Experimental Safety Assessment of Degraded EV Battery Modules: Insights from High-Current Testing and Thermal Behavior

This study presents an experimental safety assessment of degraded lithium-ion battery modules from a 2016 Nissan Leaf 30 kWh pack (106,394 km) exhibiting P33E6 faults, "turtle mode" activation, and sudden range drops. On-vehicle diagnostics using LeafSpy revealed severe cell voltage imbalance, with a 2323 mV spread across 96 cells under high-current load (165–170 A), individual cells dropping to 1.041 V. Laboratory capacity testing of modules 73–88 (16S2P configuration) measured 49.8 Ah (4.1–3.1 V) versus nominal 84 Ah, confirming SOH 59% (vs. BMS-reported 57%). High-current discharge (600 W → 40 A) of the weakest segment (cells 81–84) demonstrated critical safety limits: cell 82 voltage collapsed from 4.04 V to 1.2 V within 56 minutes, cell voltage delta escalated from 20 mV to 810 mV after 38 minutes, culminating in open-circuit failure. Infrared thermography recorded localized surface heating to 43 °C (ΔT = 24 K above 19 °C ambient) with irreversible cell swelling, indicating early thermal runaway precursors. The findings validate LeafSpy OBD diagnostics for identifying at-risk modules and underscore the necessity of high-power bench testing with thermal monitoring for aging EV packs. Fleet operators should prioritize module replacement for cells showing >200 mV load delta to prevent sudden power loss and thermal events in 30 kWh Nissan Leaf vehicles.