What is Compression Force Deflection and Why is this Important in EV Applications?
Compression Force Deflection (CFD) is a value measured on a foam to express its compressibility (strain) when subjected to load (stress). It is way to express how much force (stress) will the foam push back with when there is a compressive load.
“CFD is a material property of C2C pads that is tunable to accommodate the differing requirements of cells with a goal to maximize cell performance and extend its life.” — Senthil Jayaseelan, Global EV Application Engineering Manager
CFD allows manufacturers and analysts to precisely determine a material’s firmness at varying levels of compression.
It is one of the most important testing methods for compression/tolerance pads used in the development of battery packs destined for use in electric vehicles.
With the number of electric vehicles in use across personal transportation, public transit and industrial applications rising year on year, the demand for increasingly efficient battery technology also continues to rise.
Lithium-ion batteries remain the battery of choice for electric vehicles due to their efficient charging speeds and impressive energy density. Unless they are properly manufactured, optimized, and used, these batteries can have relatively short working lives when used in challenging automotive and other transportation applications.
The chemistries used in lithium-ion batteries rely on the principle of compression, as the cells’ physical dimensions are altered due to the electrochemical reactions taking place during charging and discharging. Batteries will expand and contract cyclically (reversible) as these are charged and discharged, while battery cells also gradually increase in thickness (irreversible) between their beginning and end of lives.
Research has shown that implementing a degree of restriction — thus forcing the cells away from a ‘free’ expansion — allows cells to maintain their efficiency for longer periods.
Restricting these cells too much can lead to safety issues, however, while allowing the cells to expand too freely could also cause the cells to lose their electrical connections or their contact to the battery’s cooling plate (necessary for thermal management and safety). This need to optimize performance while maintaining safety means that it is imperative that these restrictions are implemented correctly, and that pressure is maintained at optimal levels at all times. One of the most useful means of managing pressure on the face of a battery cell is through the use of compression pads, ideally manufactured from an appropriate material such as a microcellular polyurethane foam.
CFD is an ideal tool for determining the extent to which the foam used in a compression pad provides a consistent compression force when exposed to a wide range of pressures. Ideally, a compression pad should demonstrate a narrow range of stress, even when exposed to a wide range of strain, therefore ensuring that predictable and optimal pressure is exerted onto the cell at all times. This pressure must also remain consistent throughout a significant number of charge and discharge cycles.
It is also important that any foam selected for use in electric vehicle application is exceptionally resistant to permanent deformation (commonly referred to as compression set) when exposed to extreme pressure or compression loads. Foam should also offer sufficient electric insulation to help prevent the risk of arcing within modules.
Norseal Microcellular Polyurethane Foam from Saint-Gobain Tape Solutions are ideal as battery compression pads in demanding applications such as those found in electric vehicles. The Norseal range of microcellular polyurethane foams has been categorized according to its CFD measurements, making it easy to select an appropriate foam based on the battery cell performance requirements and the degree of compression deflection to be accommodated.
Foams are available in a range of thicknesses and CFD curves ranging from very soft to very firm. A range of specialist foams is also available.
The complete range of foams offers excellent resiliency over the entire compression range, while being sufficiently energy-absorbing to minimize vibration. Minimizing vibration is an important consideration for electric vehicle applications that may see batteries required to maintain consistent operations despite uneven terrain or significant speeds.
Norseal PF100 features premium microcellular polyurethane foam that has been specially developed to provide robust cell cushioning in electric vehicle battery packs. Most notably, PF100 offers the flattest, widest compression deflection range of the portfolio, making it an excellent option for use in electric vehicle battery packs where a robust combination of reliability, safety and high performance is vital. Available in the lowest thickness to density combination currently available, allows battery developers to maximize energy density while maintaining minimal weight requirements — essential in improving the range and efficiency of electric vehicles.
As demand for battery-powered electric vehicles continues to grow, it is increasingly important that the batteries used in these vehicles strike an optimum balance between efficiency and safety. The use of compression pads fabricated from robust foams like Norseal PF100 premium microcellular polyurethane foam offers an ideal means of optimizing the performance of the lithium-ion batteries used in electric vehicles.
The use of compression force deflection testing provides a reliable metric for determining the appropriate foam choices for compression pads, according to the specific performance and compression/restriction needs of the application in question.