In the modern era, the automotive industry has experienced rapid development, with new energy vehicles becoming a critical component due to their environmental and energy-saving advantages. During normal operation, the EV battery pack serves as the “heart” of the vehicle’s safe operation. If subjected to external forces such as compression, it can pose significant safety risks. Therefore, it is essential to conduct an in-depth analysis of the EV battery pack enclosure structure, considering its stress characteristics, to comprehensively optimize the design and manufacturing techniques. By adopting lightweight manufacturing technologies, from forming to joining, we can effectively reduce the weight of the EV battery pack, ensuring the stability and safety of the new energy battery system. This article explores the EV battery pack enclosure structure from multiple perspectives, including materials, design, manufacturing, and performance evaluation, aiming to provide insights for sustainable development in the new energy vehicle industry.
The EV battery pack is a core element in electric vehicles, influencing protection performance, overall structure, vehicle lifespan, and side-impact safety. A well-designed EV battery pack enclosure can consistently provide power and ensure safe driving. This research delves into the intricacies of the EV battery pack, focusing on lightweight strategies to enhance efficiency and reliability. The following sections cover material selection, structural analysis, manufacturing technologies, and performance standards, all centered on optimizing the EV battery pack for modern automotive demands.
Material Analysis for EV Battery Pack Enclosures
In the context of new energy vehicles, the EV battery pack accounts for 18% to 30% of the total vehicle mass, with the enclosure itself comprising 10% to 20% of the pack weight. To meet design standards, lightweight solutions are paramount. Materials for the EV battery pack enclosure must exhibit chemical stability and electrical insulation, with metals and composites being prevalent in the market. This analysis focuses on metallic materials, particularly aluminum alloys, due to their high strength-to-weight ratio. Aluminum alloys offer excellent rigidity and low density, making them ideal for EV battery pack applications. Common designs include aluminum extrusions for battery boxes, with structures like底板式 (base plate) and框架式 (frame type). For instance, vehicles such as Tesla Roadster, Nio ES8, and BMW i3 utilize aluminum plates in their EV battery pack enclosures to resist external impacts and enhance system stability.
The material properties significantly affect structural performance. Aluminum alloys provide substantial strength, and when used in full-aluminum enclosures, they maintain integrity under continuous vibration. For example, consider the properties of AL6061-T6 aluminum alloy, as shown in Table 1. These attributes ensure that the EV battery pack remains durable in various operating conditions.
| Material | Poisson’s Ratio | Elastic Modulus (MPa) | Yield Strength (MPa) | Density (g/cm³) |
|---|---|---|---|---|
| AL6061-T6 | 0.33 | 68,900 | 276 | 2.70 |
To achieve comprehensive protection for the battery modules, welding techniques are employed at critical points such as the shell, crossbeams, and mounting areas. The relationship between stress and strain in these materials can be expressed using Hooke’s Law: $$ \sigma = E \epsilon $$ where $\sigma$ is stress, $E$ is the elastic modulus, and $\epsilon$ is strain. This formula underpins the design rationale for ensuring the EV battery pack enclosure meets strength requirements. Additionally, the lightweight index for materials can be evaluated using: $$ \text{Lightweight Index} = \frac{\text{Strength}}{\text{Density}} $$ For AL6061-T6, this index is approximately 102.2 MPa·cm³/g, highlighting its suitability for EV battery pack applications.
Structural Analysis of EV Battery Pack Enclosures
The EV battery pack enclosure structure is pivotal for vehicle performance. This section examines the shape, layout, and internal design of the enclosure, emphasizing lightweight optimization.
Shape and Layout of EV Battery Pack
Typically, the EV battery pack is fixed beneath the vehicle floor, with shapes like “concave,” “T,” or “I” configurations, depending on vehicle design and model. For example, a concave shape leverages the space between the ground and floorboard to integrate seamlessly with the passenger compartment. When arranging the EV battery pack, factors such as overall vehicle space, load distribution, and ground clearance must be considered. Common layout types include underbody placement and rear placement. Underbody placement enhances stability by lowering the center of gravity and optimizing crash force paths, as seen in models like Xiaopeng G3 and Tesla Model series. The latter uses a rectangular底板 integrated with the body floor, improving overall robustness. To illustrate, Table 2 summarizes typical EV battery pack shapes and their characteristics.
| Shape | Description | Advantages | Example Vehicles |
|---|---|---|---|
| Concave | Fits into floor cavity | Space-efficient, reduces conflicts | Various sedans |
| T-shaped | Extends along vehicle axis | Balanced weight distribution | Some SUVs |
| Rectangular | Simple flat design | Easy manufacturing, good stability | Tesla Model S |
The layout optimization can be modeled using geometric constraints. For instance, the gap between the EV battery pack and body floor should minimize interference, which can be expressed as: $$ \min \Delta d = |d_{\text{pack}} – d_{\text{floor}}| $$ where $\Delta d$ is the clearance. Additionally, thermal management systems are incorporated to maintain battery temperature, often using fluid dynamics principles: $$ \dot{Q} = h A (T_{\text{battery}} – T_{\text{ambient}}) $$ where $\dot{Q}$ is heat transfer rate, $h$ is heat transfer coefficient, $A$ is surface area, and $T$ represents temperatures.
Lower Enclosure Structure of EV Battery Pack
The lower enclosure is the primary load-bearing component of the EV battery pack. Its design involves reinforcement ribs, crash-resistant structures, and internal module partitions to enhance durability. Given diverse road conditions, the enclosure must withstand various collision forms. Aluminum alloys are selected based on thickness and type to improve local deformation resistance. For example, foam sandwich panels or double-layer protective plates, based on the sandwich principle, can boost crash safety. To resist impact deformation, local reinforcement ribs are added, increasing stiffness. The bending stiffness of a ribbed plate can be approximated by: $$ D = \frac{E t^3}{12(1-\nu^2)} + \sum E_i I_i $$ where $D$ is flexural rigidity, $t$ is thickness, $\nu$ is Poisson’s ratio, and $E_i I_i$ represents contributions from ribs. In EV battery pack designs like BYD Han, the battery cells themselves act as beam structures, eliminating the need for internal crossbeams. The lower shell uses battery borders and base plates, optimizing space and weight.
Internal partitions are crucial for multi-layer battery arrays. Their design must balance stiffness and weight, often using lightweight materials with connections ensured by welding or adhesive. The natural frequency of the enclosure can be calculated to avoid resonance: $$ f_n = \frac{1}{2\pi} \sqrt{\frac{k}{m}} $$ where $f_n$ is natural frequency, $k$ is stiffness, and $m$ is mass. This ensures the EV battery pack remains stable under vibration.
Lightweight Optimization of EV Battery Pack Enclosure
Lightweight optimization is essential for improving the EV battery pack performance. Finite element simulation is a key tool, enabling topological optimization of the enclosure structure. By creating detailed models and refining them locally, we can analyze stress and deformation. For instance, using software like ANSYS, we perform simulations to determine maximum stress $\sigma_{\text{max}}$ and deflection $\delta_{\text{max}}$. The optimization goal is to minimize mass while meeting constraints: $$ \min \sum_{i=1}^{n} \rho_i V_i $$ subject to $$ \sigma_i \leq \sigma_{\text{allowable}}, \quad \delta_i \leq \delta_{\text{limit}} $$ where $\rho_i$ is density, $V_i$ is volume, and constraints ensure structural integrity. Orthogonal experimental design can streamline this process by testing multiple factors simultaneously. For example, factors like rib height, thickness, and material grade are varied, and responses are evaluated using statistical methods. Table 3 shows an orthogonal array for optimizing an EV battery pack enclosure.
| Experiment | Rib Height (mm) | Thickness (mm) | Material Grade | Mass (kg) | Max Stress (MPa) |
|---|---|---|---|---|---|
| 1 | 10 | 2.0 | AL6061 | 25.3 | 150 |
| 2 | 10 | 2.5 | AL7075 | 28.1 | 120 |
| 3 | 15 | 2.0 | AL7075 | 26.8 | 110 |
| 4 | 15 | 2.5 | AL6061 | 29.5 | 130 |
Through such analyses, we can reduce material usage by up to 20% while maintaining performance, aligning with lightweight goals for the EV battery pack.
Lightweight Manufacturing Technologies for EV Battery Pack Enclosures
Manufacturing techniques play a vital role in realizing lightweight EV battery pack enclosures. This section covers forming and joining technologies, with a focus on aluminum-based methods.
Forming Technologies for EV Battery Pack Enclosures
For aluminum materials, common forming methods include stamping, welding, and casting. Tesla Model series, for instance, uses stamped aluminum welding and integrated die-casting. Stamping involves applying force via molds to plastically deform metal sheets into desired shapes. The process can be described by the forming limit curve (FLC): $$ \text{FLC}: \epsilon_1 = f(\epsilon_2) $$ where $\epsilon_1$ and $\epsilon_2$ are principal strains. Integrated die-casting employs high-tonnage presses to form large parts in one step, reducing components. Advanced welding techniques like cold metal transfer (CMT) and resistance spot welding are used for aluminum. Resistance spot welding controls heat input precisely, with weld depth governed by: $$ Q = I^2 R t $$ where $Q$ is heat generated, $I$ is current, $R$ is resistance, and $t$ is time. This ensures strong joints in the EV battery pack enclosure. Table 4 compares forming technologies for EV battery pack enclosures.
| Technology | Process | Advantages | Challenges | Application in EV Battery Pack |
|---|---|---|---|---|
| Stamping | Press forming of sheets | High precision, good strength | Tooling cost | Tesla models |
| Die-casting | Molten metal injection | Complex shapes, lightweight | Porosity risks | Structural parts |
| Extrusion | Pushing through die | Uniform sections, efficient | Limited to constant cross-sections | Frame components |
Joining Technologies for EV Battery Pack Enclosures
Joining is critical for assembling EV battery pack enclosures, especially with hybrid materials like metal-composites. Methods include adhesive bonding, mechanical fastening, and hybrid approaches. For aluminum enclosures, fasteners like rivets or bolts with reinforcement ribs are used. Adhesive bonding avoids fiber damage in composites, with shear strength given by: $$ \tau = \frac{F}{A} $$ where $\tau$ is shear stress, $F$ is force, and $A$ is bonded area. Hybrid joining combines adhesive with mechanical fasteners for enhanced fatigue strength. In carbon fiber enclosures, metal inserts are often joined using rivet nuts or wire thread inserts. For example, rivet nuts are installed in cavity floors to secure thin plates, preventing weld burn-through and thread stripping. The pull-out strength of a rivet nut can be estimated as: $$ F_{\text{pull-out}} = \pi d t \sigma_{\text{bearing}} $$ where $d$ is diameter, $t$ is thickness, and $\sigma_{\text{bearing}}$ is bearing stress. These technologies ensure the EV battery pack enclosure remains intact under load.
Standards and Performance Evaluation Methods for EV Battery Pack
Performance evaluation is crucial for ensuring the safety and reliability of the EV battery pack. Tests cover mechanical, environmental, and electrical aspects, from cell to system level. Key methods include salt spray, low-pressure, collision, and crush tests, all vital for the EV battery pack enclosure integrity.
Salt Spray Test
The salt spray test simulates corrosive environments to assess the EV battery pack enclosure durability. A saline mist of sodium chloride solution is sprayed in a controlled chamber, with parameters like pressure and settling rate monitored. Weight loss,外观 changes, and electrical performance are recorded. The corrosion rate can be modeled by: $$ R = \frac{W_0 – W_t}{A t} $$ where $R$ is corrosion rate, $W_0$ and $W_t$ are initial and final weights, $A$ is area, and $t$ is time. Standards such as ISO 9227 define test conditions to ensure the EV battery pack meets safety requirements.
Low-Pressure Test
This test evaluates the EV battery pack under high-altitude conditions. A low-pressure chamber simulates reduced atmospheric pressure, with temperature controllers and detection systems tracking changes. Before testing, the EV battery pack undergoes charge-discharge cycles at 20±3°C. The ideal gas law relates pressure and volume: $$ PV = nRT $$ where $P$ is pressure, $V$ is volume, $n$ is moles of gas, $R$ is gas constant, and $T$ is temperature. Hazards like explosion or fire are checked, and capacity retention is measured to ensure the EV battery pack operates safely in extreme environments.
Collision Test
Collision tests assess the EV battery pack structural strength during accidents. Scenarios include frontal, side, and rear impacts, with speeds and angles set per standards like GB/T or ISO. Advanced equipment measures deformation and force. The impact energy can be calculated using: $$ E = \frac{1}{2} m v^2 $$ where $E$ is kinetic energy, $m$ is mass, and $v$ is velocity. Post-test, the enclosure is inspected for cracks, component damage, or leakage, ensuring the EV battery pack maintains integrity.
Crush Test
Crush tests determine the EV battery pack resistance to external pressure. Following IEC standards, a force is applied to simulate vehicle underbody or side impacts. The crush force versus displacement is recorded, and structural integrity is verified. The stress during crushing is: $$ \sigma_{\text{crush}} = \frac{F}{A} $$ where $F$ is applied force and $A$ is contact area. The EV battery pack must prevent short circuits or electrolyte leakage, with tests conducted at relevant temperatures to mimic real-world conditions.
Table 5 summarizes key performance tests for the EV battery pack, highlighting their purposes and criteria.
| Test | Purpose | Standard | Key Metrics | Acceptance Criteria for EV Battery Pack |
|---|---|---|---|---|
| Salt Spray | Corrosion resistance | ISO 9227 | Weight loss,外观变化 | No significant corrosion or performance drop |
| Low-Pressure | High-altitude safety | IEC 60068-2-13 | Pressure level, temperature | No explosion, fire, or capacity loss |
| Collision | Impact integrity | GB/T 31467.3 | Deformation, force | Enclosure intact, no internal damage |
| Crush | Compression resistance | IEC 62660-2 | Crush force, displacement | No leakage or thermal runaway |
These evaluations ensure that the EV battery pack enclosure can withstand rigorous conditions, contributing to overall vehicle safety.
Conclusion
In summary, the EV battery pack is central to the operation and adoption of new energy vehicles. Its lifespan and performance directly impact driving safety. By focusing on the lower enclosure structure, we can optimize design and manufacturing techniques to securely house battery modules. Lightweight manufacturing technologies, such as advanced forming and joining, enhance production efficiency for the EV battery pack. Coupled with rigorous standards and performance evaluations, including crush and collision tests, we improve structural integration and reliability. This comprehensive approach ensures that the EV battery pack provides stable power, supporting the sustainable growth of the new energy vehicle industry. Future research may explore novel materials like composites or additive manufacturing to further reduce weight and cost while maintaining safety for the EV battery pack.