In the fast-paced world of electric vehicles (EVs), every gram counts. The quest for longer range, better efficiency, and enhanced performance has led researchers to focus on a critical component: the battery box. As the primary carrier and protector of the power battery system, the battery box’s weight directly impacts an EV’s driving range. A breakthrough study has now unveiled a innovative approach to lightweighting battery boxes, achieving a significant 5.3 kg reduction while meeting all essential strength, stiffness, and low-order modal frequency requirements.
The research, which delves into the intricate balance between material science and structural engineering, offers a promising solution to one of the most pressing challenges in EV design. By combining advanced material selection methods with rigorous structural optimization techniques, the team has set a new benchmark for battery box lightweighting.
The Importance of Battery Box Lightweighting
Battery boxes are more than just containers; they are vital components that ensure the safety and functionality of an EV’s power system. Their weight plays a pivotal role in determining the vehicle’s energy efficiency and, consequently, its range. “The mass of the battery box affects the driving range of the power battery,” explains the research team. “Finding ways to lightweight the battery box while ensuring strength, stiffness, and other performance metrics is crucial for effectively extending the range, making it a topic of great research significance.”
For years, engineers and researchers have explored various avenues to reduce the weight of battery boxes. Previous efforts have included structural optimizations such as topology, size, and shape adjustments, as well as material substitutions, often replacing traditional steel with lighter alternatives like aluminum alloys. However, many of these approaches have focused on either structural changes or single material replacements, leaving room for a more comprehensive strategy that considers the unique requirements of different parts of the battery box.
A Multi-Step Approach to Lightweighting
The new study takes a holistic approach, combining advanced material selection with targeted structural optimization. The researchers employed a two-pronged strategy: first, identifying the most suitable lightweight materials using a sophisticated decision-making method, and then determining the optimal placement of these materials in different parts of the battery box through systematic testing.
Step 1: Material Selection with TOPSIS Method
The research began by evaluating nine commonly used metal materials for battery boxes. These materials, including various types of steel and aluminum alloys, were assessed based on key performance indicators: density, elastic modulus, tensile strength, yield strength, and elongation.
To make a objective and comprehensive selection, the team turned to the TOPSIS (Technique for Order Preference by Similarity to an Ideal Solution) method, enhanced with CRITIC (Criteria Importance Through Intercriteria Correlation) weighting. This combination allowed the researchers to account for both the variability in material properties and the relationships between different criteria, resulting in a more accurate assessment of each material’s overall suitability.
“The CRITIC weight method considers not only the impact of variation on indicators but also the correlation between indicators, making it more comprehensive,” notes the team. This approach helped eliminate dimensional differences between different sets of data and determine objective weights for each performance parameter.
After rigorous analysis, two materials emerged as top candidates: Al6061-T6 and AZ91D. These materials demonstrated superior comprehensive performance compared to traditional steel options, offering a promising balance of lightweight properties and mechanical strength.When combined with the original Q235 steel, they formed a trio of materials that could be strategically deployed across different parts of the battery box.
Step 2: Orthogonal Experiment for Material Matching
With the candidate materials identified, the next challenge was to determine which material should be used in which part of the battery box. The battery box, consisting of components such as the box body, cover, lifting lugs, bottom bracket, internal bracket, and electrical bracket, has varying performance requirements for each part.
To address this, the researchers designed an orthogonal experiment using the L18 (3^6) scheme, which allowed them to efficiently test different combinations of the three materials across the six main components. The experiments evaluated each combination based on critical performance metrics: overall mass, maximum displacement under vertical bump conditions, maximum stress, and first-order frequency.
Through range analysis of the experimental results, the optimal material matching scheme emerged:
- Box body and bottom bracket: Q235 steel
- Box cover and internal bracket: AZ91D
- Lifting lugs and electrical bracket: Al6061-T6
This configuration struck a balance between weight reduction and performance, ensuring that each component was made from the material best suited to its specific function.
Step 3: Structural Optimization
While the multi-material approach showed promise for weight reduction, initial tests revealed some performance issues. The multi-material battery box exhibited greater displacement and stress in various conditions compared to the original model, and its first-order frequency of 16.796 Hz was below the 28 Hz threshold needed to avoid resonance with wheel excitation frequencies.
To address these concerns, the researchers implemented two rounds of optimization:
- Topography Optimization: Focused on improving the first-order modal frequency, this step adjusted the shape of the box cover. After 14 iterations, the first-order frequency was successfully increased to 35.25 Hz, well above the critical 28 Hz mark.
- Multi-Objective Size Optimization: Using adaptive Latin hypercube sampling and global response surface methodology, the team optimized the thickness of each component. The thicknesses of the box body, cover, lifting lugs, bottom bracket, internal bracket, and electrical bracket were adjusted within specified ranges to minimize mass while ensuring maximum displacement and stress remained within acceptable limits.
The result of these optimizations was a battery box that not only met all performance requirements but also achieved a significant weight reduction.
Impressive Results: 11.37% Weight Reduction
The final optimized battery box design represented a remarkable achievement in lightweighting. Compared to the original model, the new design reduced weight by 5.3 kg, resulting in a weight reduction rate of 11.37%. This substantial reduction translates directly to improved energy efficiency and extended driving range for electric vehicles.
Performance testing confirmed that the lightweighted battery box met or exceeded all essential requirements:
- Strength: Maximum stress under various conditions remained below the yield strength of the materials used.
- Stiffness: Displacements under different operating conditions were reduced compared to the initial multi-material model.
- Dynamic performance: The first-order frequency of 39.943 Hz avoided resonance with wheel excitation frequencies, ensuring stable operation during vehicle movement.
The optimization also led to improved performance across other metrics. For example, under emergency braking conditions, maximum displacement decreased by 40.12%, and maximum stress reduced by 8.02%. Similar improvements were seen in other critical operating scenarios, demonstrating the comprehensive benefits of the design approach.
Broader Implications for the EV Industry
This research represents a significant step forward in battery box design, offering a systematic and data-driven approach to lightweighting. By combining advanced material selection with strategic placement and targeted structural optimization, the study provides a blueprint for manufacturers looking to enhance the performance of electric vehicles.
The use of multi-material designs, as demonstrated in this research, allows for a more nuanced approach to weight reduction. Rather than a one-size-fits-all material substitution, it enables engineers to tailor material choices to the specific demands of each component. This not only reduces weight but also optimizes performance, ensuring that critical areas maintain necessary strength and rigidity.
Furthermore, the methodologies employed—including the CRITIC-weighted TOPSIS method and orthogonal experiments—offer a replicable framework for lightweighting other vehicle components. This could have far-reaching implications for the automotive industry as it continues to pursue greater efficiency and sustainability.
As electric vehicles become increasingly prevalent, innovations like this battery box design will play a crucial role in addressing consumer concerns about range anxiety and performance. Every kilogram saved contributes to a more efficient and capable electric vehicle, bringing us closer to a future where EVs are the undisputed norm on our roads.
The research, conducted by Kang Yuanchun and Liu Junfeng from the School of Automotive Engineering, Hubei University of Automotive Technology, and Hubei Key Laboratory of Automotive Power Train and Electronic Control, was published in the “Journal of Chongqing University of Technology (Natural Science)” in its 2024 volume 38, issue 1. The DOI for this groundbreaking study is 10.3969/j.issn.1674-8425(z).2024.01.012.
This work not only advances the field of battery box design but also exemplifies the kind of interdisciplinary research that will drive the future of automotive engineering. By combining material science, mechanical engineering, and data analysis, the researchers have delivered a solution that is both practical and innovative, paving the way for the next generation of electric vehicles.