As the global shift toward sustainable transportation accelerates, the development of new energy vehicles, particularly electric vehicles (EVs), has become a cornerstone in addressing energy crises and environmental pollution. Central to the performance and safety of these vehicles is the EV battery pack, which serves as the primary power source. Within this system, the battery pack enclosure plays a critical role in protecting the lithium-ion cells from mechanical damage, environmental hazards, and, most importantly, thermal runaway events. With the EV battery pack contributing significantly to the overall vehicle weight, lightweighting these enclosures is essential for enhancing driving range and energy efficiency. However, reducing mass must not compromise safety, especially regarding fire resistance. Currently, the industry lacks standardized testing methods for evaluating the ablation resistance of lightweight EV battery pack enclosures, leading to inconsistent material selections and safety protocols. In this study, I explore the thermal runaway behavior of battery cells, develop a simulation-based testing methodology, and assess various lightweight enclosure materials to provide insights into factors influencing fire resistance. This work aims to establish a unified framework for evaluating EV battery pack enclosures, ensuring both lightweight design and robust safety in the face of extreme thermal events.
The importance of the EV battery pack cannot be overstated, as it directly impacts vehicle performance, safety, and longevity. Typically, the enclosure accounts for 2% to 6% of the total vehicle mass, making it a prime target for lightweighting initiatives. Materials such as polymers, composites, and metals are employed, but each presents trade-offs between weight, cost, and fire resistance. Existing standards, like QC/T 989-2014 and GB 38031-2020, offer guidelines for fire safety but fall short in specifying material-level testing for ablation resistance. This gap necessitates a reliable approach to simulate real-world thermal runaway scenarios, where the EV battery pack is subjected to high temperatures and pressurized gas emissions. My research begins by investigating the thermal runaway process of lithium-ion cells, which informs the development of a burn-through test that replicates these harsh conditions. By analyzing different enclosure materials, I seek to identify key determinants of ablation resistance, ultimately contributing to safer and lighter EV battery pack designs for the automotive industry.
To understand the demands placed on an EV battery pack enclosure during a failure, I first conducted thermal runaway tests on individual battery cells. A 3.7 V, 113 Ah ternary lithium-ion cell (622-type aluminum shell) was selected, representative of those used in many EV battery packs. The cell was wrapped with a heating band to induce thermal runaway, and a K-type thermocouple was placed between the heater and cell surface to monitor temperature. The assembly was enclosed in a metal box with a composite sheet sealed on top, simulating the confined space of an actual EV battery pack. This setup was placed within a concrete wall for safety, with a camera positioned 1 meter away to record the event. Upon powering the heater, the cell temperature rose steadily until approximately 200°C, at which point thermal runaway initiated. The process was violent and rapid: dense smoke erupted, followed by a burst of pressurized gases, all within about 35 seconds. Peak temperatures reached around 1200°C, confirming the intense thermal and mechanical loads that an EV battery pack enclosure must withstand. This observation highlighted the need for a testing method that incorporates both high-temperature flames and gas pressure, rather than static fire exposure alone.
Based on these findings, I developed a burn-through test to evaluate the ablation resistance of lightweight EV battery pack enclosure materials. The goal was to create a consistent, reproducible flame that mimics the thermal runaway conditions. I used a propane-oxygen flame torch, as propane is readily available and its combustion characteristics align with the temperatures observed in cell failures. To calibrate the flame, I referenced mica plates—a common fire barrier in early EV battery pack designs—which are expected to prevent flame penetration for at least 5 minutes during a thermal event. By adjusting the gas pressures, I aimed to achieve a flame that burns through a 1 mm mica plate in 5–10 minutes, ensuring the test’s severity is representative. The propane (C₃H₈) and oxygen (O₂) pressures were varied, and flame temperatures were measured at a distance of 100 mm from the nozzle, corresponding to the typical gap between cells and the enclosure in an EV battery pack. The complete combustion of propane requires 5 moles of oxygen per mole of fuel, so the oxygen pressure was maintained at least five times the propane pressure to approximate stoichiometric conditions. The burn-through time for the mica plate was recorded for each pressure combination, with three repetitions per setting to ensure accuracy.
| Test No. | Oxygen Pressure (MPa) | Propane Pressure (MPa) | Flame Temperature Range (°C) | Average Burn-Through Time (min) |
|---|---|---|---|---|
| 1 | 0.40 | 0.05 | 872–931 | 18.6 |
| 2 | 0.50 | 0.05 | 833–879 | 17.2 |
| 3 | 0.50 | 0.06 | 967–1024 | 12.2 |
| 4 | 0.50 | 0.07 | 1161–1247 | 9.5 |
| 5 | 0.50 | 0.10 | 1287–1355 | 7.2 |
| 6 | 0.40 | 0.07 | 1178–1263 | 11.4 |
| 7 | 0.40 | 0.10 | 1028–1087 | 13.8 |
The data revealed that flame temperature and burn-through time depend on both gas pressures. For instance, when propane pressure was fixed at 0.07 MPa, an oxygen pressure of 0.50 MPa yielded a higher temperature (1161–1247°C) and shorter burn-through time (9.5 minutes) compared to other combinations. I selected oxygen at 0.50 MPa and propane at 0.07 MPa for subsequent tests, as this produced a flame temperature near 1200°C—matching the peak thermal runaway temperature—and a burn-through time within the target range. This calibration ensures that the test accurately simulates the extreme conditions an EV battery pack enclosure might face during a cell failure.
With the testing method established, I evaluated a variety of lightweight materials commonly used or proposed for EV battery pack enclosures. These included thermoplastics like polypropylene (PP), polyphenylene sulfide (PPS), and polyamide (PA), as well as thermosets such as sheet molding compound (SMC), epoxy-based composites, and polyurethane systems. Samples were prepared or sourced to thicknesses representative of actual EV battery pack designs, typically around 2.0 mm, though some variations were included to assess thickness effects. Each material was subjected to the propane-oxygen flame at a distance of 100 mm, and the time until flame penetration (i.e., visible flame on the back side) was recorded. Three tests per material ensured reliability, and the average burn-through times are summarized below.
| Material Type | Specific Designation | Thickness (mm) | Average Burn-Through Time | Key Characteristics |
|---|---|---|---|---|
| Thermoplastic | PPS (GF8 reinforced) | 1.3 | 7 min 19 sec | High glass transition temperature |
| Thermoplastic | PPS (GF8 reinforced) | 2.0 | 8 min 15 sec | Improved resistance with thickness |
| Thermoplastic | PP (LG30 grade) | 2.0 | 58 sec | Low fire resistance, prone to dripping |
| Thermoplastic | PP (LG30 grade) | 3.5 | 2 min 18 sec | Thickness enhances performance slightly |
| Thermoplastic | PA66 (flame-retardant) | 2.0 | 18 min 29 sec | Excellent ablation due to char formation |
| Thermoplastic | PA66 (non-flame-retardant) | 2.0 | 1 min 33 sec | Poor performance without additives |
| Thermoset | SMC (A103 grade) | 2.0 | 3 min 32 sec | Non-continuous fibers, moderate resistance |
| Thermoset | Epoxy PCM (prepreg) | 2.0 | 4 min 39 sec | Continuous fibers, better structural integrity |
| Thermoset | Epoxy HPRTM | 2.0 | 5 min 7 sec | High-pressure resin transfer, good ablation |
| Thermoset | Polyurethane HPRTM | 2.0 | 5 min 18 sec | Similar to epoxy, with flexibility |
| Thermoset | Polyurethane STM | 2.0 | 3 min 49 sec | Spray transfer molding, lower resistance |
The results demonstrate significant variations in ablation resistance among materials, underscoring the importance of careful selection for EV battery pack enclosures. For example, flame-retardant PA66 outperformed all others with a burn-through time over 18 minutes, while standard PP failed in less than a minute. This disparity highlights the critical role of material composition and additives in ensuring safety for the EV battery pack. To delve deeper, I analyzed key factors influencing ablation resistance, which can guide the design of lightweight yet fire-resistant enclosures for the EV battery pack.
First, material thickness is a primary determinant of burn-through time. In general, thicker enclosures for the EV battery pack provide longer protection, as evidenced by the PPS samples: increasing thickness from 1.3 mm to 2.0 mm extended the burn-through time by about 56 seconds. This relationship can be approximated by a power-law equation, where burn-through time \( t \) is proportional to thickness \( d \) raised to an exponent \( n \):
$$ t = k \cdot d^n $$
Here, \( k \) is a material-dependent constant, and \( n \) typically ranges from 1 to 2 for polymer-based composites. For instance, from the PP data, doubling the thickness from 2.0 mm to 3.5 mm increased \( t \) by a factor of approximately 2.4, suggesting \( n \approx 1.3 \) for this material. However, excessive thickness contradicts lightweighting goals for the EV battery pack, so an optimal balance must be struck. In practice, enclosure thickness should align with vehicle-specific requirements, often between 2.0 mm and 3.0 mm, to maintain structural integrity without adding undue mass.
Second, flame-retardant properties profoundly impact ablation resistance. The comparison between flame-retardant PA66 (102fr-RG600) and its non-retardant counterpart (102-RG600) is stark: the former lasted 18+ minutes, while the latter failed in under 2 minutes. This difference arises from mechanisms such as char formation and gas-phase inhibition. Flame-retardant additives, often containing phosphorus or halogens, promote the development of a carbonaceous char layer upon heating. This char acts as a thermal insulator, reducing heat transfer to the underlying material and minimizing mass loss. Moreover, these additives release inert gases that dilute oxygen near the surface, slowing combustion. The effectiveness can be modeled using a char growth rate equation:
$$ \frac{dC}{dt} = \alpha \cdot T – \beta $$
Where \( C \) is char thickness, \( T \) is temperature, and \( \alpha \), \( \beta \) are constants related to material composition. For EV battery pack enclosures, incorporating flame-retardant grades is essential to enhance safety, as even thin sections can achieve substantial burn-through times with proper formulations.
Third, the distinction between thermoplastic and thermoset materials influences ablation behavior. Thermoplastics like PPS and PA66 have glass transition temperatures (\( T_g \)) that dictate their performance under heat. Materials with higher \( T_g \), such as PPS (\( T_g \approx 90^\circ \text{C} \)), resist softening and dripping better than those with lower \( T_g \), like PP (\( T_g \approx -20^\circ \text{C} \)). Dripping can exacerbate fire spread in an EV battery pack, making it a critical failure mode. The resistance time \( t_r \) can be correlated with \( T_g \) through an Arrhenius-type relation:
$$ t_r = A \cdot e^{\frac{E_a}{RT_g}} $$
Where \( A \) is a pre-exponential factor, \( E_a \) is activation energy, and \( R \) is the gas constant. Thermosets, such as epoxies and polyurethanes, do not melt but instead decompose, often retaining shape better under fire. This makes them suitable for EV battery pack enclosures where dimensional stability is crucial during thermal events. However, their performance depends on fiber reinforcement; continuous fibers (e.g., in PCM) offer superior ablation resistance compared to discontinuous fibers (e.g., in SMC), due to better load-bearing and heat dissipation.
Other factors include fiber content and orientation, which affect the structural resilience of the EV battery pack enclosure. Higher fiber content, especially of glass or carbon fibers, improves fire resistance by providing a scaffold that slows degradation. The burn-through time can be expressed as a function of fiber volume fraction \( V_f \):
$$ t \propto V_f^m $$
With \( m \) around 0.5–0.8 for many composites. Additionally, the pressurized gas during thermal runaway imposes mechanical stresses, so materials with higher toughness, like some thermosets, may outperform brittle alternatives in real-world EV battery pack scenarios.
In conclusion, this study establishes a simulation-based testing method for evaluating the ablation resistance of lightweight EV battery pack enclosures, addressing a gap in current standards. By replicating the high-temperature, pressurized conditions of battery thermal runaway, the method provides a reliable means to compare materials and inform design choices. Key findings indicate that material thickness, flame-retardant additives, and the thermoplastic-thermoset dichotomy are major influencers of fire performance. For optimal EV battery pack safety, enclosures should combine adequate thickness (e.g., 2.0–3.0 mm), flame-retardant formulations, and materials with high glass transition temperatures or thermoset matrices. Future work could explore the impact of particulate emissions from cell failures on enclosure integrity, as well as develop multi-scale models to predict ablation behavior under diverse scenarios. As the EV industry evolves, standardized testing based on such approaches will be vital for ensuring that lightweight EV battery pack enclosures meet stringent safety demands without compromising vehicle efficiency.