Requirements for electric vehicles (EVs) are continually evolving. With consumer expectations for longer ranges and shorter charging times increasing, it’s no wonder that battery developers are focusing on higher battery capacities (kWh), higher capacity per battery weight (kWh/kg), and faster charging solutions.
Despite significant progress in this field, challenges remain, especially in the repeatability and standardization of testing methods. Mobility suppliers around the globe are actively addressing these challenges by improving testing procedures and creating new materials that enhance the safety of EV batteries.
In some cases, minor modifications to existing materials are sufficient. The requirements for media and temperature resistance for EV batteries are often even lower than those for applications in vehicles with combustion engines. In other cases, new materials and components — such as 2D thermal barrier mats, flame barrier profiles, and heat shields — are being used that are new to the automotive industry and come with new sets of requirements.
Figure 1. Although EVs experience fewer fire accidents compared to ICE vehicles, battery safety remains a priority for automakers and consumers.
Evolving battery requirements
Although EVs experience fewer fire accidents compared to traditional combustion engine vehicles (1.2 fires per 10,000 EVs compared to 7.3 fires per 10,000 combustion vehicles), battery safety remains a priority due to the increasing energy densities and faster charging requirements.* One critical concern is the risk of thermal runaway, where a single cell’s failure can trigger a chain reaction in neighboring cells leading to a chain reaction (thermal propagation).
While new battery designs, active monitoring systems, and temperature control can mitigate these risks, the materials used to contain these reactions are vital to enhancing vehicle safety. These include flame-retardant materials that ensure the vehicle interior is protected in the event of a thermal runaway of individual cells or the entire battery. This is where it becomes apparent that existing methods for testing the thermal barrier properties of individual materials are insufficient, making them unreliable for characterizing these properties.
Proven and adapted test methods
As with ICE vehicles, all components and materials for EVs must meet precise specifications for basic mechanical parameters such as hardness, tensile strength, and elongation at break. In addition, the maximum extent to which these properties may change over a certain period under normal application temperatures is usually also specified.
Several established standard test methods have not yet been used in testing elastomer and plastic-based materials in ICE vehicles but have long been used in other industries. These methods can be transferred relatively easily to materials for e-mobility applications. Examples of these tests are thermal conductivity at different temperatures, dielectric strength and creep resistance, and a basic flammability rating (such as UL94 V or H). Nevertheless, adaptations to these established test methods for EV applications are often necessary.
For example, the heat flow meter and mechanical cycle testing are two tests used to characterize heat shields. Both are adaptations of existing test methods. The heat flow meter test checks thermal conductivity under different compression loads and temperatures. This allows insulation properties and heat flow between the cells to be mapped at different compression levels or phases of cell breathing and battery service life. The cycle test is an adaptation of classic fatigue or relaxation/hysteresis measurements and provides hysteresis of compressive force during and after different load cycles with constant data recording.
Measuring a material’s dielectric strength is also a common practice. In the development of fire protection materials for busbars, the electrical properties of the material are measured after exposure to fire. This involves investigating how well a material remains insulated after a fire to prevent short circuits that could lead to further hazards.
Testing the thermal barrier properties
However, new test methods must also be developed and standardized for material behavior during a thermal runaway. This applies to materials and components with direct and indirect exposure to flames and particles. So far, a distinction has been made between three main categories: flame contact testing, pyrotechnic testing, and heat-only testing.
Figure 2. The functional tests of materials with direct flame contact and with or without particle contact.
Figures 2 and 3 show test complexity, safety requirements, costs, and relevance for the application increasing from left to right. The tests range from material tests and component testing to testing a complete battery. Ultimately, the behavior of a complete battery system is directly relevant to passenger safety. Therefore, it is essential to test the suitability of individual components and materials at the battery system level to meet high safety standards.
Figure 3. The functional test of heat protection materials without direct exposure to flames or hot particles.
It’s important to note that these tests are relatively new to the automotive industry. Therefore, it’s not surprising that both the tests themselves and the specific test conditions vary considerably depending on the manufacturer, battery technology, and design and are still being optimized.
• Tests with flames and cold particles. The torch test, torch & grit test (TaG), and pyrotechnical test are particularly important for developing materials with direct exposure to flames or flame and particles. In the torch test, a defined flame (1,000° to 1,500° C) is aimed directly at the sample to evaluate a material’s flame and temperature resistance depending on its thickness. The TaG test supplements this with a cyclic change between flame and blasting with defined, cold particles. The most important result of both tests is the burn-through time for relative material evaluation. Almost all test providers also measure a temperature curve.
Experts in the field have carried out extensive torch and TaG tests, both internal and with various external test providers, for numerous materials. While advancements in measurement methods have been made, results still show some variability in burn-through times, with occasional deviations in repeatability and maximum values.
The temperature curve would be of greater interest for a deeper understanding of the different phases of material changes and degradation processes. Unfortunately, no current test method or test facility with sufficient repeatability can be used beyond comparative tests to support the formulation and development of material composition. This is due to different, non-standardized test procedures and setups.
Examples are the various methods and locations of temperature measurement, differences in sample holders and the flamed sample cut-outs, and the type of detection of the burn-through of the samples.
• Pyrotechnic tests. In contrast to torch and TaG tests, the pyrotechnic test simultaneously exposes the sample material to flames and hot particles. This is similar to the conditions created during a direct venting event of a battery.
However, as this test only lasts 20 seconds, it provides considerably less information and no temperature curves. To ensure that the fireworks are as consistent as possible, stage fireworks are used to burn relatively evenly for safety reasons. Nevertheless, the results are not sufficiently reproducible. They fail to provide the depth and detail required for comprehensive material developments. As a result, pyrotechnic tests are only suitable for obtaining quick feedback on a material’s basic fire protection properties.
• Testing under heat. Materials only exposed to heat but not direct flames are often tested using hot plates or hot oven tests. Although testing materials before and after heat exposure is well-established, temperatures (ranging from 400° to 800° C) are considerably higher than those typically encountered in the automotive industry. When the preheated oven is opened to load the sample, the temperature inevitably drops, and not all ovens can regain the required test temperature within the limited timeframe.
Additionally, the positioning of the sample — whether on a grate or a solid metal surface — affects the results as it alters the heat transfer rate into the material. The hot plate test is crucial for evaluating components inside and outside the battery that are not directly exposed to open flames.
Both new test methods and new materials are still under development and need further improvement in terms of their properties and reliability, presenting both a challenge and an opportunity for the automotive industry.
In conclusion, the ongoing development of EVs requires new materials and test methods that meet increasing industry requirements. Although significant progress has already been made, there is still room for improvement in test repeatability and standardization. The development of safe and efficient flame-retardant materials is crucial to the success of electro-mobility.
Through continuous research and development, the mobility industry can ensure that the batteries of the next generation will not only be more powerful but also safer.
* Source: Product development of e-mobility components RWTH Aachen University (2023), Challenges and solutions in battery safety]
Filed Under: Batteries, FAQs, Featured Contributions, Testing and Safety