As the automotive industry shifts toward electrification, the EV battery pack has become a critical component, directly impacting vehicle performance, safety, and range. Lightweighting is a key strategy to enhance the energy efficiency of electric vehicles, often achieved through advanced materials like sheet molding compound (SMC) for the cover. However, structural integrity issues, such as cracking during durability tests, pose significant challenges. In this study, I investigate a cracking incident in an EV battery pack cover observed during road endurance testing for an SUV model. Using finite element simulation, I aim to identify the root cause by analyzing modal responses, vehicle-level strength, random vibrations, and assembly-induced stresses. The goal is to provide insights into design and manufacturing improvements for EV battery pack reliability.
The EV battery pack is a complex assembly consisting of battery modules, an upper cover, a lower tray, sealing gaskets, structural adhesives, and auxiliary components. The cover, typically made from SMC composite, serves as a protective enclosure that ensures insulation and sealing against environmental factors. In this case, the EV battery pack exhibited symmetrical micro-cracks on the upper cover after prolonged durability testing, as shown in the image above. These cracks were located near bolt attachment points, prompting a detailed analysis to prevent potential safety hazards and warranty claims. The EV battery pack in question weighed approximately 370 kg and was mounted underneath the vehicle body via ten bolt connections, making it susceptible to various dynamic loads.
To model the EV battery pack for simulation, I employed HyperWorks software, specifically the HyperMesh module for finite element modeling. The geometry was discretized using shell elements for the cover and tray, with solid elements for the battery modules to account for mass distribution. The SMC cover material was assigned orthotropic properties, while the lower tray and internal beams were modeled as steel with spot welds. Battery modules were simulated as concentrated masses connected via bar elements representing mounting bolts. The complete finite element model included constraints at the mounting points to replicate real-world boundary conditions. Material properties for the EV battery pack components are summarized in Table 1.
| Component | Material | Density (t/mm³) | Elastic Modulus (GPa) | Poisson’s Ratio | Tensile Strength (MPa) |
|---|---|---|---|---|---|
| Upper Cover | SMC | 1.70 × 10⁻⁹ | 12.0 | 0.38 | 65 |
| Lower Tray | DC06 Steel | 7.85 × 10⁻⁹ | 210 | 0.30 | 120 |
| Internal Beams | HC340LA Steel | 7.85 × 10⁻⁹ | 210 | 0.30 | 480 |
| Battery Modules | Generic Composite | 2.30 × 10⁻⁹ | 28.0 | 0.30 | N/A |
| Structural Adhesive | Epoxy | 1.32 × 10⁻⁹ | 1.003 | 0.48 | N/A |
Modal analysis was first conducted to assess whether road-induced resonance could cause the cracking in the EV battery pack cover. The natural frequencies of the structure must avoid the typical road excitation range of 10–30 Hz to prevent resonant amplification. The constrained modal analysis involved fixing the mounting points in all six degrees of freedom, as per GB 38031-2020 standards. The governing equation for free vibration is given by:
$$ M\ddot{q} + Kq = 0 $$
where \( M \) is the mass matrix, \( K \) is the stiffness matrix, and \( q \) is the displacement vector. Solving the eigenvalue problem:
$$ (K – \omega^2 M)q = 0 $$
yields the natural frequencies \( \omega \). Using the Block Lanczos method, the first mode of the EV battery pack cover was found at 48 Hz, with a shape corresponding to vertical bending. This frequency exceeds the 30 Hz threshold, indicating sufficient stiffness to avoid resonance with road irregularities. Thus, modal analysis ruled out vibration-induced cracking for this EV battery pack.
Next, I performed vehicle-level strength analysis to evaluate the EV battery pack cover under extreme driving conditions, such as pothole braking, wheel hop, and cornering. These scenarios generate inertial forces and body deformations that transmit loads to the EV battery pack. A full-vehicle finite element model was constructed, incorporating the body-in-white, suspension, and the EV battery pack assembly. Load cases were derived from standard durability profiles, applying accelerations and displacements at tire contact patches. The stress distribution on the EV battery pack cover was examined, with results summarized in Table 2.
| Load Case | Maximum Stress on Cover (MPa) | Material Allowable Stress (MPa) | Safety Margin |
|---|---|---|---|
| Pothole Braking | 12.8 | 40–60 (based on tensile strength) | Adequate |
| Wheel Hop | 10.6 | Adequate | |
| Cornering | 9.8 | Adequate |
The stresses remained well below the SMC material’s tensile strength of 65 MPa, with a safety factor exceeding 3.0. This confirmed that extreme road loads were not the primary cause of cracking in the EV battery pack cover. However, the stress concentrations near bolt holes aligned with crack locations, hinting at other factors.
Random vibration analysis was then employed to account for stochastic road excitations that the EV battery pack encounters during operation. Road roughness is characterized by a power spectral density (PSD) function, which describes the energy distribution across frequencies. The response of the EV battery pack to random inputs was computed using modal superposition. The PSD of stress response \( S_{xx}(\omega) \) is derived from:
$$ S_{xx}(\omega) = |H(\omega)|^2 S_{ff}(\omega) $$
where \( H(\omega) \) is the frequency response function and \( S_{ff}(\omega) \) is the input PSD. The root mean square (RMS) stress \( \sigma_{\text{rms}} \) is obtained by integrating the PSD over the frequency range. For reliability, the 3σ value (three times the RMS) is compared against material limits, representing a 99.7% confidence level. The random vibration analysis for the EV battery pack cover yielded the results in Table 3.
| Direction | RMS Stress (MPa) | 3σ Stress (MPa) | Allowable Stress (MPa) |
|---|---|---|---|
| Vertical (Z) | 9.1 | 27.3 | 40–60 |
| Lateral (Y) | 4.2 | 12.6 | |
| Longitudinal (X) | 1.6 | 4.8 |
While the maximum 3σ stress of 27.3 MPa occurred at the crack sites, it still fell within the allowable range. Thus, random vibrations alone were insufficient to cause failure in the EV battery pack cover. This led me to suspect assembly-related issues, as the EV battery pack is bolted to the vehicle body with potential misalignment.
I focused on installation-induced stresses, considering that the EV battery pack mounting points on the body might have planar deviations due to manufacturing tolerances. According to GB 50205-2001, high-strength bolt connections require flatness errors within 1 mm. I measured the actual positions of the ten mounting points on the body using coordinate measuring equipment. The deviations in x, y, and z directions are listed in Table 4, showing exceedances of the standard.
| Mounting Point ID | Deviation in X (mm) | Deviation in Y (mm) | Deviation in Z (mm) |
|---|---|---|---|
| 1 (Left) | 0 | 0 | -0.613 |
| 2 (Right) | 0 | 0 | 0.591 |
| 3 (Left) | 0 | 0 | -0.079 |
| 4 (Right) | -5.411 | -0.187 | 0.862 |
| 5 (Left) | 0 | 0 | 0.562 |
| 6 (Right) | 0 | 0 | -0.539 |
| 7 (Left) | -0.215 | 0 | 1.769 |
| 8 (Right) | -4.736 | 0 | 0.038 |
| 9 (Left) | 0 | 3.51 | -0.228 |
| 10 (Right) | -3.529 | 4.616 | -5.009 |
These deviations were applied as enforced displacements in the finite element model to simulate the assembly condition. The analysis solved for stresses due to geometric misfit, using the linear static equation:
$$ K u = F $$
where \( u \) is the displacement vector from installation errors, and \( F \) is the resulting force vector. The stress results, shown in Table 5, revealed peak values exceeding the material’s endurance limit.
| Analysis Case | Maximum Stress on Cover (MPa) | Material Failure Criteria (MPa) | Conclusion |
|---|---|---|---|
| Forced Displacement (Pre-optimization) | 65.9 | 40–60 | Cracking Likely |
| Forced Displacement (Post-optimization) | 25.2 | 40–60 | Safe |
The pre-optimization stress of 65.9 MPa localized precisely at the observed crack positions, confirming that installation errors were the primary driver of failure in the EV battery pack cover. This highlights the sensitivity of large EV battery pack structures to assembly tolerances.
To rectify the issue, I recommended adjustments to the body assembly process for the EV battery pack. This involved refining welding sequences and adding shims to reduce planar deviations at mounting points, ensuring flatness within 1 mm per standards. After implementation, the forced displacement analysis showed a stress reduction to 25.2 MPa, well within safe limits. The modified EV battery pack was then subjected to a 45,000 km road durability test, with no cracks appearing on the cover. This validated the effectiveness of process controls in enhancing the reliability of the EV battery pack.
In conclusion, this study demonstrates a comprehensive approach to diagnosing cracking in an EV battery pack cover using finite element simulation. Modal, strength, and random vibration analyses eliminated road-induced factors, while forced displacement analysis pinpointed installation errors as the critical cause. The EV battery pack’s large span and multiple attachment points make it vulnerable to manufacturing variances, emphasizing the need for tight tolerances in production. The solution involved simple工艺 adjustments, avoiding costly design changes. This case underscores the importance of integrating assembly considerations into the design phase for EV battery pack systems. Future work could explore advanced materials or adaptive mounting systems to further robustness. Overall, the methodology provides a valuable framework for ensuring the structural integrity of EV battery packs in the evolving automotive landscape.