Heatshield Technology
The Best Combination of Formability and Burn Through Time in the E-Mobility Market
Where the engine was once the powerhouse of the vehicles, battery cells are largely taking over. With more and more functionalities being fuelled by electrical power, the components become larger and require new arrangements. The size and placing depends on the model and the constitution, if it is a hybrid, plug-in, or electric vehicle. However, the majority have reached a size where these either share the hood with a smaller engine in PHEVs or are placed directly beneath the passenger cabin in between the axis with a large quantity of modules. When large amounts of energy are placed in closer proximity to the passengers, also safety regulations increase in case of system failure.
The most commonly used batteries are Li-batteries and currently pack between 58 – 260 kW/h. In normal use, the energy is well preserved. However, what happens, if one battery cell or the entire battery system is damaged by mechanical stress, overcharge or a short circuit? In short, the thermal event can lead to a thermal runaway or even to a thermal propagation, discharging the energy stored in the battery system causing an inflammation of the battery cells or even an electric arc in the weakest part of the system. In the worst case, the particle beam enters the passenger cabin and becomes a life-threatening danger to the passengers. As a result, safety measures in the overall battery system have become a number 1 priority.
To minimize risk, the Battery Management System (BMS) and the Thermal Management System (TMS) continuously monitor the battery conditions in terms of voltage, current and temperatures. Any changes above a certain threshold which the systems cannot deter, trigger a passenger warning. Despite these safety measures, the passenger currently has less than 30 seconds to exit the vehicle. Legal requirements such as mentioned in the ECE-R100 now specify a minimum time of 5 minutes for the battery system to withstand, after the passenger warning is issued.
Granting minimum 5 minutes requires a more resilient battery system overall. Each component adds to the resilience, however not all changes serve the automotive requirements. First, the battery cells could contain flame retardant chemicals which can reduce the runaway effect, however cannot impede it. Second, the battery shell could become sturdier by exchanging aluminium with steel frames, which in turn is further from the goal of building a lightweight, long range and non-corrosive vehicle. None of the existing components bear the relevant characteristics of flame retardance and dielectric strength required.
An additional component is required to add significant resilience. Though which capabilities must it have to best achieve it?
- It must be fireproof at temperatures >1000 °C when cells enflame
- It must prevent particles from passing through before forming a particle beam
- It must have isolation properties to shield thermal sensitive components, such as connectors, cables, etc.
- It must withstand dielectric strength between 5 – 10 KV at the occurrence of an electric arc
Fireproof
Within milliseconds of a thermal runaway, the battery temperature spikes up to 400 °C. A software-based solution is likely fried at this point. A mechanical fireproof solution can be considered the only option. The regularly used Aluminium or composite lids can only withstand up to 600 °C and 400-500 °C respectively before reaching their melting points. This however, does not suffice to withstand the between 1000 – 1200 °C the fire reaches at its peak. Fortifying this component by applying mineral coating on steal or refractory bricks comes with downsides such as high weight, thermal conductivity, and possible cracking upon too strong vibrations. A light weight, and fireproof material that can withstand temperatures higher than 1000 °C would be ideal. Glass fibres are a well-known insulation material with high temperature resistance. In combination with a mineral coating, it cannot only withstand the needed temperatures, but even has a cooling effect leading to a longer burn-through-time. However, not all glass fibres are the same. Glass fibre tissue cannot withstand temperature for as long as a denser version – glass fibre fleece. Different layers are compared in Fig 1. We tested different layered compositions to find the optimal combination between burn through time and thickness for the automotive sector. The version 1.1 consists of several layers of fleece, the mineral coating, and an additional fire barrier yet still withstands the temperatures for longer than 5 minutes, while being thinner than 5 mm. This version of VitriShield Battery was already chosen by customers as a new battery component on the market. By adding an additional layer of fleece in sample 1.3, the temperatures measured can be reduced even further, still leaving room for customization.
Insulation properties
A lack in insulation properties in the battery system can also damage components with low thermal resistance. Without additional protection, the battery system may heat up connectors or cables to the point they induce a short circuit – one possible cause for a thermal event. Thus, prevention of danger inducing stress is equally as important as the other properties. TBMS strive to maintain a low battery temperature between 20°C and 40°C. Only at temperatures above 130°C does it become critical for the battery cells. The surrounding components reach their melting points around the same temperature. In fact, regular polymer-based connectors melt around 125°C and their high temperature resistant counterparts withstanding around 150°C, only marginally adding to prevention. Whichever type of connector is used, an additional layer with isolation properties decreases the risk of a short circuit through damaged connectors, wires, etc. significantly.
Particle burning
In addition to high temperature development, the cathode and anode also reach their meting points between 1000 °C – 1200 °C and disintegrate into burning metal particles. These are ejected into the battery system and cause tears wherever they touch. Even several layers of glass fibre fleece lack the mechanical strength on their own. According to own customer requirements, the battery system must withstand the pyro particle burning test for 20 seconds, i.e., a continuous particle stream. Additional fire barriers allow VitriShield Battery to pass the test successfully.
Dielectric strength
Not all battery systems are built equally. Weak spots widely differ and where for some the occurrence of electric arcs is negligible, for others this is an important and frequent occurrence. The component only needs to fortify what the system is lacking, which makes customization all the more important and also ensures the right safety precautions. To overcome an electric arc with strengths between 5 – 10 KV, the used materials may not conduct electricity, reducing the available alternatives. Many types of metals are not suitable; however glass textiles neutralize the electric arc and prevent it from spreading to the passenger cabin.
In addition to the legal requirements, the component must meet manufacturing standards. Diederik Cuylits, Chief Technology Officer and one of the owners of Culimeta, stated: “In the automotive industry, all suppliers are held to extremely high standards, not only in terms of product quality and size, but most importantly the technical cleanliness. With our latest innovation in heatshield technology, we held ourselves to equally high standards so VitriShield Battery can meet and exceed the customer needs.” In the assembly lines, product quality can be divided into two characteristics: product fit and technical cleanliness according to VDA. As part of product fit, the component must be thinner than 5mm and must conform well to the battery lid’s 2D and 3D shapes. The technical cleanliness is a challenge when working with fibres. An additional composite film helps prevent filaments from spreading during the assembly and ensure meeting the highest standards. Finally, complying with all requirements and ensuring consistent quality for large quantities requires state-of-the-art manufacturing processes with a certain level of automation.
To test the component capabilities there are types which test the component on its own and those that test the entire battery system resilience. To prove the latter and ultimately pass the ECE-R100, the fuel fire test is conducted. Here, the complete battery system is exposed to an enflamed fuel fire test for 5 minutes and may only continue burning for a maximum of 15 minutes after being taken off the fire. However, before conducting this final test, components can be tested for the compatibility by conducting a particle burning test as previously described. The passing thresholds largely depend on the individual system requirements, however withstanding a consistent stream for a minimum of 2 minutes can be considered appropriate. Due to the lack in testing regulations and the largely varying system requirements, the uniquely applied tests must be met and additional development time needs to be taken into consideration.
Meeting both, the overarching safety requirements and the unique technical requirements is a challenge that few in the market can satisfy. The largest difficulties are faced on delaying the particle beam for long enough. Despite the difficulties, VitriShield Battery meets all requirements and even exceeds the prerequisite with a system resilience of up to 13 minutes. With this, Culimeta offers the best combination of formability and burn through time in the e-mobility market, thereby generating maximum safety for the passengers with a thin, customizable system component.
Dr. Florian Winter Head of R&D Culimeta Textilglas-Technologie GmbH & Co. KG
