In‐Depth Investigation of Fire Hazards in Lithium‐Ion Batteries Based Solely on Protection Circuit Modules for Reliable Operation in Small Appliances

This study investigates the fire hazards of lithium‐ion battery (LIB) products that rely solely on a protection circuit module (PCM) instead of the battery management system (BMS). Although PCMs can provide basic overcharge and overdischarge cut‐off functions, their voltage‐based control is insufficient to ensure battery safety under severe operating conditions. To evaluate these limitations, repeated charge/discharge experiments were conducted using a commercial cylindrical 18,650 LIB cell coupled with a PCM under extreme temperature environments (40 and −20°C), and sequential mixed‐temperature conditions. Electrochemical impedance spectroscopy (EIS) analysis revealed that loss of lithium inventory (LLI) was the dominant degradation mechanism in all cases, while computed tomography (CT) analysis confirmed internal structural deformation caused by pressure buildup during cycling. In particular, the low‐temperature condition produced more severe electrochemical degradation and mechanical damage than the high‐temperature condition, leading to earlier failure. In addition, MATLAB‐based state‐of‐health (SOH) analysis and comparison with a normal cell simulation showed that extreme temperature cycling drastically accelerated degradation; the 80% SOH level was reached after about 25 cycles at 40°C and 15 cycles at −20°C, compared with about 400 cycles under normal conditions. These results demonstrate that PCM‐only battery systems are highly vulnerable to hazardous degradation under harsh environments, especially at low temperatures, and indicate the need for strengthened certification standards and safety regulations for small consumer products using LIBs.