The global electric vehicle (EV) industry has witnessed explosive growth in recent years. This rapid adoption, heavily driven by policy mandates and technological advances, has brought vehicle safety, particularly for high-mass commercial vehicles like pickup trucks, into sharp focus. A paramount concern is the integrity of the high-voltage EV battery pack during collision events. Unlike a conventional fuel tank, the EV battery pack is a complex, heavy, and energy-dense component intolerant to significant mechanical deformation. Compromise of its enclosure can lead to internal cell short circuits, electrolyte leakage, and potentially catastrophic thermal runaway. Statistics indicate that side impacts pose a disproportionately high risk for such failures. Therefore, ensuring the crashworthiness of the vehicle structure to protect the EV battery pack is not merely a regulatory compliance issue but a fundamental requirement for consumer confidence and sustainable market growth. This study presents a systematic investigation into optimizing the side impact safety of a pure electric pickup truck, with the core objective of safeguarding the EV battery pack through targeted body structure enhancement without altering the battery’s layout.
The design challenge is multifaceted. The EV battery pack is typically a large, flat module mounted longitudinally along the vehicle’s underbody, occupying significant space and contributing 20-30% of the vehicle’s total mass. This configuration lowers the center of gravity but creates a large, rigid “target” area vulnerable to intrusion from a side impact. The primary protective structure for the side-mounted EV battery pack consists of the rocker (sill), the floor crossmembers, and the B-pillar. A disruption in the load path or insufficient stiffness in any of these members can funnel collision energy directly into the EV battery pack region. Traditional approaches often focus on reinforcing the EV battery pack enclosure itself, adding weight and cost. This research adopts a novel, holistic perspective: optimizing the coupled body-battery system. The central hypothesis is that by strategically strengthening the vehicle’s inherent load path—specifically the floor crossmember—the collision energy can be managed and dissipated more effectively by the body structure itself, thereby creating a “safety zone” around the EV battery pack.
1. Regulatory Framework and Load Path Analysis
The design process is governed by stringent safety regulations which define the test conditions and performance criteria. For side impact, key standards include China’s GB 20071-2006 and the C-NCAP protocol, which prescribe a moving deformable barrier (MDB) test at 50 km/h. Other regimes, like the US IIHS, employ even more severe conditions. A comparative analysis is essential for developing a robust design.
| Regulatory Body / Standard | Test Type | Impact Speed | Key Performance Criteria | Implication for EV Battery Pack Protection |
|---|---|---|---|---|
| GB 20071-2006 / C-NCAP | Moving Deformable Barrier (MDB) | 50 km/h | Thoracic Injury Index (TTI), Abdomen Force, Pelvis Acceleration, Door Intrusion | Sets the baseline for structural integrity; intrusion limits indirectly protect the EV battery pack. |
| Euro NCAP | MDB & Pole Side Impact | 50 km/h (MDB), 32 km/h (Pole) | Similar biomechanical metrics, plus additional assessment for head protection. | Pole test creates a highly localized load, challenging the stiffness of specific members near the EV battery pack. |
| US IIHS | MDB (updated geometry) | 62 km/h | Measures intrusion at multiple points (B-pillar, sills); higher speed demands greater structural energy absorption. | Directly necessitates higher global body stiffness and efficient load paths to prevent EV battery pack enclosure crush. |
From a mechanics perspective, a side impact is a dynamic event governed by momentum transfer and energy dissipation. The initial kinetic energy of the barrier, $E_{k}$, is given by:
$$E_{k} = \frac{1}{2} m_{b} v_{b}^{2}$$
where $m_{b}$ is the barrier mass (e.g., 950 kg) and $v_{b}$ is the impact velocity (e.g., 13.89 m/s for 50 km/h). This energy must be absorbed through plastic deformation of the vehicle’s side structure and converted into internal energy (strain energy). The primary load path forms a continuous loop: Barrier -> Rocker -> Floor Crossmember/B-Pillar -> Roof Rail -> Opposite-side structure. The effectiveness of this path can be analyzed by the force flow and the energy absorption distribution among components.
If a component like the floor crossmember has insufficient bending stiffness, it may buckle prematurely, creating a local “soft spot.” This interrupts the load path, causing stress concentration and excessive local deformation. The intrusion velocity and displacement at the EV battery pack location become critical metrics. A simplified model for the force resisted by the rocker and crossmember system can be expressed as a function of crush distance:
$$F_{int}(x) = k_{1}x + k_{2}x^{2} + C_{d}\dot{x}$$
where $F_{int}$ is the intrusion force, $x$ is the intrusion distance, $k_{1}$ and $k_{2}$ represent structural stiffness coefficients (linear and nonlinear), and $C_{d}$ is a damping coefficient related to the material’s plastic deformation characteristics. The goal of optimization is to tailor $F_{int}(x)$ to maintain a high enough force level to divert energy, while ensuring the peak intrusion $x_{max}$ at the EV battery pack interface remains below a critical threshold (e.g., 5 mm).
2. Finite Element Model Development and Baseline Analysis
Computer-aided engineering (CAE) simulation is indispensable for modern vehicle safety development. A high-fidelity finite element model (FEM) of the complete vehicle was constructed to analyze the side impact scenario.
2.1 Modeling Methodology and Material Data
The model was built according to standard automotive CAE practices. Key structural components were meshed primarily with quadrilateral shell elements (S4R in LS-DYNA nomenclature) with a characteristic size of 5 mm. The mesh for the EV battery pack enclosure was refined to 3 mm to accurately capture local stresses and deformations. Non-structural masses were represented using discrete mass elements positioned at their respective centers of gravity to ensure correct inertial properties. The total model mass was calibrated to the vehicle’s design curb weight. Material properties were assigned based on engineering datasheets, with key high-strength steel grades defined using piecewise linear plasticity models. The original floor crossmember material was HC420/780DP.
| Material Designation | Young’s Modulus (E) | Poisson’s Ratio (ν) | Density (ρ) | Yield Strength (σy) | Application in Model |
|---|---|---|---|---|---|
| HC420/780DP | 210 GPa | 0.3 | 7.85e-9 tonne/mm³ | ~420 MPa | Original Floor Crossmember, Body Panels |
| HC550/980DP | 210 GPa | 0.3 | 7.85e-9 tonne/mm³ | ~550 MPa | Optimized Crossmember Reinforcement |
| Battery Enclosure Alloy | 70 GPa | 0.33 | 2.70e-9 tonne/mm³ | ~180 MPa | EV Battery Pack Housing |
The contact between all deformable parts was managed using an automatic single-surface algorithm. The MDB was modeled according to regulatory specifications, and an initial velocity of 50 km/h was applied. The simulation ran for 120 ms to capture the full dynamic event.
2.2 Baseline Simulation Results and Failure Mode Identification
The analysis of the initial design revealed a critical weakness. The floor crossmember, a key link between the rocker and the central tunnel, exhibited excessive bending deformation due to inadequate sectional stiffness and a suboptimal connection detail. The maximum intrusion displacement at the crossmember measured 43.9 mm. This large deformation directly translated into a severe inward punch of the crossmember towards the EV battery pack.
The consequence for the EV battery pack was immediate and unacceptable. The simulation showed the crossmember intruding into the space reserved for the EV battery pack, causing a calculated crush of the battery enclosure of approximately 16.6 mm. Local von Mises stress on the enclosure shell exceeded 200 MPa, far above its yield point, indicating a high probability of breach. The energy absorption analysis was revealing:
- Rocker: Absorbed ~40% of total energy (as intended).
- Floor Crossmember: Absorbed only ~15% before buckling.
- Excess Energy Path: Approximately 45% of the energy, not properly managed by the disrupted crossmember path, effectively became a concentrated load on the EV battery pack area.
The root cause was identified as a discontinuity in the load path. The force from the rocker could not be smoothly transferred through the crossmember to the tunnel and the opposite side, creating a local stress concentration factor ($K_{t}$) estimated from the simulation to be greater than 3 at the joint. The governing equation for bending stress ($\sigma_{b}$) highlights the problem:
$$\sigma_{b} = \frac{M y}{I}$$
where $M$ is the bending moment, $y$ is the distance from the neutral axis, and $I$ is the area moment of inertia of the cross-section. For the original single-sheet crossmember, $I$ was too low for the imposed moment $M$, leading to high stress $\sigma_{b} > \sigma_{y}$ and plastic hinge formation. This confirmed that reinforcing the floor crossmember to increase its $I$ and connection robustness was the most direct solution to protect the EV battery pack.
3. Structural Optimization Design and Simulation Validation
3.1 Proposed Design Enhancement
The optimization strategy focused solely on the floor crossmember assembly, adhering to the constraint of not modifying the EV battery pack or its mounting points. The solution was elegantly simple yet effective: transform the single-layer crossmember into a structured, reinforced closed-section.
Key Design Changes:
- Internal Reinforcement Plate: A new insert plate, stamped from higher-grade HC550/980DP steel with a thickness of 2.0 mm, was added inside the main crossmember profile.
- Section Stiffening: The central span of the crossmember, which aligned with the impact zone and the widest part of the EV battery pack, was redesigned with added embossments (beads). These geometric features significantly increase the bending and buckling resistance without adding substantial mass by enhancing the section’s moment of inertia.
The principle can be summarized by revisiting the bending stress formula. For a composite section, the effective moment of inertia $I_{eff}$ increases dramatically. If the original crossmember has inertia $I_{orig}$, adding a reinforcement plate at a distance $d$ from the original neutral axis contributes an additional term via the parallel axis theorem. The new inertia $I_{new}$ is approximately:
$$I_{new} \approx I_{orig} + I_{reinforcement} + A_{reinforcement} \cdot d^{2}$$
where $A_{reinforcement}$ is the area of the added plate. The $A \cdot d^{2}$ term is dominant, leading to a substantial increase in $I_{new}$. Consequently, for the same bending moment $M$, the resulting bending stress is greatly reduced: $\sigma_{b,new} \ll \sigma_{b,orig}$, delaying or preventing yield and buckling.
3.2 Simulation of the Optimized Design
The updated crossmember model was integrated into the full vehicle FEM, and the side impact simulation was rerun under identical conditions. The results demonstrated a transformative improvement:
| Performance Metric | Baseline Design | Optimized Design | Improvement |
|---|---|---|---|
| Max. Floor Crossmember Intrusion | 43.9 mm | 14.8 mm | 66% Reduction |
| EV Battery Pack Enclosure Crush | 16.6 mm | ≤ 2.0 mm | Deemed negligible; no risk of breach. |
| Peak Stress on Battery Enclosure | > 200 MPa | < 80 MPa | Well below yield strength. |
| Energy Absorbed by Floor Crossmember | ~15% | ~35% | Became a major, controlled energy absorption member. |
The post-processed results showed a completely different deformation mode. The reinforced crossmember now acted as a robust beam, resisting bending and effectively transferring load into the central tunnel and across the vehicle. The load path was continuous, with no evidence of localized buckling. The stress contour plots showed a smooth, distributed pattern of energy absorption through controlled plastic deformation of the crossmember and rocker. The EV battery pack enclosure was now situated in a stable, minimally deformed space, fulfilling the primary safety objective.
4. Physical Test Validation
To conclusively verify the simulation predictions and the real-world efficacy of the design in protecting the EV battery pack, a full-scale physical crash test was conducted according to GB 20071-2006.
4.1 Test Setup and Execution
A prototype vehicle, built to the optimized design specifications and with a mass of 2342 kg, was prepared. The high-voltage EV battery pack was installed and charged to a 50% state of charge (SOC). An array of instrumentation was deployed: accelerometers on the B-pillar and barrier, string potentiometers to measure intrusion at key locations (rocker, crossmember, door), and specialized sensors to monitor the EV battery pack voltage and temperature during the event. The vehicle was subjected to a 50 km/h side impact from a standardized 950 kg MDB, targeting the B-pillar area.
4.2 Test Results and Correlation with Simulation
The test was successful. Visual inspection post-impact showed significant but controlled deformation of the rocker and door, with no catastrophic failure. Crucially, the passenger compartment and the zone housing the EV battery pack remained largely intact. Subsequent detailed measurement and teardown analysis provided quantitative validation.
The physical intrusion measurements showed excellent correlation with the CAE predictions:
- Floor Crossmember Max Intrusion: 15.2 mm (measured) vs. 14.8 mm (simulated). The error rate was less than 3%, demonstrating high model fidelity.
- EV Battery Pack Enclosure Deformation: Direct measurement after careful removal of the pack revealed a maximum permanent set of 1.8 mm, consistent with the simulated prediction of ≤2.0 mm. No cracks, leaks, or internal cell displacements were observed.
- Battery Electrical Integrity: Voltage monitoring showed only minor, within-specification fluctuations (±2%). The temperature rise was negligible (<5°C), confirming no internal short circuits or onset of thermal events.
The energy management strategy was also validated. Post-test analysis of deformation patterns and calculated forces confirmed the intended load path was active: the rocker absorbed approximately 42% of the energy, and the optimized floor crossmember absorbed about 36%. This balanced distribution confirmed the creation of a robust, continuous load path that shielded the EV battery pack.
| Validation Aspect | Simulation Prediction | Physical Test Result | Correlation & Conclusion |
|---|---|---|---|
| Structural Intrusion (Crossmember) | 14.8 mm | 15.2 mm | Excellent (<3% error). Model is predictive. |
| EV Battery Pack Crush | ≤ 2.0 mm | 1.8 mm | Excellent. Safety margin confirmed. |
| Battery Functional Safety | No breach predicted | No leakage, stable voltage/temperature | EV battery pack integrity fully maintained. |
| Primary Load Path Members | Rocker & Crossmember | Rocker & Crossmember (by deformation analysis) | Design intent for energy distribution achieved. |
5. Conclusion
This study successfully addressed the critical challenge of protecting a high-voltage EV battery pack during a side impact collision for a pure electric pickup truck. By shifting the design paradigm from isolated component protection to a coupled body-battery system approach, an efficient and cost-effective solution was developed. The core of the solution lay in the targeted optimization of a single body structure—the floor crossmember—through the addition of an internal reinforcement plate and local section stiffening. This modification dramatically increased the crossmember’s bending stiffness and energy absorption capacity, quantified by a 66% reduction in its maximum intrusion (from 43.9 mm to 14.8 mm).
The finite element simulation proved to be a highly accurate development tool, with its predictions for both structural deformation and EV battery pack clearance validated by physical testing with an error margin below 3%. The optimized design ensured the EV battery pack experienced less than 2 mm of enclosure deformation, a level deemed safe against electrolyte leakage or internal cell damage. Importantly, this safety enhancement was achieved without any alteration to the EV battery pack itself—its layout, size, or mounting remained unchanged—minimizing cost and complexity. The added mass from the reinforcement was marginal, and the cost increase was estimated at less than 5%, demonstrating high engineering feasibility.
In conclusion, the research establishes a validated methodology for EV battery pack safety development. It underscores that strategic reinforcement of existing vehicle load paths can create a robust protective cage around the EV battery pack, effectively managing collision energy before it reaches the battery enclosure. This “body structure-first” strategy provides a scalable and economical reference for the safety design of electric vehicles, particularly light commercial vehicles like pickup trucks where payload, cost, and ruggedness are paramount. Future work will explore the application of this principle under more complex multi-directional impact loading and its integration with advanced battery pack monitoring systems for holistic safety management.