In the rapidly evolving automotive industry, the shift toward battery electric cars has become a global priority due to environmental concerns and energy sustainability. A critical component in these vehicles is the battery box, which houses the energy storage systems and ensures safety and performance. However, the substantial weight of traditional battery boxes, often made from metals like steel or aluminum, poses challenges for overall vehicle efficiency, including reduced range and increased energy consumption. Therefore, lightweight design has emerged as a key research area to enhance the competitiveness of battery electric cars. This study focuses on optimizing a battery box for a battery electric car by employing composite materials and structural optimization techniques, aiming to reduce mass while maintaining or improving safety and dynamic performance. The approach integrates finite element analysis (FEA) and multi-objective optimization to achieve a balance between strength, stiffness, and weight reduction, ultimately contributing to the advancement of battery electric car technology.
The battery box in a battery electric car typically consists of an upper cover, lower body, and attachment components, such as lugs, all designed to protect battery modules from mechanical shocks, vibrations, and environmental factors. Traditional designs use materials like Q235 steel or 6082-T6 aluminum, which offer good mechanical properties but add significant mass. For instance, in the reference model, the total mass of the battery box is 86.72 kg, with the battery modules weighing 204.08 kg, highlighting the potential for weight savings. Lightweighting the battery box can directly improve the energy efficiency and driving range of battery electric cars, making it a crucial aspect of vehicle design. This research proposes a comprehensive optimization framework, starting with FEA to assess the baseline performance, followed by topology and size optimization for the upper cover, and material substitution with carbon fiber composites for the lower body, along with free-size, size, and ply sequence optimizations. The goal is to achieve a lightweight design that meets stringent safety standards, such as GB 38031-2020 and GB/T 31467.3-2015, for battery electric cars.
To begin, I developed a geometric model of the battery box using UG software, focusing on key components: the upper cover, lower body, battery modules, and lugs. Non-essential parts like electrical systems and thermal management components were omitted to simplify the analysis while retaining structural integrity. The overall dimensions of the battery box are 1600 mm × 850 mm × 175 mm, representative of typical designs for battery electric cars. For finite element modeling, I used Optistruct software, where 2D shell elements were applied to the upper cover, lower body, and lugs due to their thin-walled nature, with an average element size of 5 mm to ensure accuracy. The mesh quality was verified to avoid distortion, and connections were simulated using RBE2 rigid elements for bolted joints (e.g., between the upper cover and lower body) and tetrahedral shell elements for welded joints (e.g., between the lugs and battery box body). Washer treatments were applied to bolt holes to prevent stress concentrations. This modeling approach provides a robust foundation for subsequent analyses, enabling detailed simulations of the battery box’s behavior under various loading conditions in battery electric cars.
The material properties assigned to the components are essential for accurate FEA. The baseline materials were selected based on common practices in battery electric car manufacturing. For the upper cover, 6082-T6 aluminum was used, with a density of 2.7 g/cm³, Young’s modulus of 72 GPa, Poisson’s ratio of 0.32, and yield strength of 285 MPa. The lower body and lugs were made of Q235 steel, with a density of 7.85 g/cm³, Young’s modulus of 210 GPa, Poisson’s ratio of 0.3, and yield strength of 235 MPa. The thickness for all components was initially set to 4 mm. The battery modules were modeled as mass elements with a total weight of 204.08 kg, distributed uniformly to represent their inertial effects. These properties are summarized in Table 1, which provides a clear comparison for later optimization stages. The use of such materials in battery electric cars often results in high safety margins but also contributes to excessive weight, prompting the need for alternative solutions like composites.
| Component | Material | Density (g/cm³) | Young’s Modulus (GPa) | Poisson’s Ratio | Yield Strength (MPa) | Thickness (mm) | Mass (kg) |
|---|---|---|---|---|---|---|---|
| Upper Cover | 6082-T6 Aluminum | 2.7 | 72 | 0.32 | 285 | 4 | 17.23 |
| Lower Body | Q235 Steel | 7.85 | 210 | 0.3 | 235 | 4 | 67.77 |
| Lugs | Q235 Steel | 7.85 | 210 | 0.3 | 235 | 4 | 1.72 |
| Battery Modules | N/A | N/A | N/A | N/A | N/A | N/A | 204.08 |
Next, I conducted static strength analysis under three extreme operating conditions that battery electric cars may encounter: bumpy road, bumpy road with sharp turning, and bumpy road with emergency braking. These scenarios simulate inertial forces from the battery modules due to accelerations, which are critical for assessing the structural integrity of the battery box. The accelerations applied in each direction (X, Y, Z) are based on standard specifications for battery electric cars, as shown in Table 2. Here, g represents gravitational acceleration (9.8 m/s²). The analysis aims to ensure that the maximum stress remains below the material yield strength with a safety factor of 1.2 to 1.5, and the maximum deformation is less than 3 mm, which are common criteria for battery electric car components to prevent failure and ensure occupant safety.
| Operating Condition | X-direction Acceleration | Y-direction Acceleration | Z-direction Acceleration |
|---|---|---|---|
| Bumpy Road | 0 | 0 | 2g |
| Bumpy Road with Sharp Turning | 0.8g | 1g | 0 |
| Bumpy Road with Emergency Braking | 1g | 0 | 1g |
The FEA results for the baseline battery box are presented in Table 3. The maximum stress of 84.785 MPa and maximum strain of 2.989 mm occur under the bumpy road condition, primarily at the connections between the lower body and lugs, and the center of the upper cover, respectively. These values are within acceptable limits, as the stress is well below the yield strength of Q235 steel (235 MPa) and 6082-T6 aluminum (285 MPa), and the deformation is under 3 mm. However, the high mass of the steel components indicates potential for weight reduction without compromising safety. This analysis underscores the importance of optimizing the battery box for battery electric cars to enhance performance while adhering to industry standards.
| Operating Condition | Maximum Stress (MPa) | Maximum Strain (mm) |
|---|---|---|
| Bumpy Road | 84.785 | 2.989 |
| Bumpy Road with Sharp Turning | 74.891 | 2.626 |
| Bumpy Road with Emergency Braking | 74.745 | 2.627 |
In addition to static analysis, modal analysis was performed to evaluate the dynamic characteristics of the battery box, which is crucial for avoiding resonance in battery electric cars. Resonance can occur when the natural frequency of the structure matches external excitation frequencies, such as those from road irregularities or motor vibrations, leading to amplified vibrations and potential damage. The first four natural frequencies of the baseline battery box were extracted: 27.40 Hz, 31.37 Hz, 38.54 Hz, and 42.27 Hz. The first-order mode shape involves bending of the upper cover, which is a common weak point in battery electric car battery boxes. To assess resonance risk, I calculated the road excitation frequency using the formula from literature: $$f = \frac{v}{L \times 3.6}$$ where \(v\) is the vehicle speed (100 km/h) and \(L\) is the road wavelength (1 m for bumpy roads). This gives: $$f = \frac{100}{1 \times 3.6} \approx 27.78 \text{ Hz}$$ Since the first natural frequency (27.40 Hz) is close to this excitation frequency, there is a potential for resonance, highlighting the need for optimization to increase the first-order frequency and improve dynamic stability in battery electric cars.
Based on the FEA results, I proceeded with structural optimization to achieve lightweight design goals. The optimization was divided into two parts: for the upper cover, I retained 6082-T6 aluminum but applied topology and size optimization; for the lower body, I replaced Q235 steel with carbon fiber composite material and conducted a series of optimizations including free-size, size, and ply sequence optimization. This approach leverages the anisotropic properties of composites to reduce weight while maintaining strength, which is advantageous for battery electric cars where every kilogram saved can extend driving range.
For the upper cover, topology optimization was first performed to maximize the first natural frequency without adding mass. The design variable was the shape of the upper surface, with a constraint that nodal displacements should not exceed 3 mm. The objective function was to maximize the first modal frequency. After 5 iterations, the frequency increased from 27.40 Hz to 31.16 Hz, and the optimized shape included reinforcing ribs distributed across the cover. Using OSSmooth in HyperMesh, I extracted the topology results and refined the geometry in UG for manufacturability. The ribs were designed to enhance stiffness, which is critical for battery electric cars to withstand vibrations. Subsequently, size optimization was applied to minimize mass while keeping the first natural frequency above 30 Hz. The thickness of the upper cover was reduced from 4 mm to an optimal value of 2.156 mm, and after rounding for practical manufacturing, it was set to 2.2 mm. This reduced the mass of the upper cover from 17.23 kg to 9.71 kg, contributing significantly to lightweighting for battery electric cars.
For the lower body, I switched to carbon fiber composite material due to its high strength-to-weight ratio, which is ideal for battery electric cars. The material properties are summarized in Table 4. The composite was modeled with symmetric layups using common ply angles: 0°, +45°, -45°, and 90°. This layup configuration helps balance mechanical performance and manufacturing feasibility for components in battery electric cars.
| Parameter | Value |
|---|---|
| Density, \(\rho\) (t/mm³) | 1.8 × 10⁻⁹ |
| Poisson’s Ratio | 0.27 |
| Longitudinal Elastic Modulus, \(E_1\) (GPa) | 140 |
| Transverse Elastic Modulus, \(E_2\) (GPa) | 8.4 |
| In-plane Shear Modulus, \(G_{12}\) (GPa) | 6.8 |
| In-plane Shear Strength, \(S\) (MPa) | 75 |
| Transverse Tensile Strength, \(Y_t\) (MPa) | 60 |
| Transverse Compressive Strength, \(Y_c\) (MPa) | 193 |
| Longitudinal Tensile Strength, \(X_t\) (MPa) | 1520 |
| Longitudinal Compressive Strength, \(X_c\) (MPa) | 1200 |
The optimization of the composite lower body involved three stages. First, free-size optimization (or ply shape optimization) was conducted to determine the optimal thickness distribution of the plies. The design variables were the ply thicknesses, with constraints on maximum displacement (< 3 mm) and first natural frequency (> 30 Hz). The objective was to maximize stiffness (minimize deflection) under the three static loading conditions. After 29 iterations, the thickness distribution was obtained, showing varied thickness across the lower body to meet performance requirements for battery electric cars. Second, size optimization (ply thickness optimization) was performed to refine the ply thicknesses into manufacturable values. The design variables were the thicknesses of 16 ply bundles, with the same constraints as before, and the objective was to minimize mass. After 2 iterations, the optimal thicknesses were determined, as listed in Table 5. These values were rounded to practical levels for production in battery electric cars.
| Ply Bundle | Thickness (mm) | Ply Bundle | Thickness (mm) |
|---|---|---|---|
| PLY1100 | 0.24 | PLY3100 | 0.28 |
| PLY1200 | 0.13 | PLY3200 | 0.14 |
| PLY1300 | 0.41 | PLY3300 | 0.56 |
| PLY1400 | 2.71 | PLY3400 | 2.59 |
| PLY2100 | 0.28 | PLY4100 | 0.36 |
| PLY2200 | 0.14 | PLY4200 | 0.28 |
| PLY2300 | 0.56 | PLY4300 | 0.72 |
| PLY2400 | 2.59 | PLY4400 | 2.24 |
Third, ply sequence optimization was carried out to determine the stacking order of the plies, considering manufacturing constraints such as the maximum number of consecutive plies (MSUCC). The design variables were the ply sequences, with constraints on displacement and frequency, and the objective was to minimize mass. After 4 iterations, the optimal ply sequence was found to be [90/45/-45/0/0/45/-45/90/90/45/-45/0], which provides balanced mechanical properties for the battery box in battery electric cars. This sequence ensures good stiffness and strength in multiple directions, crucial for handling complex loads in battery electric cars.
After optimization, I validated the performance of the lightweight battery box through multi-condition analyses. Static strength analysis under the same three extreme conditions showed significant improvements, as summarized in Table 6. The maximum stress reduced to 53.304 MPa, and the maximum strain to 2.623 mm, both lower than the baseline values and within safety limits for battery electric cars. This demonstrates that the optimized design maintains structural integrity while reducing weight, which is essential for the efficiency and safety of battery electric cars.
| Operating Condition | Maximum Stress (MPa) | Maximum Strain (mm) |
|---|---|---|
| Bumpy Road | 53.304 | 2.623 |
| Bumpy Road with Sharp Turning | 50.893 | 2.505 |
| Bumpy Road with Emergency Braking | 50.909 | 2.505 |
Modal analysis of the optimized battery box revealed a notable increase in the first natural frequency to 30.74 Hz, which is above the road excitation frequency of 27.78 Hz, effectively mitigating resonance risks in battery electric cars. The first four natural frequencies are compared with the baseline in Table 7. The higher frequencies indicate improved dynamic stiffness, which enhances the durability and comfort of battery electric cars by reducing vibration transmission.
| Mode Order | Baseline Frequency (Hz) | Optimized Frequency (Hz) |
|---|---|---|
| 1 | 27.40 | 30.74 |
| 2 | 31.37 | 42.78 |
| 3 | 38.54 | 51.89 |
| 4 | 42.27 | 54.87 |
Additionally, random vibration analysis was conducted to assess the battery box’s performance under stochastic loads, which are common in real-world driving conditions for battery electric cars. The power spectral density (PSD) profiles for X, Y, and Z axes were based on GB/T 31467.3-2015, as shown in Table 8. This standard is stricter than others, ensuring robust validation for battery electric cars. The root mean square (RMS) stresses were computed, and the 3σ stresses (three times RMS) were compared to the material yield strengths. The results indicated that the maximum RMS stresses were 1.996 MPa (X-axis), 1.688 MPa (Y-axis), and 50.368 MPa (Z-axis), all with 3σ values well below the yield strengths of the materials used. This confirms that the optimized battery box can withstand random vibrations without fatigue failure, meeting the safety requirements for battery electric cars.
| Frequency (Hz) | X-axis PSD (g²/Hz) | Y-axis PSD (g²/Hz) | Z-axis PSD (g²/Hz) |
|---|---|---|---|
| 5 | 0.0125 | 0.01 | 0.05 |
| 10 | 0.03 | 0.015 | 0.06 |
| 20 | 0.03 | 0.015 | 0.06 |
| 50 | 0.01 | N/A | N/A |
| 200 | 0.00025 | 0.0004 | 0.0008 |
The lightweight design achieved a significant mass reduction. The upper cover mass decreased from 17.23 kg to 9.71 kg, and the lower body mass was reduced from 67.77 kg to approximately 24.5 kg (estimated based on composite density and optimized thicknesses), leading to a total battery box mass reduction of over 50%. This weight saving directly contributes to improved energy efficiency and extended range for battery electric cars, addressing key challenges in their adoption. Moreover, the use of carbon fiber composites offers additional benefits such as corrosion resistance and design flexibility, which are advantageous for battery electric cars operating in diverse environments.
In conclusion, this study presents a comprehensive lightweight design methodology for a battery box in battery electric cars, combining FEA and multi-stage optimization. The approach successfully reduced weight while enhancing static and dynamic performance, as validated through rigorous analyses. The optimized battery box meets all safety standards, with lower stresses and strains, higher natural frequencies, and robust random vibration resistance. These improvements are crucial for the reliability and efficiency of battery electric cars, supporting the global transition to sustainable transportation. Future work could explore advanced composite materials or integrate thermal management considerations to further optimize battery boxes for battery electric cars. Overall, this research demonstrates the potential of lightweight design to advance battery electric car technology, contributing to a greener automotive future.
To summarize the key equations used in this study, the road excitation frequency formula is essential for resonance avoidance in battery electric cars: $$f = \frac{v}{L \times 3.6}$$ where \(f\) is the frequency in Hz, \(v\) is the vehicle speed in km/h, and \(L\) is the road wavelength in meters. Additionally, the optimization objectives can be expressed mathematically. For topology optimization, the objective was to maximize the first natural frequency \(f_1\): $$\text{maximize } f_1 \text{ subject to } u_{\text{max}} \leq 3 \text{ mm}$$ where \(u_{\text{max}}\) is the maximum nodal displacement. For size optimization, the goal was to minimize mass \(m\): $$\text{minimize } m \text{ subject to } f_1 \geq 30 \text{ Hz and } u_{\text{max}} \leq 3 \text{ mm}$$ These formulations guided the design process to achieve a balanced performance for battery electric cars.
The integration of composite materials and optimization techniques highlights a forward-thinking approach to automotive design, particularly for battery electric cars where weight reduction is paramount. As the demand for battery electric cars grows, such innovations will play a pivotal role in enhancing their competitiveness and sustainability. This study provides a framework that can be adapted to other vehicle components, fostering further advancements in lightweight design for battery electric cars and beyond.