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.

Aim: To simulate the thermal effects and compare life cycle performance at various temperatures, charge & discharge rates of 10 series cells using MATLAB.

Theory:

lithium-ion (Li-ion) battery

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.

Li-ion batteries can use several different materials as electrodes. The most common combination is that of lithium cobalt oxide (cathode) and graphite (anode), which is most commonly found in portable electronic devices such as cellphones and laptops. Other cathode materials include lithium manganese oxide (used in hybrid electric and electric automobiles) and lithium iron phosphate. Li-ion batteries typically use ether (a class of organic compounds) as an electrolyte.

Construction of the circuit:

A detailed design of the MATLAB Simulink model for thermal modelling is given as:-

Cell 01 to 04 Subsystem:

Cell subsystem basically consists of a Thermal Model and Lithium Cell 1RC block which calculates the temperature and thermal effect for cells and overall calculates for the battery pack.

The block which is inside the cell subsystem is the PS Gain block which multiplies the input physical signal by a constant i.e. y = u.* gain. Other blocks are the voltage sensor and current-voltage source.

The voltage sensor block represents an ideal voltage sensor, that is, a device that converts voltage measured between any electrical connections into a physical signal proportional to the voltage.

The current-voltage source represents an ideal voltage source that is powerful enough to maintain the specified voltage at its output regardless of the current passing through it. The output voltage is V = Vs, where Vs is the numerical value presented at the physical signal port.

Thermal Model Subsystem:

The important block which is inside the thermal model subsystem are:

• Thermal Reference: This block represents a reference point in a thermal network where the temperature is equal to absolute zero.
• Controlled heat flow rate source: This block represents an ideal energy source in a thermal network that can maintain a controlled heat flow rate regardless of the temperature difference. The heat flow rate is set by the physical signal port S. A positive heat flow rate flows from port A to port B.
• Thermal Mass: This block models internal energy storage in a thermal network. The rate of temperature increase is proportional to the heat flow rate into the material and inversely proportional to the mass and specific heat of the material.
• 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 the temperature at port A is greater than the temperature at port B.

Lithium Cell 1RC Subsystem:

This block basically consists of a capacitance value table which depends on an external physical signal inputs SOC and T. It is assumed that the capacitance value is slowly varying with time, and hence the equation i = C*dv/dt holds.

Resistance value (R) table which depends on an external physical signal inputs SOC and T.

Voltage value (EM) implements the cell's main branch voltage source and determines values for capacity (C) and state of charge (SOC). The defining equations depend on cell temperature, T.

Thermal behaviour and fault setup:

Lithium-ion cell one RC-Branch equivalent circuit detail for 10 cells.

For cell 01 to 04

• The number of series-connected cells is 4
• The initial temperature is 299.1 K

For cell 05 (fault setup) In this cell, I have done some changes in the Electrical system of the cell so that we can see the fault.

• The number of series-connected cells is 1
• The initial temperature is 299.1 K
• 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
• C1 capacitance (farads) = C1_LUT*0.95

For cell 06 to 10

• The number of series-connected cells is 5
• The initial temperature is 299.1 K

Here the output of one cell is connected with the SOC calculation whereas another output is connected with the AC source for cyclic charge/discharge profile.

The peak amplitudes set to 50 A for cyclic charge and discharge. A basic parameter of AC source is as

The ideal AC source maintains the sinusoidal current through it, independent of the voltage across its terminals. The output current is defined by I = I0 * sin(2*pi*f*t + PHI), where I0 is the peak amplitude, f is the frequency in Hz, and PHI is the phase shift in radians.

The other block which is used in a lithium-ion battery pack is as:

Temperature Source:

This 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.

This block is used for ambient temperature and which I set as 299.1 K or 25.95 C.

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 the temperature difference, heat transfer coefficient, and surface area in contact with the fluid.

Used 3 blocks for convection.

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'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.

The conduction is done from the 2 sets of blocks, 1 block is used for 01-04 and the other is used for cell 06-10.

For SOC calculation Gain block, Integrator and Sum block with PS Simulink converter was used.

Results:

Simulation-based on different cell temperature:

In this simulation, ambient temperatures were set at 293 K or 25.95 C and other cell temperatures are as follows:

Calculated SOC:

As we see that the battery is fluctuating every 291 seconds which depend upon the thermal behaviour of the battery pack.

Cyclic charge / discharge profile:

Charge and discharge profile are depending upon Current (I) and Voltage (V) vs Time. The cyclic charge and discharge were calculated at 50A peak amplitude.

As we can see that in cell 5 the temperature rise to 320 and it goes fluctuating from 295 K to 323 K and end at 321 k.