3. Battery pack architecture and crash protection requirements

An electric vehicle battery pack is not a single component but a hierarchical assembly. Crash protection requirements propagate downward through that hierarchy: vehicle-level crash pulses define what the pack must survive, the pack-level structure distributes those loads, the module frames and cell cans absorb what remains, and the enclosure keeps the whole system sealed, cooled, and electrically isolated. Understanding this chain is a prerequisite for any LS-DYNA model, because the fidelity of each level determines which physical phenomena you can actually capture.

The Five-Level Hierarchy

The conventional decomposition runs from smallest to largest: cell → module → pack → enclosure → vehicle integration. Each level has distinct structural behavior and distinct failure modes.

• Cell: The electrochemical unit. Common formats are cylindrical (e.g., 21700, 4680), prismatic (stacked electrode pouch in a hard can), and pouch (laminated aluminum film). The cell can is a thin-walled pressure vessel; its dominant crash concerns are can buckling, internal short circuit from separator rupture, and jelly-roll or electrode-stack displacement.
• Module: A group of cells mechanically restrained by frames, end plates, busbars, and often a thermal interface. The module is where cell-to-cell spacing, compression preload, and electrical isolation are established.
• Pack: The complete energy storage assembly including modules, interconnects, busbars, contactors, BMS harness, cooling plates, and structural cross-members.
• Enclosure: The tray and lid that carry the pack, seal it against water and dust (typically IP67 or IP6K9K), and provide the primary load path to the vehicle body.
• Vehicle integration: The interface between pack and body-in-white (BIW) — mounting points, rocker panels, floor cross-members, and the underbody shield.

The critical insight for simulation is that load path continuity matters more than any single component's strength. A pack that is stiff in isolation but mounted through compliant bushings will still allow large relative displacement between the enclosure and the BIW, which can rupture coolant lines or high-voltage cabling.

Crash Load Cases and Their Physical Demands

Battery pack crash requirements are derived from regulatory and internal load cases. The most consequential for pack design are:

Each case produces a different stress state. Frontal and rear impacts are dominated by axial crush and bending of the enclosure. Side impacts are dominated by local indentation and shear of the module frames. Underbody strikes are local contact problems where a small rigid object penetrates the tray. A single LS-DYNA model rarely captures all of these well; the mesh density and material models that resolve a pole intrusion are different from those needed for a full-vehicle frontal pulse.

From Requirement to Simulation Model

Translating a crash requirement into a model involves three decisions: what to represent explicitly, what to lump, and what to constrain.

For a full-vehicle frontal model, the pack is often represented with shell elements for the enclosure, solid or shell elements for module frames, and lumped masses for cell stacks. This is adequate when the question is whether the enclosure buckles or whether mounts fail. It is not adequate when the question is whether a specific cell is crushed.

For a side pole intrusion study, the model must resolve the cell stack. A common approach is to model the pole, the rocker, the enclosure side wall, the module frame, and a representative cell block with a crushable material model. The cell block is often represented with *MAT_CRUSHABLE_FOAM or *MAT_SIMPLIFIED_JOHNSON_COOK calibrated to cell-level compression tests, because modeling every cell can is computationally prohibitive.

For a module-level compression study, the model can be detailed: individual cell cans with shell elements, end plates, and busbars. This is where you calibrate the homogenized material properties used in the larger models.

Enclosure Design and Its Crash Role

The enclosure is the pack's primary structural element. It must simultaneously:

• Carry inertial loads from the modules (which can exceed 20 g in a severe crash) without excessive deflection.
• Provide a sealed environment — any breach of the tray or lid compromises IP rating and can expose live conductors.
• Integrate with the vehicle's crash load paths, typically through the rocker panels and floor cross-members.
• Manage thermal loads from the cooling plate, which is often bonded or welded to the tray floor.

In LS-DYNA, the enclosure is usually modeled with shell elements using *MAT_024 (piecewise linear plasticity) for steel or aluminum, or *MAT_036 for high-strength steel with strain-rate effects. The choice of material model matters: aluminum trays fail by tearing at weld lines, while steel trays fail by buckling and plastic hinge formation. Spot welds and structural adhesives are represented with *MAT_100 (spot weld) or cohesive elements respectively.

Vehicle Integration and Load Path Continuity

The pack-to-body interface is where many crash protection strategies succeed or fail. Key considerations:

• Mounting strategy: Rigid bolted mounts transfer load directly but transmit vibration; elastomeric bushings isolate vibration but allow relative motion. In crash, bushing compliance can cause the pack to shift and rupture coolant or HV lines.
• Load path alignment: The pack's internal cross-members should align with the vehicle's floor cross-members so that crash loads flow through the pack rather than into it.
• Ground clearance and underbody shielding: A shield or skid plate can distribute underbody impact loads over a larger area, reducing local puncture risk.

The figure below summarizes how a side-impact load path propagates from the pole through the vehicle structure into the pack, and where each level of the hierarchy is engaged.

Practical Modeling Implications

Three practical consequences follow from this architecture.

First, sub-modeling is essential. A full-vehicle model cannot resolve cell-level failure. The standard workflow is to run the full-vehicle model with a homogenized pack, extract the displacement or force boundary condition at the pack interface, then apply that to a detailed pack or module sub-model. In LS-DYNA this is done with *INTERFACE_COMPONENT or by mapping nodal displacements from the global run.

Second, material calibration must match the level. A cell stack homogenized as crushable foam needs a stress-strain curve from cell-level compression tests, not from coupon tests of the can material. Mixing calibration levels is a common source of error.

Third, boundary conditions dominate results. Whether the pack is modeled as rigidly mounted or bushing-mounted changes the predicted intrusion by tens of millimeters. The mounting representation should match the actual vehicle build, including bushing stiffness and preload.

With the hierarchy and load paths established, the next step is to select the appropriate LS-DYNA element formulations and material models for each level — which is the subject of the following section.