EV Battery Pack 8D — Thermal Runaway Root Cause & Pr...

Problem Overview Product: 400V / 60kWh EV battery pack with 96s3p configuration using NMC pouch cells Failure Mode: Battery pack shut down during DC fast charging at an ambient temperature of 38°C. BMS reported cell module #7 over-temperature (65°C internal estimated). Post-event inspection showed 3 pouch cells in module #7 with 10-15% swelling. Detection: BMS over-temperature protection triggered pack disconnect at 62°C. No thermal runaway occurred — the protection system functioned correctly. Customer Impact: Vehicle disabled during road trip. Fast charging unavailable until root cause resolved. D2 Problem Description (5W2H) What: 3 cells in module #7 swollen 10-15% with capacity degradation to 78% SOH (other modules at 97-99% SOH) Where: Module #7 (center-rear position in pack, lowest airflow zone) When: During 2C DC fast charge at 38°C ambient, after 14 months and 45,000 km of service How Many: 3 cells affected. Module #7 total cell count: 12 cells in 3p4s configuration How Detected: BMS cell voltage deviation + internal temperature estimate exceeded threshold Severity: Vehicle immobilized, no safety incident, warranty claim D4 Root Cause Analysis 5-Why Chain Why did cells swell? → Internal gas generation — GC-MS confirmed electrolyte decomposition products (CO₂, ethylene, propylene) Why electrolyte decomposition? → Module #7...