The automotive industry serves as a cornerstone of national economic development, with electric vehicles (EVs) epitomizing the convergence of low-carbon initiatives and intelligent technology. In daily life, the presence of new energy vehicles is ubiquitous, representing a critical breakthrough for green development, economic growth, and industrial restructuring under policy guidance. Within this context, the EV battery pack stands as the most vital component, directly determining key performance parameters such as driving range and safety. The structural integrity and reliability of the EV battery pack are paramount, ensuring operational stability and occupant safety during vehicle operation. During collision events, the EV battery pack is subjected to immense impact forces within milliseconds, potentially leading to mechanical deformation, internal short circuits, or electrolyte leakage. Traditionally, addressing such safety concerns relied on physical vehicle testing or empirical methods, which were not only resource-intensive but also lacked precision. However, advancements in computer simulation software and the finite element method (FEM) have revolutionized this domain, enabling virtual crash analysis that significantly reduces development costs and offers engineers diverse solutions. This article, from my perspective as a researcher in automotive passive safety, delves into a comprehensive structural safety assessment of an EV battery pack using finite element analysis, focusing on static, modal, and crush simulations, followed by an optimization design to enhance performance.
Finite element modeling forms the foundation of this analysis. The three-dimensional geometry of a specific EV battery pack was initially created in CATIA and subsequently imported into ANSA for geometry cleanup and preprocessing. The EV battery pack structure primarily consists of sheet metal components. The modeling process involved several key steps: First, two-dimensional shell elements were used to discretize the thin-walled structures. Second, a mesh size standard of 6 mm was applied, employing a mixed meshing strategy to balance accuracy and computational efficiency. Third, to accurately represent mass distribution during strength analysis, concentrated mass points (MASS entries) were utilized. The total mass of the EV battery pack, including all internal modules, was set at 800 kg. This mass was distributed appropriately across the pack’s lower body floor based on the actual installation points of the battery modules. Material properties were assigned critically: the connection brackets between the EV battery pack and the vehicle body, which bear significant loads, were made from Q355 steel. This low-cost material offers excellent bending and torsional resistance, making it prevalent in automotive design. The primary enclosure or case of the EV battery pack, responsible for protecting the internal cells, was modeled using DC01 steel, a material with higher density suitable for structural protection. The final finite element model of the EV battery pack comprised 191,412 elements and 175,595 nodes, with a total calculated mass of 805 kg.
Static analysis was performed to evaluate the EV battery pack’s strength under various driving conditions. Vehicles encounter diverse road scenarios, transmitting loads through the suspension to the body and attached components like the EV battery pack. Four typical load cases were defined to simulate extreme but realistic scenarios, as summarized in the table below. These cases involve specific constraints at the mounting points (degrees of freedom 1 through 6 are constrained) and inertial loads representing acceleration forces.
| Load Case | Constraints | Applied Loads |
|---|---|---|
| Left Turn on Bumpy Road | Mounting points fixed (DOF 1-6) | Y-direction: 0.75g, Z-direction: 1g |
| Right Turn on Bumpy Road | Mounting points fixed (DOF 1-6) | Y-direction: -0.75g, Z-direction: 1g |
| Braking on Bumpy Road | Mounting points fixed (DOF 1-6) | Z-direction: 3g |
| Emergency Braking on Bumpy Road | Mounting points fixed (DOF 1-6) | X-direction: 1.02g, Z-direction: 1g |
These load cases were solved using the OptiStruct solver, and results were post-processed in HyperView. The analysis revealed critical insights. For the left and right turn cases, the maximum displacement of approximately 10.90 mm occurred in the central region of the EV battery pack’s lower cover, an area lacking direct structural support. Stress was primarily concentrated on the internal module support brackets, with peak values of 196.21 MPa and 192.86 MPa, respectively, both below the yield strength of Q355 (355 MPa). The braking case proved most severe. The lower cover displacement reached 32.69 mm, and the maximum stress on the internal brackets soared to 583.61 MPa, exceeding the material’s yield limit and indicating a high risk of plastic deformation or failure. The emergency braking case showed a displacement of 11.01 mm and a stress of 194.42 MPa, within the safe limit. The fundamental stress relationship is governed by $$ \sigma = \frac{F}{A} $$ where $\sigma$ is stress, $F$ is force, and $A$ is cross-sectional area. The excessive stress in the braking case highlighted a design flaw: insufficient structural stiffness in the EV battery pack’s lower cover, particularly due to the absence of transverse reinforcing elements.
Modal analysis is crucial for the EV battery pack to avoid resonant vibrations that could accelerate fatigue or damage. During operation, the EV battery pack is excited by frequencies from the road (typically up to 30 Hz) and the electric drive motor (around 25 Hz). Resonance occurs when an excitation frequency coincides with a natural frequency of the EV battery pack structure. The national standard GB/T 31467 specifies a vibration frequency range of 0-200 Hz for such analysis. A free-free modal analysis was conducted, extracting the first ten modes, with the first six being most relevant. The natural frequency $f_n$ for a simple system relates to stiffness $k$ and mass $m$ by $$ f_n = \frac{1}{2\pi} \sqrt{\frac{k}{m}} $$. For complex structures like an EV battery pack, FEM solves the eigenvalue problem $$ ( [K] – \omega_n^2 [M] ) \{\phi_n\} = 0 $$ where $[K]$ is the stiffness matrix, $[M]$ is the mass matrix, $\omega_n$ is the angular frequency ($\omega_n = 2\pi f_n$), and $\{\phi_n\}$ is the mode shape vector.
| Mode Order | Frequency (Hz) | Mode Shape Description |
|---|---|---|
| 1 | 16.22 | Vertical vibration of the upper cover |
| 2 | 32.45 | Lateral “breathing” vibration of the upper cover |
| 3 | 32.48 | Longitudinal “breathing” vibration of the upper cover |
| 4 | 39.09 | Vertical vibration of the lower cover |
| 5 | 47.91 | Combined breathing vibration of the upper cover |
| 6 | 52.46 | Vertical vibration at the rear section of the upper cover |
The first natural frequency of the EV battery pack at 16.22 Hz is sufficiently separated from the motor’s idle frequency. However, the second and third modes (32.45 Hz and 32.48 Hz) are perilously close to the common road excitation frequency of 30 Hz, posing a potential resonance risk that necessitates design modification.
Crush analysis simulates the mechanical abuse an EV battery pack might suffer in a side impact or undercarriage collision. The simulation follows the Chinese national standard GB/T 31467.3. The setup involves a rigid cylindrical or semi-cylindrical indenter with a radius of 75 mm. The crush is applied along two principal directions: the X-direction (vehicle longitudinal/front-rear) and the Y-direction (vehicle lateral/side-to-side). The simulation continues until either the crush force reaches 100 kN or the deformation reaches 30% of the EV battery pack’s dimension in the crush direction. The safety criterion requires that the EV battery pack does not catch fire or explode, and ideally, the internal modules should not be contacted by the deforming enclosure at the 100 kN force level. The force-displacement curves from the initial design analysis showed that the maximum contact force in both X and Y directions failed to reach 100 kN before excessive deformation occurred. This indicates that the structural stiffness of the EV battery pack’s enclosure was inadequate to withstand the regulatory force threshold, thus failing the safety requirement. The contact force $F_c$ during crushing is a complex function of material hardening, geometry, and deformation: $$ F_c = \int_{A_c} \sigma(\epsilon) \, dA $$ where $A_c$ is the contact area and $\sigma(\epsilon)$ is the stress as a function of strain.
Based on the findings from static, modal, and crush analyses, a structural optimization was imperative to enhance the safety of the EV battery pack. The core weakness was identified as the insufficient stiffness of the lower cover. To address this, a reinforcing plate or bolster was designed and integrated into the EV battery pack structure. This plate is positioned beneath the lower cover, extending laterally to connect with the side walls of the EV battery pack enclosure, forming a unified, robust structure through welding. The geometry of this reinforcing plate is strategically shaped to provide maximal support to the unsupported central region of the lower cover where large displacements occurred.
The optimized EV battery pack model was subjected to the same battery of analyses. For static strength, all four load cases now showed stresses within the safe limits of the materials. The maximum stress on the lower cover (DC01 material, yield strength ~270 MPa) and the internal brackets (Q355) for each case are summarized below, demonstrating a significant improvement, especially for the critical braking case.
| Load Case | Max Stress on Lower Cover (MPa) | Max Stress on Internal Brackets (MPa) |
|---|---|---|
| Left Turn on Bumpy Road | 89.93 | 88.75 |
| Right Turn on Bumpy Road | 83.06 | 92.33 |
| Braking on Bumpy Road | 259.40 | 265.73 |
| Emergency Braking on Bumpy Road | 86.53 | 89.74 |
The modal analysis of the optimized EV battery pack revealed a positive shift in natural frequencies. The first six modal frequencies showed a slight increase, which is expected due to the added stiffness from the reinforcing plate. The first mode increased to 16.32 Hz, and crucially, the second and third modes shifted to 34.59 Hz and 34.86 Hz, respectively. This creates a more substantial margin from the 30 Hz road excitation frequency, significantly mitigating the resonance risk. The change in natural frequency due to added stiffness $\Delta k$ can be approximated by $$ \Delta f_n \approx \frac{f_n}{2k} \Delta k $$ for a single-degree-of-freedom system, illustrating the direct relationship.
The crush simulation of the optimized EV battery pack yielded compliant results. In the X-direction, the contact force reached the 100 kN threshold at approximately 0.072 seconds, exhibiting a stable rising curve. At this force level, the strain field within the internal battery modules was extracted. The maximum principal strain value was effectively zero, confirming that the deformed enclosure had not yet contacted the sensitive internal components of the EV battery pack. Similarly, in the Y-direction, the 100 kN force level was attained at around 0.013 seconds. Again, the strain analysis of the internal modules showed a maximum value of zero, indicating no encroachment. This satisfies the regulatory intent, demonstrating that the optimized EV battery pack structure can withstand significant mechanical abuse while protecting its core energy storage elements. The improvement in crush performance stems from the increased global bending stiffness provided by the reinforcing plate, which resists indentation more effectively. The effective stiffness $K_{eff}$ of a plate under bending is proportional to the cube of its thickness and its moment of inertia $I$: $$ K_{eff} \propto E \cdot I $$ where $E$ is the Young’s modulus. The reinforcing plate effectively increases the moment of inertia of the EV battery pack’s lower section.
In conclusion, this systematic analysis underscores the critical importance of rigorous virtual testing in the design phase of an EV battery pack. The initial design, while adequate for some loads, revealed vulnerabilities under specific braking conditions, potential resonance with road vibrations, and insufficient crush resistance. By integrating a strategically designed reinforcing plate into the EV battery pack’s lower structure, all identified issues were mitigated. The optimized EV battery pack exhibited safe stress levels across all typical driving loads, improved modal frequencies that avoided key excitation bands, and met the stringent 100 kN crush force requirement while maintaining isolation for the internal battery modules. This holistic approach to EV battery pack structural safety, employing finite element analysis for static, modal, and crush simulations, provides a robust and cost-effective framework. It offers practical significance for automotive OEMs, enabling them to design EV battery packs that are not only lightweight and efficient but also fundamentally reliable and safe, thereby supporting the broader adoption and trust in electric vehicles. The iterative process of analysis, identification, and optimization highlighted here is essential for advancing the structural integrity of every EV battery pack deployed on the road.