Battery Swelling — Failure Analysis & 8D Report Guide

Symptoms Visible bulging; internal pressure > design limit SOH 4.2–4.35V → electrolyte oxidation → gas Overdischarge → Cu dissolution → dendrite → short High temp → SEI decomposition/reformation → gas Manufacturing defects: metal particles, poor wetting Li plating: low temp / high current charging H₂O >20 ppm → HF generation → cathode dissolution → O₂ 5-Why Example Problem: Pouch cell 15% swelling after 6 months Why swollen? → Gas: 60% CO₂, 30% CO, 10% C₂H₄ Why gas? → Electrolyte decomposition at cathode interface Why decompose? → 50°C internal (35°C design max) Why hot? → 2C fast charge at 40°C ambient exceeded thermal limit Why allowed? → BMS derating not validated for combined fast-charge + high-ambient TRC: Electrolyte decomposition from combined fast-charge + high-temperature MRC: BMS thermal derating validation overlooked worst-case combined stress Fishbone (6M) Man: Hot fast-charging habit Machine: Linear BMS derating curve Method: Lab 25°C qualification Material: Cycle-life optimized electrolyte Measurement: Sensor 5°C thermal lag Environment: Arizona summer 45°C D3 Containment OTA BMS: 0.5C limit above 40°C; CT scan stock D5 Permanent Fix Exponential BMS derating above 35°C; high-temp electrolyte additives Internal temp estimation via impedance tracking; active liquid cooling D7 Prevention BMS validation: combined stress scenarios 60°C/90-day storage...