The relentless evolution of the electric vehicle (EV) sector is fundamentally tied to advancements in core technologies, with the EV battery pack standing as the most critical and costly component. Its design directly influences vehicle range, performance, and, most importantly, safety. Among various crash scenarios, the side pole impact presents a particularly severe challenge. Unlike frontal or offset impacts, the vehicle’s side structure offers minimal crumple zone, causing concentrated forces that can directly threaten the integrity of the floor-mounted EV battery pack, potentially leading to catastrophic thermal runaway. This study delves into a critical engineering trade-off: material selection for the EV battery pack enclosure. While aluminum alloy packs are celebrated for their excellent strength-to-weight ratio, they contribute significantly to overall vehicle cost. A shift to steel, a more economical material, presents an attractive avenue for cost reduction but introduces new challenges in vehicle dynamics and crash safety due to increased mass and differing material properties.
This investigation was initiated with the objective of evaluating the feasibility of substituting an existing aluminum alloy EV battery pack with a steel counterpart in a specific vehicle platform to achieve cost savings. The initial step involved a detailed Finite Element Analysis (FEA) of the complete vehicle under side pole impact conditions, adhering to the protocol outlined in C-NCAP 2021. The rigid pole was modeled as a *RIGIDWALL in LS-DYNA with an initial velocity of 32 km/h. The integrity of the simulation model was paramount. The mesh consisted of approximately 1.85 million elements, with 2D shell elements (primarily quadrilaterals) representing the body-in-white and 3D solid elements for complex geometries. Triangle elements were rigorously controlled to be less than 5% of the total shell elements to ensure computational accuracy. Model validation included checks for artificial mass scaling, which was maintained below 5% of total mass, and energy balance, where hourglass energy was confirmed to be less than 5% of internal energy, ensuring a physically credible result.
The simulation of the vehicle equipped with the new steel EV battery pack revealed a critical issue. The increased mass of the steel pack altered the vehicle’s dynamic response. The side structure, particularly the door and the rocker (or sill) assembly, underwent excessive deformation. The collision load path transferred significant force through the rocker into the mounting brackets (lifting lugs) of the EV battery pack. Consequently, the steel enclosure itself experienced substantial inward deformation. The key metric, the lateral (Y-direction) intrusion into the EV battery pack shell, was measured. The allowable intrusion limit, defined by the physical clearance between the battery modules and the inner wall of the pack, was 60 mm. The initial steel design yielded a maximum intrusion of 66.67 mm, breaching the safety threshold and indicating a high risk of module short-circuit and subsequent thermal runaway. This finding necessitated a structural reinforcement strategy.
The rocker assembly was identified as the primary load-bearing and energy-absorbing component in this loading condition. Its performance is governed by the synergistic interaction of its constituent panels. To enhance the crashworthiness without a complete redesign, a parametric optimization focused on the thickness of three key rocker components was undertaken: Outer Panel 1 (A), Outer Panel 2 (B), and the Internal Reinforcement (C). A full-factorial design of experiments for three factors at three levels would require 27 simulations, which is computationally expensive and time-consuming. Therefore, an efficient Orthogonal Experimental Design (OED) was employed. OED is based on mathematical principles of factorial design and allows for the evaluation of multiple factors and their interactions with a significantly reduced number of trials. The selected orthogonal array was L9(3^4), accommodating three factors at three levels. The factors and their levels (thickness in mm) are defined below:
| Factor | Description | Level 1 | Level 2 | Level 3 |
|---|---|---|---|---|
| A | Outer Panel 1 Thickness | 1.8 | 2.0 | 2.2 |
| B | Outer Panel 2 Thickness | 1.0 | 1.2 | 1.4 |
| C | Internal Reinforcement Thickness | 1.6 | 1.8 | 2.0 |
The optimization pursued a dual-objective function: to minimize both the lateral intrusion of the EV battery pack shell (I) and the mass increase of the reinforced rocker components (M). This can be formulated as finding the design vector X = [A, B, C] that minimizes a composite objective. A simple weighted formulation, though not explicitly solved algebraically here, guided the selection:
$$ F(\mathbf{X}) = w_1 \cdot I(\mathbf{X}) + w_2 \cdot M(\mathbf{X}) $$
where $w_1$ and $w_2$ are weights prioritizing safety and lightweighting, respectively. The nine orthogonal trials were simulated, and the results for intrusion and component mass were recorded.
| Trial No. | A (mm) | B (mm) | C (mm) | Rocker Mass (kg) | Pack Intrusion (mm) |
|---|---|---|---|---|---|
| 1 | 1.8 | 1.0 | 1.6 | 3.759 | 63.98 |
| 2 | 1.8 | 1.2 | 1.8 | 4.183 | 57.89 |
| 3 | 1.8 | 1.4 | 2.0 | 4.601 | 52.17 |
| 4 | 2.0 | 1.0 | 1.8 | 4.005 | 63.60 |
| 5 | 2.0 | 1.2 | 2.0 | 3.951 | 55.20 |
| 6 | 2.0 | 1.4 | 1.6 | 4.568 | 57.05 |
| 7 | 2.2 | 1.0 | 2.0 | 4.250 | 58.95 |
| 8 | 2.2 | 1.2 | 1.6 | 4.390 | 61.15 |
| 9 | 2.2 | 1.4 | 1.8 | 4.814 | 56.08 |
Analysis of the orthogonal results is crucial. While Trial 3 achieved the lowest intrusion (52.17 mm), it came with the highest rocker mass (4.601 kg) among the promising candidates. Trial 5 offered a compelling balance: its intrusion of 55.20 mm was well within the 60 mm safety limit and represented a drastic improvement from the initial 66.67 mm, while its associated rocker mass (3.951 kg) was notably lower than that of Trial 3. The mass difference of 0.65 kg, though seemingly small, contributes to overall vehicle weight reduction, aligning with lightweighting principles and mitigating some of the added mass from the steel EV battery pack. Therefore, based on the dual objectives of safety (minimized intrusion) and mass efficiency, the configuration from Trial 5 (A=2.0 mm, B=1.2 mm, C=2.0 mm) was selected as the optimal solution for the steel EV battery pack vehicle.
The final step involved physical validation. Full-scale side pole impact tests were conducted according to C-NCAP 2021 specifications on two vehicles: the baseline with the aluminum alloy EV battery pack and the modified vehicle with the steel EV battery pack and the optimized rocker design (Trial 5). Both vehicles showed controlled deformation of the A-pillar and B-pillar, maintaining survival space for occupants. As predicted, the side doors exhibited significant crushing. Post-crash inspection of the underbody revealed distinct differences. The aluminum EV battery pack showed localized deformation only at the mounting lugs, with its enclosure remaining largely undeformed. The steel EV battery pack vehicle showed more pronounced deformation at the lugs and a slight bulge on the pack enclosure; however, the intrusion was visually confirmed to be less severe than the initial unoptimized simulation, and critically, it did not compromise the battery modules. Both vehicles underwent the standard post-crash electrical safety observation period, and the results related to electrical isolation were as per protocol, confirming that the optimized design met core safety requirements.
The discussion now shifts from specific design optimization to a broader, multi-perspective comparison between aluminum alloy and steel for EV battery pack enclosures. The choice of material is a strategic decision with far-reaching implications.
1. Crash Safety and Structural Integration: Aluminum alloys (e.g., 6xxx series) typically offer higher specific strength (strength-to-density ratio) than mild steels. This allows an aluminum EV battery pack to achieve required stiffness and strength with thinner gauges, directly contributing to vehicle lightweighting. A lighter vehicle has lower kinetic energy, which generally translates to lower crash forces. The steel EV battery pack, while offering high absolute strength, has a higher density. Its integration increases the vehicle’s sprung mass and alters the inertial response during a crash. As demonstrated, this often necessitates reinforced surrounding structures (like the rocker) to manage the altered load paths and protect the pack, adding complexity and partial mass back. The optimal solution lies in integrating the pack as a structural member, an area where both materials are being actively developed with tailored alloys and advanced joining techniques.
2. Economic and Manufacturing Considerations: This is the primary driver for considering steel. The raw material cost differential is substantial. While prices fluctuate, high-strength steel typically costs significantly less per kilogram than automotive-grade aluminum alloy. For a large component like an EV battery pack, this translates into direct and significant savings in the Bill of Materials (BOM). Furthermore, the manufacturing supply chain for steel stamping, welding, and corrosion protection is mature and widespread, potentially offering lower processing costs compared to the more specialized techniques often required for aluminum, such as friction stir welding or self-piercing rivets. The economic argument for steel is powerful, especially for cost-sensitive vehicle segments.
3. Environmental Impact and Lifecycle Assessment: The “green” credential of an EV is increasingly scrutinized through a full lifecycle lens, from material production to end-of-life recycling. The production of primary aluminum is extremely energy-intensive and is a major source of direct (process) CO2 emissions, particularly if the electricity grid is carbon-intensive. Estimates suggest the carbon footprint per kilogram of primary aluminum can be multiples higher than that of steel. Using a steel EV battery pack enclosure can substantially reduce the embedded manufacturing emissions of the vehicle. The equation evolves when considering recycled content and use-phase emissions (where lightweight aluminum improves efficiency), but for the production phase, steel holds a clear advantage, supporting broader carbon neutrality goals.
| Aspect | Aluminum Alloy EV Battery Pack | Steel EV Battery Pack |
|---|---|---|
| Primary Advantage | High specific strength, excellent lightweighting potential. | Low raw material cost, mature manufacturing supply chain. |
| Key Challenge | High material and sometimes processing cost. | Higher density requires careful crash management and can impact efficiency. |
| Impact on Vehicle Mass | Lower, beneficial for driving range and performance. | Higher, may necessitate compensatory lightweighting elsewhere. |
| Embedded Production Carbon | Generally higher per kilogram of material. | Generally lower per kilogram of material. |
| Design Flexibility | Often used in extruded or cast forms for complex structures. | Excellent for high-strength stamped and roll-formed sections. |
In conclusion, this integrated study demonstrates that a direct substitution of an aluminum EV battery pack with a steel one is not a straightforward task due to the consequential impact on vehicle crash dynamics. However, through systematic simulation and efficient optimization techniques like Orthogonal Experimental Design, the safety deficits can be effectively addressed. The optimal solution (Trial 5) successfully reduced the lateral intrusion of the steel EV battery pack to a safe level while managing the associated mass increase. The subsequent multi-angle comparison reveals a non-trivial trade-off. The aluminum EV battery pack excels in lightweighting and specific performance, a traditional advantage. The steel EV battery pack presents a compelling case based on cost economics and a lower production-phase carbon footprint, which is becoming critically important. There is no universally superior material. The optimal choice for an EV battery pack enclosure depends on the specific vehicle platform’s priorities: a premium or range-focused model may justify the cost of aluminum, while a high-volume, cost-conscious model may find the steel solution, especially when paired with optimized peripheral structures like the rocker, to be the most balanced and strategic answer. This research provides a methodological framework and comparative insights to guide that critical decision-making process for automotive engineers and strategists.