COMSOL 6.3 - Thermal Runaway Propagation in a Battery Pack

Thermal Runaway Propagation in a Battery Pack

Due to abuse, such as internal or external short circuits or excessive heating, an individual battery cell may go into thermal runaway, during which the battery cell generates a significant amount of heat. If enough heat is transferred between adjacent cells during a thermal runaway event, neighboring cells will also go into thermal runaway.

A thermal runaway propagating through the whole pack constitutes a severe safety hazard. When designing a battery pack, measures need to be taken in order to mitigate runaway propagation.

This tutorial simulates the heat transfer and resulting thermal runaway propagation in a pack consisting of 20 cylindrical cells using event-based heat sources.

For a general introduction to the battery pack interface, the user is also referred to the tutorial Thermal Distribution in a Pack of Cylindrical Batteries.

Model Definition

Figure 1 shows the model geometry. The pack consists of 20 cylindrical batteries in a 5s4p configuration. In a 5s4p configuration, 4 sets of battery cells are connected in parallel, and each set contains 5 serially connected individual battery cells. Two plastic holder frames are used for fixating the battery locations and cell-to-cell distances. Serial connectors are welded to the battery terminals, and parallel connectors are in turn welded to the serial connectors, midway between the battery cylinders. A thin plastic wrapping encloses the whole pack, thereby forming a compartment of quiescent air surrounding the battery cylinders.

The following materials are used from the built-in material library:

•

Plastic holders: Acrylic Plastic

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Connectors and battery terminals: Steel AISI 4340

•

Air compartment: Air

The battery pack model is set up in a similar manner as in the Thermal Distribution in a Pack of Cylindrical Batteries tutorial, using the interface together with a interface and an multiphysics node. The node in heat transfer is used to define the anisotropic properties of the batter cylinders.

The pack is charged from an initial state of charge of 25% at a 2C rate. The node is used to control the load current so that it is turned of when the pack voltage exceeds 21 V. The initial pack temperature is set to equal the surrounding temperature of 30°C.

Figure 1: Model geometry.

The cell thermal runaway is modeled by the use of nodes in the battery pack interface. At the triggering of a thermal event in a battery cylinder, a time-dependent heat source is released as shown in Figure 2 with a total released heat equal to 22 kJ. The heat profile was taken from Ref. 1. Due to the loss of electrolyte and the resulting increase of the internal battery resistances, the ohmic resistance of the corresponding battery model in each cylinder is increased about two orders of magnitude when a thermal event is triggered.

To initiate the propagation of the thermal runaway, one cell is assumed to be start malfunctioning at an early stage in the charge cycle (due to an internal short circuit, or similar). The triggering of the malfunctioning cell is set explicitly to begin at t = 1 min into the simulation. For the remaining cells, the thermal events are set to be triggered when the maximum temperature of an individual cell exceeds 80°C.

Figure 2: Heat source released in an individual battery cell when a thermal event is triggered.

Results and Discussion

Figure 3 shows the pack voltage and maximum battery temperature in the pack versus time. The steep gradients in the temperature curve indicate the triggering of the thermal events. At the triggering of each event, a small shift in the pack voltage curve can also be seen, which is related to the corresponding increase in cell resistance, impacting the current distribution in the pack.

During the first minute, only a small temperature increase can be observed. This is due to Joule heating and other processes during normal charging operation. After triggering of the first cell runaway at 1 min, there is a steep increase in the maximum temperature, after which there is an incubation time of several minutes until enough heat has spread to a neighboring cell in order to trigger consecutive runaways. After triggering of this second runaway, the intervals between successive cell runaways become shorter.

After about 10 min, the maximum charging voltage limit has been reached, and the charging of the pack is turned off. However, the thermal runaway continues to propagate through the pack also after this.

A few minutes later, all cells have gone through thermal runaway, and the maximum temperature slowly starts to relax toward the surrounding temperature of 30°C.

Figure 3: Pack voltage and maximum battery temperature in the pack.

The temperature in the pack, excluding the air domain, at various times is shown in Figure 4.

Figure 4: Temperature surface plots at various times.

Reference

1. P.T. Coman, E.C. Darcy, and R.E. White, “Simplified Thermal Runaway Model for Assisting the Design of a Novel Safe Li-Ion Battery Pack,” J. Electrochem. Soc., vol. 169, p. 040516, 2022.

Battery_Design_Module/Thermal_Management/thermal_runaway_propagation

Modeling Instructions

From the menu, choose .

New

In the window, click

Model Wizard

In the window, click

In the tree, select > > .

Click .

In the tree, select > .

Click .

Click

In the tree, select > .

Click

Geometry 1

Import the geometry sequence from a file.

In the toolbar, click and choose .

Browse to the model’s Application Libraries folder and double-click the file thermal_runaway_propagation_geom_sequence.mph.

In the dialog, select in the list.

Click .

In the toolbar, click

Inspect the imported geometry. You can enable and disable wireframe rendering and transparency to view internal geometry features.

Click the

button in the toolbar.

In the window, under click .

Disable the analysis of the geometry as the remaining small geometric details can be kept.

In the window, click .

In the window for , locate the section.

Clear the checkbox.

Geometry 1

In the window, collapse the > node.

Global Definitions

Geometry Parameters

Some geometry parameters were imported together with the geometry sequence.

In the window, under click .

In the window for , type Geometry Parameters in the text field.

Physics Parameters

Import some additional parameters needed for setting up the physics.

In the toolbar, click

and choose > .

In the window for , type Physics Parameters in the text field.

Locate the section. Click

Browse to the model’s Application Libraries folder and double-click the file thermal_runaway_propagation_physics_parameters.txt.

Add Material

In the toolbar, click

to open the window.

Go to the window.

Add some materials from the built-in material library.

In the tree, select > .

Right-click and choose .

In the tree, select > .

Right-click and choose .

In the tree, select > .

Right-click and choose .

In the toolbar, click

to close the window.

Materials

Air (mat1)

In the window for , locate the section.

From the list, choose .

Acrylic plastic (mat2)

In the window, click .

In the window for , locate the section.

From the list, choose .

Steel AISI 4340 (mat3)

In the window, click .

In the window for , locate the section.

From the list, choose .

Battery Pack (bp)

The battery pack interface is active on the battery and conductor domains only.

In the window, under click .

In the window for , locate the section.

From the list, choose .

Batteries

In the window, under > click .

In the window for , locate the section.

From the list, choose .

Locate the section. In the Qcell,0 text field, type Q_cell.

In the SOCpack,0 text field, type SOC_0.

Cell Equilibrium Potential 1

In the window, click .

In the window for , locate the section.

Click

Browse to the model’s Application Libraries folder and double-click the file thermal_runaway_propagation_E_OCP_data.txt.

In this tutorial we will keep the default value of 0 for the temperature derivative of the open circuit voltage for all SOC values. The reason is that during the thermal runaway, very high temperatures are expected, which in turn would result in inaccurate cell open circuit voltage values for nonzero values of the derivative.

Voltage Losses 1

In the window, click .

In the window for , locate the section.

In the ηIR,1C text field, type eta_1C.

Locate the section. In the J0 text field, type J0.

Locate the section. Select the checkbox.

In the τ text field, type tau.

Batteries

This model will make use of two thermal events. The first event triggers a heat source in a malfunctioning cell at an explicit time.

In the window, click .

Thermal Event 1

In the toolbar, click

and choose .

In the window for , locate the section.

Click

Select Domains 24–26 only.

Locate the section. From the list, choose .

In the texp text field, type t_start.

In the table, enter the following settings:


Time after event (s)	Heat source (W)
t_peak	Qh_peak

Click

In the table, enter the following settings:


Time after event (s)	Heat source (W)
t_tr	0

Select the checkbox.

Locate the section. Click

Create a named selection of the malfunctioning cell. This will make it easy to select the complementary (nonmalfunctioning) battery cells.

In the dialog, type Malfunctioning Cell in the text field.

Click .

Definitions (comp1)

In the window, expand the > node.

Nonmalfunctioning Cells

In the window, expand the > > node.

Right-click > and choose > .

In the window for , type Nonmalfunctioning Cells in the text field.

Locate the section. Under , click

In the dialog, select in the list.

Click .

In the window for , locate the section.

Under , click

In the dialog, select in the list.

Click .

Battery Pack (bp)

Thermal Event 1

Now duplicate the first thermal event to create a second one for the complementary cells that is to be triggered if the maximum temperature exceeds a certain trigger level.

Thermal Event 2

In the window, under > > right-click and choose .

In the window for , locate the section.

From the list, choose .

Locate the section. From the list, choose .

In the Tmax,te text field, type T_trigger.

Current Conductors

Ground the negative terminals of the pack as follows:

In the window, under > click .

In the window for , locate the section.

From the list, choose .

Ground 1

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Current Conductors

Use a charge-discharge cycling condition on the positive terminals of the pack. This will turn off the charging current when a specified voltage maximum level is reached.

In the window, click .

Charge-Discharge Cycling 1

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Locate the section. In the Ich text field, type C_rate*I_1C_pack.

In the Vmax text field, type E_max_pack.

Select the checkbox.

In the trest,ch text field, type 1[h].

Locate the section. From the list, choose .

Negative Connectors

Finalize the pack part of the model by specifying how the batteries are connected to the conductor domains.

In the window, under > click .

In the window for , locate the section.

From the list, choose .

Positive Connectors

In the window, click .

In the window for , locate the section.

From the list, choose .

Heat Transfer in Solids (ht)

Now setup the heat transfer part of the model. Start with specifying the properties of the layers (jelly roll) of the active battery materials.

In the window, under click .

Battery Layers 1

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Locate the section. From the list, choose .

In the ktl text field, type kT_batt_tl.

In the kil text field, type kT_batt_il.

In the ρeff text field, type rho_batt.

In the Cp,eff text field, type Cp_batt.

Initial Values 1

In the window, click .

In the window for , locate the section.

In the T text field, type T_ext.

Heat Flux 1

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Locate the section. From the list, choose .

In the h text field, type h_conv.

In the Text text field, type T_ext.

Multiphysics

Electrochemical Heating 1 (ech1)

In the toolbar, click

and choose > .

Mesh 1

For this model we will create a swept mesh in the longitudinal axis direction of the battery cylinders.

Identical Mesh 1

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Locate the section. From the list, choose .

Free Triangular 1

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Size 1

Right-click and choose .

In the window for , locate the section.

Click the button.

Locate the section.

Select the checkbox. In the associated text field, type gap/2.

Select the checkbox. In the associated text field, type 1.5.

Size

In the window, under > click .

In the window for , locate the section.

From the list, choose .

Click

Swept 1

In the toolbar, click

In the window for , locate the section.

From the list, choose .

Click

Free Tetrahedral 1

In the toolbar, click

In the window for , click

Definitions (comp1)

Add probes for the pack voltage and the maximum and average battery temperatures before solving. The probes will output their corresponding values to a table for each step taken by the time-dependent solver.

Global Variable Probe 1 (var1)

In the toolbar, click

and choose .

In the window for , click in the upper-right corner of the section. From the menu, choose > > > .

Locate the section.

Select the checkbox. In the associated text field, type Pack voltage.

Click to expand the section. Click

Click

Domain Probe 1 (dom1)

In the toolbar, click

and choose .

In the window for , locate the section.

From the list, choose .

Locate the section. From the list, choose .

Click in the upper-right corner of the section. From the menu, choose > > > .

Locate the section. In the field, type degC.

Select the checkbox. In the associated text field, type Maximum temperature in batteries.

Click to expand the section. Click

From the list, choose .

Domain Probe 2 (dom2)

In the window, right-click and choose .

In the window for , locate the section.

From the list, choose .

Locate the section. In the text field, type Average temperature in batteries.

Study 1

Step 1: Time Dependent

In the window, under click .

In the window for , locate the section.

From the list, choose .

In the text field, type range(0,5,20).

Solution 1 (sol1)

In order to improve the accuracy of the solution, set the initial step of the time-dependent solver to 0.1 s. This will ensure that the heat released by each thermal event is properly resolved in the time domain.

In the toolbar, click

In the window, expand the node, then click .

In the window for , click to expand the section.

In the text field, type 0.1[s].

Click to expand the section. Also disable storing of the time-derivatives. This will avoid numerical artifacts when interpolating between solutions at different times when creating an animation of the runaway propagation. It will also reduce the required disk space for storing the model.

Clear the checkbox.

In the toolbar, click

Results

Temperature (ht)

Modify the default temperature plot as follows:

In the window, under click .

In the window for , locate the section.

Clear the checkbox.

Volume 1

In the window, expand the node.

Right-click and choose .

Temperature (ht)

In the window for , click to expand the section.

From the list, choose .

Surface 1

Right-click and choose .

In the window for , locate the section.

In the text field, type T.

In the field, type degC.

Locate the section. From the list, choose .

In the toolbar, click

Temperature (ht)

In the window, click .

In the window for , click to expand the section.

From the list, choose .

In the text field, type Time=eval(round(t)) s.

Click the

button in the toolbar.

In the toolbar, click

You may now change the time in the section to plot the temperature at various times. By default the solver is set to output the solution before and after triggering of an event hence the dual entries for some of the times in the list.

Locate the section. From the list, choose .

In the toolbar, click