In the mobility sector, the transition from internal combustion engine to electric powertrain not only presents a huge opportunity for OEMs, but also a series of challenges. One of those challenges is maintaining the integrity and safety of the battery – something that is vital for the increased ranges consumers are demanding, as well as the industry’s confidence in the safety of a vehicle’s driver and passengers.
With the majority of electric vehicles using Li-ion batteries, the main issue for manufacturers is the risk of thermal runaway. Li-ion batteries may offer higher power and energy density, but they also generate a lot of heat, which if not controlled, can dramatically increase and eventually cause thermal runaway, resulting in the battery catching fire or even exploding. Not only will this cause extensive damage to the car, but potentially put the vehicle’s occupants in danger.
Therefore, one of the key requirements for sealing materials in the batteries of electric vehicles to contribute to overall thermal management is fire resistance. As the batteries have evolved, the legislation around fire safety has been continuously changing, and now all modern battery seals are required to have a V0 rating, which means the materials are self-extinguishing and can survive exposure to fire for certain time.
However, by their very nature, organic elastomers that make up standard sealing compounds are highly flammable, thus fire resistance must be achieved by other means, such as using fire-retardant additives or fire-retardant coatings.
in batteries of electric vehicles
In this article we focus on introduction of various methods evaluating fire resistance of materials.
The four major fire resistance tests
While the physical properties of rubber compounds such as tensile strength, elongation, and compression set are relatively simple to assess and compare, testing their fire resistance is more difficult to quantify. In addition, the tests have certain practical difficulties and potential health and safety hazards.
That said, several key tests have emerged, each one offering different piece of information about the compound that add up to a larger picture of fire resistance.
Among the many sealing solutions Datwyler offers for the e-powertrain, we will focus in this article on three specific products: the housing gasket, oil baffle and LSR 2K gasket.
#1
Limiting Oxygen Index
Defined as the minimum concentration of oxygen in a mixture of oxygen and nitrogen that is needed to support the flaming combustion of a material, the Limiting Oxygen Index (LOI) is expressed in volume percent of oxygen. Materials with LOI values less than 21% are classified as combustible, since their combustion can be sustained at ambient temperature without any external energy contribution. Those with an LOI greater than 21% are classified as self-extinguishing. The higher the percentage, the greater the fire resistance.
The test involves supporting a sample of the material vertically in a transparent chimney while a mixture of oxygen and nitrogen flows upwards. The upper end of the sample is ignited, and the subsequent burning of the specimen is observed to compare how long the burning continues or the length of the sample burnt, both within specified limits. By testing a series of samples in different oxygen concentrations, the minimum oxygen concentration is determined.