Evaluating Polyolefin Separator Friction Limits in Lubricated Cell Stacks
Sufficient stack pre-load retention and ceramic coating friction prevent wetted polyolefin separator sliding and tab shear in lubricated cell stacks.
Stack Pre-Load Decay
Meaning
Gradual reduction over time of the initial clamping force applied to a group of battery cells describes a primary mechanical aging effect in module assemblies. Sourcing agreements address stack pre-load decay because it can lead to cell movement, increased contact resistance, or accelerated electrode degradation. This loss of force stems from the relaxation of the polymer foam spacers and the creeping of the structural module bands.
It must be modeled accurately to ensure that the remaining pressure at the end of life is sufficient to hold the cells securely.
Decay Mechanism
Continuous exposure to elevated temperatures and cyclic pressure changes accelerates the viscoelastic deformation of the elastomeric materials used for cell compression. Over thousands of hours, the polymers within the spacer foam reorganize their molecular structure to relieve the stress, reducing their elasticity. At the same time, the physical expansion and contraction of the cells during charging and discharging cycles exert high cyclic forces that slowly stretch the metal or composite retaining straps.
The combination of these two phenomena results in a steady downward trend of the holding pressure.
Engineering Mitigation
Engineers counteract this behavior by selecting materials with low creep rates and designing structural systems with built-in spring-like compliance. Sourcing specifications often demand the use of advanced silicone foams or spring washers that maintain a relatively flat force-deflection curve over their operational lifetime. These high-performance components absorb the initial relaxation without allowing the force to drop below the threshold needed to prevent electrode delamination.
Sourcing Criterion
Contracts require suppliers to provide long-term force retention curves under simulated thermal and mechanical aging conditions. This documentation allows procurement teams to verify that the assembly will remain mechanically stable over the entire ten-year or fifteen-year vehicle lifecycle.