Parts and Instances functionality in Optistruct

instancing of CELL IN battery pack modules in optistruct

Manohara Mariyappa, Altair / Narendran Subramanian­, Altair / Rejeesh Rajan, Altair October 27, 2021


Large finite element models typically consist of many elements and nodes. One of the ways to reduce the size and complexity of the finite element model is to use the Parts and Instances (PI) functionality in Optistruct. The PI method is used to combine independently created substructures or parts into a single model. The locations of the instances are defined using translations and rotational transformations of the global part(s) or independent substructures. Besides allowing for changes on independent substructure to be carried forward to the dependent instances automatically, the PI method provides a modular environment for different teams to work on independent substructures that can be assembled into the final finite element model.

Problem Setup

Fig 1 shows the FE model of 1 cell instanced 90 times to create an assembly of a 7 x 13 array. The model consists of the Battery cell, which is the repeating part that can be instanced, and the Module cover and Bus bar that are the non-repeating parts of the Module. The size of the model is reduced by following a few simple numbering requirements.

Figure 1 – Instancing of Battery Pack to create the Module

The process of creating the final .fem file is shown in Fig 2. The images for different include files show the example for 2 instances of the Battery cell. This is the .fem file that needs to be run along with the different include files.

Figure 2 – Contents of final .fem file with different include files

Simulations and Results

The primary intention of this study is to demonstrate Parts and Instances capabilities in Optistruct in static and linear dynamic solutions. Optistruct 2021.2 is used to run the analysis. The out file shows the model information as seen in Fig 3. This kind of approach is more relevant today than ever because of EV revolution where high fidelity battery pack FE models could run upwards of 8 million elements.  Parts and instances can significantly reduce modeling efforts and model size.


Figure 3 – Finite Element Model Data Information from out file


The first loadcase is the modal analysis requesting natural frequencies between 0 and 300 Hz. The results are as shown in Fig 4

Figure 4 – Modes of the Battery Pack Module in 0-200 Hz

The second loadcase involves a pressure load of 0.5 MPa applied on the top surface of the Battery Pack Module in conjunction with a temperature load of 130 deg to the entire assembly. The Fig 5 shows the displacement and stress contours on the Full assembly.

Figure 5 – Displacement and Stress on Full Assembly


The Parts and instances functionality allows greater flexibility in the creation of a finite element model by using simple transformations which alleviates modeling burden and time. This functionality is available for all analysis solution sequences in Optistruct.


[1] Optistruct user guide