Optimizing Lightweight and Crashworthy Electric Vehicle Battery Box Designs with a Novel Hybrid Structure

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Comprehensive Optimization and Design of an Electric Vehicle Battery Box Side Profile for Lightweight and Crashworthiness Using a Novel Hybrid Structure

Abstract: Lightweighting is a critical focus in the transportation sector, directly enhancing efficiency and significantly reducing costs. In electric vehicle (EV) design, the body surrounding the battery must effectively absorb impact, especially during crashes. This study aims to improve the crash performance of the side profiles in the battery box of an M1 category vehicle, based on the crash test in Annex 8D of the ECE R100 regulation. The safe displacement at which the battery will not deform is set at 20 mm, and the maximum force and energy absorption at this displacement are compared. A total of 33 different electric and hybrid vehicle models were benchmarked. L-shaped geometry and aluminum materials are generally preferred; this study focuses on using glass-fiber-reinforced polymer (GFRP) pultruded profiles to enhance battery durability while reducing weight. The GF800 material was selected for its superior mechanical strength among glass fiber composites. A virtual tensile test verified its properties. A unique hybrid model combining honeycomb and auxetic geometries was developed, showing crash performance improvements of approximately 360% over honeycomb structures and 88% over auxetic structures. Through multi-objective optimization using artificial neural networks (ANNs), 27 models were analyzed, leading to an optimized design. The final design results in a battery box side profile that is 23.9% lighter, 38.6% cheaper, and exhibits 3% higher performance. This study demonstrates significant advancements in EV safety and cost efficiency, highlighting the practical benefits of innovative material and design approaches.

Keywords: hybrid structure; multi-objective optimization; glass-fiber-reinforced polymer (GFRP); battery box; crashworthiness; pultrusion

1. Introduction

Companies in the automotive sector are increasingly focused on reducing fuel consumption and enhancing vehicle energy efficiency. One of the most effective strategies for achieving this is weight reduction. These lightweighting efforts must align with international safety standards while also considering travel comfort.

Various studies have been conducted across the transportation sector to reduce weight, employing different materials, optimization techniques, and methods such as minimizing the number of parts. Literature highlights the growing significance of lightweighting, particularly in land transportation, driven by factors like decreasing oil reserves and increasing global warming due to exhaust emissions.

Sustainability is becoming increasingly critical in electric mobility, with efforts aimed at reducing carbon emissions and enhancing urban quality of life. This study explores the current state of electro-mobility in Europe, examining its social, economic, and environmental aspects within a circular economy framework, arguing that integrating these principles is essential for sustainable decarbonization.

The automotive sector plays a pivotal role in the economy and society but significantly impacts global emissions. A life cycle assessment comparing petrol internal combustion engines and battery electric vehicles analyzes 17 impact categories related to climate change and ecosystems. Electric vehicles (EVs) are crucial for reducing greenhouse gas emissions and air pollution, which enhances quality of life globally.

In electric vehicle studies, batteries serve as the power source, and factors such as distance, weight, and safety measures are vital. In crash situations, the battery’s surrounding body must effectively absorb impacts, making safety a crucial aspect of electric vehicle design. Measures such as robust battery compartment protection and the use of impact-absorbing materials are essential.

This study presents a design, analysis, and optimization of a fiberglass side profile, evaluating its performance in collision scenarios through numerical simulations. GFRP, a composite material combining glass fibers with thermosetting resin, is produced using the pultrusion process, which ensures uniform fiber distribution and optimal mechanical properties.

1. Materials and Methods

2.1 Materials
GFRP exhibits anisotropic properties, meaning its mechanical characteristics vary across different axes. Due to the pultrusion process, material distribution will be uniform throughout the component, behaving like an orthotropic material. GF800 was selected for its mechanical properties similar to aluminum, with a fiber content of 60.5% and a matrix of 39.5%.

A virtual tensile test model was established to verify the material properties at various orientations. The finite element analysis (FEA) results aligned well with experimental data, supporting the use of GF800 in further analyses.

2.2 Benchmarks
A total of 33 electric and hybrid vehicle models were examined using a reverse engineering application, revealing that L-shaped structures are commonly utilized. The benchmark findings, including material types and dimensions, informed the redesign of the existing L structure, which predominantly uses aluminum 6082 material.

2.3 Test Requirements and Heat Dissipation
The crash performance improvement of the battery box side profiles was evaluated through tests according to Annex 8D of the ECE R100 regulation. A complete battery pack, including its cover and base, was subjected to a load of 100 kN. The maximum safe displacement for the battery to remain undamaged is 20 mm.

2.4 Structure Design
The study created a hybrid structure by combining regular honeycomb and auxetic structures to enhance crash performance. This novel hybrid design exhibited superior mechanical properties compared to the individual structures.

2.5 Finite Element Analysis (FEA) Model
The FEA was conducted using HyperMesh-Radioss to evaluate the structural performance of the battery box side profile under various conditions, with 27 different sections analyzed.

2.6 Optimization
The optimization study involved FEA results processed through MATLAB to identify optimal design variables and interpolate the relationships between input parameters and FEA outputs using ANNs. The NSGA-II algorithm was employed for multi-objective optimization, resulting in a comprehensive set of Pareto-optimal solutions.

1. Verification
   The optimized model was validated against the current design, achieving a peak force of 45,909 N and an absorbed energy of 658 J, with a verification accuracy of 97%. The new design is 23.9% lighter and 38.6% cheaper, showcasing substantial improvements in performance and cost efficiency.

2. Conclusion
   This study introduces an innovative methodology combining ANN, NSGA-II, and TOPSIS for optimizing the structural design of battery side profiles in electric vehicles. The hybrid model integrating honeycomb and auxetic structures achieved significant advancements in crashworthiness and weight reduction, demonstrating the effectiveness of this design approach in meeting the evolving needs of the automotive industry for safety, energy efficiency, and sustainability.
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