Voltage Collapse and Early Failure Indicators in a Degraded EV Battery Under High-Current Load

This paper investigates the safety behavior of degraded lithium-ion battery modules taken from a 2016 Nissan Leaf (30 kWh, 106,394 km). The vehicle exhibited typical failure symptoms, including P33E6 faults, sudden range drops, and activation of turtle mode under load. Initial diagnostics based on LeafSpy data revealed strong cell imbalance, with a voltage spread exceeding 2.3 V under high current (≈170 A). The weakest cells dropped close to 1 V, suggesting severe internal degradation. To better understand this behavior, selected modules (cells 73–88) were removed and tested under controlled laboratory conditions. Capacity measurements in a 16S2P configuration showed 49.8 Ah in the 4.1–3.1 V range, corresponding to a state of health of about 59%, which is consistent with BMS estimates. However, high-current discharge tests on the weakest segment revealed a much more critical picture. One cell experienced rapid voltage collapse (from ~4.0 V to ~1.2 V), accompanied by a sharp increase in voltage divergence and visible thermal effects. Infrared observations indicated localized heating up to 43 °C and irreversible swelling, pointing to early-stage electro-thermal instability. These results suggest that moderate SOH values do not necessarily reflect actual safety margins under dynamic load conditions. Overall, the study shows that simple OBD-based diagnostics can help identify problematic modules, but additional load testing is necessary to assess real safety risks in aged EV battery systems.