Meaning
Time dependent deformation property identifies how a porous polymer material slowly loses its ability to push back against a constant load over long durations within a battery stack. Systematic elastomeric compression foam creep reduces the steady pressure applied to the cells which can lead to electrical disconnects or changes in the internal resistance of the electrochemical storage unit.
Deformation Progress
Mechanism begins with the initial collapse of individual cell walls within the foam matrix when the compression is first applied during the module assembly stage. Progressive elastomeric compression foam creep follows as the polymer molecules slowly reorganize under the sustained tension and allow the material to become permanently thinner over time. This loss of thickness decreases the effective clamping force which is necessary to keep pouch cells from swelling and delaminating during their operational lifespan.
Engineers account for this gradual thinning by over sizing the initial foam thickness so that the end of life pressure remains above the minimum safety requirements.
Thermal Acceleration
Higher operating temperatures increase the speed at which this structural thinning occurs because heat provides the energy needed for molecular chains to move past each other more easily. Any study of elastomeric compression foam creep shows that materials which perform well in room temperature tests may fail rapidly when exposed to the fifty degree conditions found inside an active battery. Designing around this limitation requires testing across several months to plot the decay curve of the force to ensure the hardware maintains its mechanical grip on the internal components.
Advanced silicone based foams are often chosen for these applications due to their high resistance to thermal softening compared to standard polyurethanes or rubbers.
Mechanical Consequence
Reduction in compression force can lead to increased gaps between the cell and the cooling plate which decreases the efficiency of heat transfer during fast charging events. Detecting significant elastomeric compression foam creep involves monitoring the internal pressure of the battery pack through specialized sensors installed in test modules during development. If the creep levels exceed engineering limits, the loose internal components become more susceptible to damage from road vibration and external mechanical shocks.
Design specifications for these foams specify the maximum allowable percentage of permanent set to guarantee that the pack remains tight and quiet throughout the full service life of the vehicle.
