Metal foreign matter defect method for lithium-ion batteries based on free-discharge high-potential test

Metal foreign matter defects in lithium-ion batteries can cause internal short circuits and thermal runaway, posing serious safety risks. However, conventional high potential (Hi-pot) testing often fails to reliably detect such defects in real production lines. This paper proposes a detection method based on free-discharge features during Hi-pot testing. The free-discharge initiation voltage U 1 and termination voltage U 2 are extracted as characteristic indicators, and the local outlier factor (LOF) algorithm is used to identify defective batteries. Copper particles with diameters of 150, 250, and 350 µm were implanted into 270 defective batteries, alongside 3000 normal batteries, and tested at 250, 350, and 450 V. Results show that increasing test voltage and particle diameter improves detection performance. At 450 V, the method achieves recall rates of 56.67% (17/30) and 80.00% (24/30) for 150 and 350 µm defects, respectively. The proposed method offers a practical and reliable approach for detecting metal foreign matter defects in battery manufacturing.