Acceptance Quality Limit Sampling Plans for Lithium Cell Receiving Facilities
Enforce ANSI ASQ Z1.4 Level II sampling with AQL 0.010 for critical safety flaws and Kelvin OCV IR screening to reject defective lithium cells at receiving dock.
Prismatic Cell Swell
Meaning
Mechanical expansion of battery housings occurs due to the physical changes in the electrode materials during charging and discharging. This phenomenon, known as prismatic cell swell, causes the thick metal walls of the cell to bulge outward. It is a critical factor in module design, requiring engineers to allocate physical space and apply compression to control the growth.
Compression Force
External pressure must be managed to maintain the structural integrity of the cell and maximize performance. To mitigate prismatic cell swell, modules are designed with rigid endplates and spring elements that apply a constant compression force. This prevents the electrodes from delaminating or deforming.
Excessive pressure must be avoided, as it can accelerate the depletion of the liquid electrolyte. By optimizing the spring rate of the compression foam, engineers can keep the cell pressure within the ideal range throughout the entire life of the pack, extending cycle life.
Cycle Life
Long-term swelling behavior affects the overall lifespan of the battery pack. Over hundreds of cycles, prismatic cell swell increases as the active materials undergo irreversible structural changes and gas accumulates. This progressive growth must be monitored to ensure the module housing does not rupture.
Sourcing decisions for pouch and prismatic cells must account for these expansion limits.
Degradation Analysis
Measuring the expansion rate provides valuable insights into the internal degradation state of the cell. Tracking prismatic cell swell during testing helps engineers identify anomalous degradation modes, such as lithium plating or binder breakdown. This data is used to optimize the charging algorithm, preventing excessive swell at low temperatures or high rates.