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Build Model of Battery Pack with Cell Balancing Circuit

This example shows how to create and build a Simscape™ system model of a battery pack with cell balancing circuits in Simscape™ Battery™. High voltage (> 60V) battery pack systems typically consist of multiple parallel assemblies or cells connected electrically in series. In these systems, the state-of-charge of individual parallel assemblies or cells often becomes unbalanced over time due to a variety of causes.

To create the system model of a battery pack, you must first create the Cell, ParallelAssembly, Module, and ModuleAssembly objects that comprise the battery pack, and then use the buildBattery function. The buildBattery function allows you to automatically generate Simscape models for these Simscape Battery objects:

This function creates a library in your working folder that contains a system model block of a battery pack that you can use as reference in your simulations. The run-time parameters for these models, such as the battery cell impedance or the battery open-circuit voltage, are defined after the model creation and are therefore not covered by the Battery Pack Builder classes. To define the run-time parameters, you can either specify them in the block mask of the generated Simscape models or use the MaskParameters argument of the buildBattery function.

To use the functions and objects in Simscape Battery, first import the required Simscape Battery package:

import simscape.battery.builder.*

Create Battery Pack Object in MATLAB

To create a battery pack, you must first design and create the foundational elements of the battery pack.

This figure shows the overall process to create a battery pack object in a bottom-up approach:

A battery pack comprises multiple module assemblies. These module assemblies, in turn, comprise a number of battery modules connected electrically in series or in parallel. The battery modules are made of multiple parallel assemblies which, in turn, comprise a number of battery cells connected electrically in parallel under a specific topological configuration or geometrical arrangement.

Create Cell Object

To create the battery Module object, first create a Cell object of cylindrical format.

cylindricalGeometry = CylindricalGeometry(Height = simscape.Value(0.07,"m"),...
    Radius = simscape.Value(0.0105,"m"));

The CylindricalGeometry object allows you to define the cylindrical geometrical arrangement of the battery cell. You can specify the height and radius of the cell by setting the Height and Radius properties of the CylindricalGeometry object. For more information on the possible geometrical arrangements of a battery cell, see the PouchGeometry and PrismaticGeometry documentation pages.

Now use this CylindricalGeometry object to create a cylindrical battery cell.

batteryCell = Cell(Geometry = cylindricalGeometry)
batteryCell = 
  Cell with properties:

            Geometry: [1x1 simscape.battery.builder.CylindricalGeometry]
    CellModelOptions: [1x1 simscape.battery.builder.CellModelBlock]
                Mass: [1x1 simscape.Value]

Show all properties

For more information, see the Cell documentation page.

Create ParallelAssembly Object

A battery parallel assembly comprise multiple battery cells connected electrically in parallel under a specific topological configuration or geometrical arrangement. In this example, you create a parallel assembly of four cylindrical cells stacked in a square topology over four rows.

To create the ParallelAssembly object, use the Cell object you created before and specify the NumParallelCells, Rows, and Topology properties according to your design.

batteryParallelAssembly = ParallelAssembly(Cell = batteryCell,...
    NumParallelCells = 4, ...
    Rows = 4, ...
    Topology = "Square", ...
    ModelResolution = "Detailed");

For more information, see the ParallelAssembly documentation page.

Create Module Object

A battery module comprises multiple parallel assemblies connected in series. In this example, you create a battery module of 4 parallel assemblies with an intergap between each assembly of 0.005 meters. You also define the model resolution of the module and add an ambient thermal boundary condition.

To create the Module object, use the ParallelAssembly object you created in the previous step and specify the NumSeriesAssemblies, InterParallelAssemblyGap, and ModelResolution properties.

batteryModule = Module(ParallelAssembly = batteryParallelAssembly,...
    NumSeriesAssemblies = 4, ...
    InterParallelAssemblyGap = simscape.Value(0.005,"m"), ...
    ModelResolution = "Detailed");

For more information, see the Module documentation page.

Create ModuleAssembly Object

A battery module assembly comprises multiple battery modules connected in series or in parallel. In this example, you create a battery module assembly of two identical modules with an intergap between each module equal to 0.005 meters. By default, the ModuleAssembly object electrically connects the modules in series.

To create the ModuleAssembly object, use the Module object you created in the previous step and specify the InterModuleGap property.

batteryModuleAssembly= ModuleAssembly(Module = repmat(batteryModule,1,2),...
    InterModuleGap = simscape.Value(0.005,"m"));

For more information, see the ModuleAssembly documentation page.

Create Pack Object

You now have all the foundational elements to create your battery pack. A battery pack comprises multiple module assemblies connected in series or in parallel. In this example, you create a battery pack of 2 identical module assemblies with an intergap between each module assembly of 0.005 meters.

To create the Pack object, use the ModuleAssembly object you created in the previous step and specify the InterModuleAssemblyGap property.

batteryPack= Pack(ModuleAssembly = repmat(batteryModuleAssembly,1,2),...
    InterModuleAssemblyGap = simscape.Value(0.005,"m"));

For more information, see the Pack documentation page.

Define Cell Balancing Strategy

The Pack object allows you to define a cell balancing strategy. Specifying a balancing strategy adds an ideal passive balancing circuit to every parallel assembly inside the battery pack. The balancing circuit consists of a balancing resistor connected in series to a signal controlled switch.

To define the balancing strategy of your battery, set the BalancingStrategy property of the batteryPack object to "Passive".

batteryPack.BalancingStrategy = "Passive";

Visualize Battery Pack and Check Model Resolution

To obtain the number of Simscape Battery Battery(Table-based) blocks used for the pack simulation, use the NumModels property of your Pack object.

disp(batteryPack.NumModels);
    64

To visualize the battery pack before you build the system model and to view its model resolution, use the BatteryChart object. Create the figure where you want to visualize your battery pack.

f = uifigure(Color="w");

Then use the BatteryChart object to visualize the battery module. To view the model resolution of the module, set the SimulationStrategyVisible property of the BatteryChart object to "On".

tl = tiledlayout(1,2,"Parent",f,"TileSpacing","Compact");
nexttile(tl)
batteryPackChart = BatteryChart(Parent = tl, Battery = batteryPack);
nexttile(tl)
batteryPackChart = BatteryChart(Parent = tl, Battery = batteryPack, SimulationStrategyVisible = "On");

{"String":"Figure contains objects of type simscape.battery.builder.batterychart.","Tex":[],"LaTex":[]}

For more information, see the BatteryChart documentation page.

Build Simscape Model for the Battery Module Object

After you have created your battery objects, you need to convert them into Simscape models to be able to use them in block diagrams. You can then use these models as reference for your system integration and requirement evaluation, cooling system design, control strategy development, hardware-in-the-loop, and much more.

To create a library that contains the Simscape Battery model of the Module object you created in this example, use the buildBattery function.

buildBattery(batteryPack,"LibraryName","packBalancingExample");
Generating Simulink library 'packBalancingExample_lib' in the current directory '/tmp/Bdoc22b_2134332_1821819/tpacc0024e/simscapebattery-ex75279834' ...

This function creates the packBalancingExample_lib and packBalancingExample SLX library files in your working directory. The packBalancingExample_lib library contains the Modules and ParallelAssemblies sublibraries.

To access the Simscape models of your Module and ParallelAssembly objects, open the packBalancingExample_lib. SLX file, double-click the sublibrary, and drag the Simscape blocks in your model.

The packBalancingExample library contains the Simscape models of your ModuleAssembly and Pack objects.

The Simscape models of your ModuleAssembly and Pack objects are subsystems. You can look inside these subsystems by opening the packLibrary SLX file and double-click the subsystem.

To learn how to implement a passive cell balancing strategy for a lithium-ion batery pack, see the Size Resistor for Battery Passive Cell Balancing example.