Meaning
High-pressure gas exposure methods force a tracer medium into the internal cavities of sealed components to detect subsequent escape. In battery cell quality assurance, vacuum bombing involves placing completed cells inside a pressurized vessel filled with helium gas for a set period. If a cell has a micro-crack, the external pressure forces the helium into the housing, where it can later be detected during chamber testing.
This non-destructive method is particularly value-producing for sealed pouch cells that do not have an integrated fill port or valve for direct gas injection.
Test Procedure
The process starts with loading the cells into the bombing chamber and pulling a vacuum to remove air. High-pressure helium is then introduced and maintained to allow gas penetration through any defects. After venting, the cells are transferred to a separate detection chamber to measure the rate of helium effusion from the defects.
Tracer Diffusion
The amount of tracer gas that enters the cell depends on the applied pressure and exposure time. This relationship is modeled using gas flow equations to ensure that the internal gas concentration is high enough for reliable detection. A longer exposure is needed for very small defects or low pressures.
Process Constraint
A primary limitation is the potential for helium to remain trapped on the outer surfaces of the cell, leading to false rejects. This surface helium must be removed by a dry-air wash before the detection step. The additional step increases the overall cycle time in the quality control process.
