The relentless pursuit of extended driving range and enhanced energy efficiency in electric vehicles has placed immense emphasis on the lightweight design of critical components. The EV battery pack, a substantial mass located at the vehicle’s underbody, represents a prime target for mass reduction. However, this objective must be rigorously balanced against stringent safety requirements. The underbody of an EV battery pack is perpetually exposed to road hazards such as curbstones, road debris, and uneven surfaces. Statistics indicate a significant proportion of underbody impact events involve obstacles with heights between 150mm and 250mm. Such impacts pose a severe threat, potentially causing internal cell damage, short circuits, thermal runaway, and consequently, fire or explosion. Therefore, developing a bottom protective structure for the EV battery pack that is both lightweight and possesses high mechanical resistance is a critical engineering challenge.
Traditionally, steel plates have been the dominant material for EV battery pack bottom protection due to their high strength and formability. Common solutions include high-strength steel (e.g., HC340-590DP), ultra-high-strength hot-stamped steel (e.g., B1500HS), and combinations of steel with cushioning foams. Alternative designs employ aluminum extrusions or aluminum panels. While effective, these metallic solutions contribute significantly to the overall mass of the EV battery pack. In this context, advanced polymer-based and cellular metal composites emerge as promising candidates. This study investigates several novel composite materials for the bottom plate of an EV battery pack, aiming to evaluate their performance against standardized mechanical abuse tests while achieving a substantial mass reduction compared to conventional steel solutions.
Our research methodology follows a comprehensive, experiment-simulation integrated approach. We begin with material-level characterization, proceed to component-level validation, and finally conduct full EV battery pack system tests. The primary mechanical abuse condition under investigation is the “bottom ball strike,” which simulates the pack running over a hemispherical obstacle. Two key load cases are defined: an energy-based impact (e.g., 120 J) and a force-based quasi-static indentation (e.g., 25 kN). The latter, being more severe, is selected as the primary load case for this study. The performance requirement is to prevent coolant leakage from the cold plate and, most critically, to minimize intrusion into the battery cells to avoid internal short circuits.
We selected five distinct composite material samples for the bottom plate application, all with a nominal thickness of 8 mm, to facilitate a direct comparison:
- 8 mm PPA Composite: A polyphthalimide (PP) honeycomb structure reinforced with glass fibers, manufactured via molding.
- 8 mm Aramid A: A composite panel primarily based on aramid fibers.
- 8 mm Aramid B: A different formulation or layup of aramid-based composite.
- 0.4 g/mL Foamed Aluminum: A cellular aluminum structure with a density of 0.4 grams per milliliter.
- 0.5 g/mL Foamed Aluminum: A cellular aluminum structure with a higher density of 0.5 grams per milliliter.
The key properties of these materials compared to traditional options are summarized in Table 1.
| Material Type | Exemplary Grade | Tensile Strength (MPa) | Typical Thickness (mm) | Key Characteristics |
|---|---|---|---|---|
| High-Strength Steel | HC340-590DP | 590 | 1.0 – 1.2 | High strength, ductile, heavy |
| Hot-Stamped Steel | B1500HS | ~1500 | ~0.8 | Very high strength, lightweight for steel |
| Aluminum Panel | Al6061-T6 | ~270 | ~2.0 | Lightweight, good specific strength |
| PPA Composite (This Study) | – | ~31* | 8.0 | Very lightweight, high specific energy absorption |
| Aramid Composite (This Study) | – | High* | 8.0 | High strength-to-weight, prone to tearing |
| Foamed Aluminum (This Study) | – | Low* | 8.0 | Lightweight, excellent energy absorption, plastic deformation |
*Note: Representative values; actual composite properties are direction-dependent and characterized via testing.
Material-Level Characterization and Model Calibration
To accurately predict the performance of an EV battery pack with these composite plates, we first must develop precise computational material models. This was achieved through sheet-level quasi-static indentation tests and subsequent finite element model calibration. A 150 mm diameter hemispherical indenter was pressed into a simply supported sample of each material at a constant speed of 0.1 mm/s. The reaction force versus displacement curve was recorded for each test. Figure 1 shows the test setup for the PPA composite sample.
The force-displacement response, $F(u)$, is governed by the combined structural and material stiffness of the sample. For a homogeneous material model in simulation, the key is to derive an accurate effective stress-strain ($\sigma-\epsilon$) relationship. The total strain can be related to displacement via a geometric matrix $\mathbf{B}$:
$$\boldsymbol{\epsilon}_{total} = \mathbf{B} \cdot \mathbf{u}$$
For linear elastic behavior, stress is proportional to elastic strain: $\boldsymbol{\sigma} = \mathbf{E} \cdot \boldsymbol{\epsilon}_e$, where $\mathbf{E}$ is the elasticity tensor. Upon yielding, the strain decomposes into elastic and plastic parts:
$$\boldsymbol{\epsilon}_{total} = \boldsymbol{\epsilon}_e + \boldsymbol{\epsilon}_p$$
Therefore, the plastic strain can be expressed as:
$$\boldsymbol{\epsilon}_p = \boldsymbol{\epsilon}_{total} – \mathbf{E}^{-1} \cdot \boldsymbol{\sigma}$$
The core challenge in simulation is that the initial material parameters (like the elastic modulus and default plastic curve) often do not capture the complex crushing behavior of composites or foams. Our calibration process involves iteratively adjusting the material’s constitutive law in the finite element model until the simulated $F(u)$ curve matches the experimental one. The excellent match post-calibration for the PPA composite is shown in Figure 2. This calibrated model now reliably represents the material’s mechanical response for system-level simulation. We repeated this rigorous calibration process for all five composite materials.
| Material Sample | Post-Test Condition | Calibration Outcome | Key Observations |
|---|---|---|---|
| 8 mm PPA | Minor permanent dent, significant elastic recovery. | Excellent match achieved. | High energy absorption through controlled crushing. |
| 8 mm Aramid A | Catastrophic tearing/fracture. | Match achieved up to point of failure. | High initial stiffness, but fails by tearing, compromising seal integrity. |
| 8 mm Aramid B | Catastrophic tearing/fracture. | Match achieved up to point of failure. | Similar to Aramid A, with slightly different force plateau. |
| 0.4 g/mL Foamed Al | Large permanent crush zone. | Excellent match achieved. | Plastic collapse of cell structure, stable force plateau. |
| 0.5 g/mL Foamed Al | Large permanent crush zone (less than 0.4). | Excellent match achieved. | Higher crush stress than lower density foam. |
Full EV Battery Pack Ball Strike Simulation
With calibrated material models, we constructed a detailed finite element model of the complete EV battery pack. The model included the battery box (lower tray), the various composite bottom plates (modeled separately), the battery modules (cells housed in a module frame), the cooling plate, and the top cover. Connections such as adhesives and bolts were appropriately modeled. The EV battery pack was supported at its mounting points, and the same 150mm diameter sphere was driven into the most critical location on the bottom plate with a quasi-static displacement to a peak force of 25 kN, as mandated by the safety standard.
The primary metric for evaluation is the intrusion into the battery cell located directly beneath the impact point. Excessive intrusion can crush the cell, leading to internal short circuits. The original cell height was 118.22 mm. The simulated intrusion values for the different bottom plate options are critical for down-selection. The governing equation for the system’s resistance can be conceptualized as a series of stiffnesses, where the bottom plate is the first line of defense. The force transmitted to the cell, $F_{cell}$, is a function of the crushing response of the bottom plate $R_{plate}(u)$ and the global structural stiffness $K_{pack}$:
$$F_{cell} = f(R_{plate}(u), K_{pack}, u)$$
Our simulation directly solves this complex interaction. The intrusion $\delta$ is then:
$$\delta = \max(u_{cell}) – u_{cell,initial}$$
The simulation results for cell intrusion are summarized in Table 3. The performance target was set at an intrusion of less than 6 mm to ensure cell integrity.
| Bottom Plate Material | Simulated Cell Intrusion (mm) | Performance vs. Target (6 mm) | Remarks on Deformation Mode |
|---|---|---|---|
| 8 mm PPA Composite | 5.21 | Pass | Plate crushes progressively, distributing load. |
| 8 mm Aramid A | 5.87 | Pass (Marginal) | Plate fractures, leading to concentrated load on cell. |
| 8 mm Aramid B | 5.92 | Pass (Marginal) | Similar fracture mode as Aramid A. |
| 0.4 g/mL Foamed Al | 5.36 | Pass | Plate collapses plastically, absorbing energy. |
| 0.5 g/mL Foamed Al | 5.51 | Pass | Higher strength than 0.4 foam, slightly less crushing. |
Experimental Validation on EV Battery Pack Prototypes
To validate the simulation findings, we manufactured physical prototypes of the EV battery pack fitted with each of the five composite bottom plates. Each complete EV battery pack was instrumented with voltage and temperature sensors on the critical cells. The pack was mounted on a test rig with bottom supports replicating the simulation boundary conditions. A servo-hydraulic test system with a 150 mm diameter indenter was used to apply a quasi-static load up to 25 kN at a speed of 0.1 mm/s, exactly mirroring the simulation setup.
The experimental outcomes provided crucial insights not fully captured by simulation alone, particularly regarding failure modes and post-impact condition. The key results are consolidated in Table 4, which includes mass data—a fundamental parameter for the lightweight objective of the EV battery pack.
| Bottom Plate Material | Plate Mass (kg) | Experimental Cell Intrusion (mm) | Post-Test Plate Condition | Critical Observation |
|---|---|---|---|---|
| 8 mm PPA Composite | 1.63 | 3.36 | Dent with significant elastic recovery | Lowest intrusion, lightest, maintains seal integrity. |
| 8 mm Aramid A | 1.06 | 6.61 | Catastrophic tearing / cracking | Exceeds intrusion limit. Failure compromises pack sealing. |
| 8 mm Aramid B | 1.38 | 5.13 | Catastrophic tearing / cracking | Passes intrusion limit marginally, but cracking is unacceptable for waterproofing. |
| 0.4 g/mL Foamed Al | 3.02 | 3.74 | Large permanent plastic crush | Good intrusion performance, but mass is ~85% higher than PPA. |
| 0.5 g/mL Foamed Al | 3.19 | 3.86 | Large permanent plastic crush | Good intrusion performance, but mass is ~96% higher than PPA. |
The correlation between simulation and experiment was satisfactory, with both showing the same performance ranking: the aramid composites allowed the highest intrusion (with Aramid A exceeding the limit in testing), the foamed aluminum plates performed better, and the PPA composite provided the best protection. The absolute intrusion values differed, which is common due to modeling simplifications and variability in material properties and assembly; however, the relative trends were consistent, validating our simulation-based down-selection process.
Discussion and Conclusion
This integrated study provides a clear framework for selecting and validating bottom protection materials for lightweight EV battery pack designs. The experimental data reveals critical trade-offs:
- Aramid Composites: While very lightweight, their brittle failure mode (tearing) is a significant disadvantage. A torn bottom plate fails to maintain the EV battery pack‘s required ingress protection (IP rating), potentially allowing water and contaminants to enter. This makes them unsuitable despite marginal passing on intrusion in one case.
- Foamed Aluminum: These materials offer excellent energy absorption and good intrusion performance, behaving predictably through plastic collapse. Their primary drawback is mass. At over 3 kg for the 8 mm plate, they are nearly twice as heavy as the PPA alternative, negating a key lightweighting goal for the EV battery pack.
- PPA (Glass-Fiber Reinforced Polypropylene Honeycomb) Composite: This material demonstrated the most favorable overall performance. It achieved the lowest cell intrusion in both simulation and experiment (3.36 mm vs. a 6 mm limit). It exhibited a stable, progressive crushing mechanism that effectively distributed the impact load. Crucially, it showed significant elastic recovery, meaning it did not fracture or retain a large permanent set, helping to preserve the pack’s sealing integrity. Furthermore, at 1.63 kg, it was the lightest solution among the high-performing options.
The success of the PPA composite can be attributed to its high specific energy absorption, a property quantified by the area under its force-displacement curve divided by its mass. Its structural design allows it to absorb the impact energy through controlled micro-fracturing and buckling of the honeycomb walls, rather than catastrophic failure.
In conclusion, our analysis and experimental study confirm that a composite-based approach is viable for achieving lightweight bottom protection in an EV battery pack. Among the candidates evaluated, the 8 mm thick glass-fiber reinforced PPA honeycomb composite stands out as the optimal solution. It successfully meets the stringent 25 kN bottom ball strike safety requirement by minimizing cell intrusion, while also contributing significantly to the overall mass reduction of the EV battery pack. This research underscores the importance of a combined testing and simulation methodology for developing and validating new materials in the critical field of EV battery pack safety engineering. The findings offer a valuable reference and a practical engineering pathway for designers aiming to enhance the safety and efficiency of future electric vehicles.