Meaning
Mechanical forces are applied to battery cells within a pack assembly to control volumetric expansion and maintain structural integrity. This system of pack preload compression uses structural plates and springs to resist the swelling that occurs during cycling. It is designed to accommodate both reversible breathing and irreversible expansion over the lifetime of the cells.
The force is carefully calculated to prevent cell deformation.
Mechanical Requirement
Pouch and prismatic cells experience internal pressure changes as lithium ions move between the electrodes and cause lattice volume changes. Applying pack preload compression prevents the expansion from delaminating the internal electrode layers or causing mechanical deformation of the cell casing. This compression must remain within a specified range, as excessive force damages the separator and induces internal short circuits.
The mounting hardware must adapt to these force variations.
Performance Benefit
Maintaining uniform contact between the electrodes and the separator optimizes the ionic transport pathway within each cell. Consistent pack preload compression reduces the rate of capacity fade by preventing the loss of electrical contact between active material particles. This uniform pressure also discourages the formation of lithium dendrites, extending the safe operating life of the pack.
Sourcing Metric
Procuring engineers specify the required spring rates and end-plate stiffness based on the cell’s expansion data. High-quality materials for pack preload compression must maintain their elastic properties over the vehicle lifetime. This durability reduces maintenance costs.
