Project 2 Thermal modeling of battery pack

Aim: Thermal modeling of the battery pack

Objective:

• To develop a model for a 10-cell series lithium-ion battery.
• To simulate the thermal effects of the battery.
• To compare life cycle performance at various temperatures, charge & discharge rates using MATLAB.

Solution:

A lithium-ion (Li-ion) battery is an advanced battery technology that uses lithium ions as a key component of its electrochemistry. During a discharge cycle, lithium atoms in the anode are ionized and separated from their electrons. The lithium ions move from the anode and pass through the electrolyte until they reach the cathode, where they recombine with their electrons and electrically neutralize. The lithium ions are small enough to be able to move through a micro-permeable separator between the anode and cathode. In part because of lithium’s small size (third only to hydrogen and helium), Li-ion batteries are capable of having a very high voltage and charge storage per unit mass and unit volume.

A battery consists of multiple connected lithium-ion cells. Battery packs for large consumer electronics like laptop computers also contain temperature sensors, voltage regulator circuits, voltage taps, and charge-state monitors. These components minimize safety risks like overheating and short-circuiting. To power, larger devices, such as electric cars, connecting many small batteries in a parallel circuit is more effective and more efficient than connecting a single large battery.

And when 10 cells are connected in series so their voltage becomes 10 times but the Current remains constant. If we want to more efficient battery pack so we need to keep more batteries in series because less amount of heat loss occurs. And if heat loss is less so we get more power and their life span also increases.

Model: This model, which is a 10-cell lithium-ion battery pack, was made using Simscape and Simulink blocks. Additionally, it outputs information on the battery's temperature, level of charge, and voltage.

Explanation :

• First shorted out all the required Simscpe/Simulink blocks.
• Constant block: To define the rate of discharge of the battery in Amperes (4.2 A).
• Controlled Current Source block: To monitor and control the flow of current from the battery.
• Table-based battery block: Instrumented + Show Thermal port.
• Temperature Source block: To define the temperature (298.15K) input to the system under study.
• Convective Heat Transfer block: To define the Area (0.1 m^2) and the Heat transfer coefficient (20 w/[m^2*K]) of the system under study.
• Temperature Sensor block: To sense and measure the temperature of the system.
• Goto Tag block: To define the value of a parameter of a system under study and assign it to the 'From Tag'.
• From Tag block: To define the value of a parameter of a system under study received from the 'Goto Tag'.
• Voltage sensor: It measures the voltage of a battery that is connected in parallel.
• Current sensor: it measures the current of a battery that is connected in series.
• SOC estimation: This estimation is done by using the coulomb counting method where we use gain, constant, integrator, and add blocks.
• Configuration Solver block: To support the simulation of the system under study.
• PS-Simulink Converter block: To convert the Physical system into a Simulink signal.
• Electrical Reference block: Electrical reference for the electrical blocks of the system under study.
• Scope block: To display the results of the system under study.

Battery: 

SOC estimation: Here we estimate the SOC using the coulomb counting method which has initial SOC minus integration of current with respect to time and is inversely proportional to the capacity (Ah).

For temperature measurement: First, we took the temperature source, and this source passed through a convective heat transfer and connected to the thermal port and temp. Measurement sensor to give out a thermal effect on the battery model.

We used a small area (0.1 m^2) to measure the temperature, but in order to assess the thermal effect, we had to adjust and observe via a scope what would happen if we increased the size.

Result:

This graph displays the battery pack's SOC, temperature increase, and voltage.

Case 1: Temperature at 298.15 K and surface area is 0.1 m^2.

We can clearly notice within an hour that there are some temperature rises to 298.54 K from 298.15 K in a battery when the SOC level of the battery system reaches 100% to 0% at around 1 C rate. When the battery's voltage falls between 41.5V and 34.6V over time, this indicates that the battery is draining.

Case 2: Temperature at 300 K and surface area is 1 m^2.

The battery system's SOC drops from 100% to 0% within an hour, and when the temperature is set at 300 K, the temperature rises exponentially from 298.15 K to 300.1 K due to the increase in surface area from 0.1 m^2 to 1 m^2.

Case 3: Temperature at 250K and surface area is 1 m^2.

The system's SOC drops to zero after an hour, and when the temperature is set at 250 K, it drops exponentially from 298.15 K to 250 K as a result of an increase in surface area from 0.1 m^2 to 1 m^2. When the C-rate is regulated at 0.5C or 0.25C, the temperature in the cell rises somewhat, and simulation time changes as a result of the C-rate regulation.

Case 4: Temperature at 310K and surface area is 10 m^2.

The battery system's SOC drops from 100% to 0% within an hour, and when the temperature is set at 300 K, the temperature rises exponentially from 298.15 K to 310 K due to the increase in surface area from 0.1 m^2 to 10 m^2.

Summary:

As seen in cases 1, 2, 3, and 4, when temperatures are set at various Kelvins, they rise and fall in accordance with the temperature source, but the graph only exhibits an exponential trend due to an increase in surface area. When the battery's surface area grows, there is a space between the cells, where the convective heat flow rate comes into contact, cooling or heating the entire battery pack.

Model for life span: We must now develop a new model in order to understand how the life span is changing and to check the life span.

Explanation :       

• Square wave generator block: It gives the output as a waveform to see the charging and discharging effect.
• Controlled current source block: To monitor and control the flow of current from the battery.
• Generic battery block: It gives the aging effect of the battery.
• Scope block: To display the results of the system under study.

Parameter:

Generic Battery: 

Simulation time: 3600*10 (see f = 1/T) We provide 3600 seconds as the input time for the life cycle, thus charging and discharging are finished in an hour. It simulates five life cycles for ten hours.

Result:

Fully charging and discharging the battery once is referred to as one cycle. When the voltage plot indicates that a battery's voltage limit has not been exceeded, it is under the limit.

Conclusion:

• The thermal effects of the battery pack during cycling charging and discharging have been analyzed and plotted at various ambient temperatures.
• As you can see, the SOC reduces as temperature rises, meaning that if we operate the battery at a higher temperature than the rated temperature, the battery will discharge quickly and its life will decrease as a result.
• The amount of time needed to reach the desired temperature will decrease as the surface area in contact with convective heat transfer rises.