some values.  

 

For Cells 6-10:

                          

 

                                   

 

D) Conductive Heat Transfer (For all cells):

 

 

E) Convective Heat Transfer:

For Cells 1-4:

 

For Cell 5:

 

For Cells 6-10:

 

F) Temperature Source: 

 

G) Defined Workspace Parameters:

                 

For the Temperature time cycle, refer to the excel sheet present in the workspace.

 

H) Gain value:

 

 

RESULT:

After simulating the model for 7200 secs, we get the following results

 

1) Ambient Temperature:-

2) Cell Temperature:-

 

3) SOC:-

 

 

Conclusion:

1. The Ambient temperature given by the Temperature source is ${\mathrm{293.15°K.}}^{}$

2. The Cell pack 1-4 reaches the maximum temperature of ${\mathrm{324°K}}^{}$, Cell pack 5 having faulty cell reaches the maximum temperature of ${\mathrm{333°K}}^{}$, and the Cell pack 6-10 reaches the maximum temperature of ${\mathrm{325°K}}^{}$.

3. The SOC of the model starts from 0 & goes up to 0.95 within 300 secs and again discharges to 0 in again 300 secs and the cycle keeps on repeating.

4. The Life cycle of battery pack is 6 cycles per hour.

 

 

The following figure shows Simulink model of a 10 cell series lithium-ion battery using a Signal Builder which has a signal input using a UDDS drive cycle of the Current profile.

 

The additional fundamental blocks used in this circuit are 

1. Signal builder: The Signal Builder block allows us to create interchangeable groups of piecewise linear signal sources and use them in a model.

Here, we are using a signal builder block to create the drive cycle data of UDDS cycle(1369 Sec). The limitation of the signal builder is that it accepts data in the form of only the current profile. So, the speed v/s time data from drive cycle should be converted to its corresponding current profile with respect to time. 

The UDDS drive cycle data with the current profile is attached below

 

2. Controlled Current Source: This block represents an ideal current source that is powerful enough to maintain the specified current through it regardless of the voltage across it. The output current is I = Is, where Is is the numerical value presented at the physical signal port.

 

MODEL PARAMETERS:

A) Signal Builder (UDDS drive cycle data with Current profile):

 

B) Gain value:

 

Rest all the parameters remain the same as the above model using a Cyclic AC current source.

 

 

RESULT:

 After simulatating the model for 1400 secs, we get the following results

 

1) Ambient Temperature:-

2) Cell Temperature:-

 

3) SOC:-

 

 

Conclusion:

1. The Ambient temperature provided by the Temperature source is ${\mathrm{293.15°K.}}^{}$

2. The Cell pack 1-4 reaches maximum temperature of ${\mathrm{293.183°K}}^{}$, Cell pack 5 which is the faulty cell reaches maximum temperature of ${\mathrm{293.19°K}}^{}$, and the Cell pack 6-10 reaches maximum temperature of ${\mathrm{293.185°K}}^{}$.

3. The SOC of the model starts from 0 and goes up to 0.9 in 1400 secs.