Cause Analysis of Thermal Runaway in Military Lithium Secondary Batteries and Circuit Design for Enhanced Surge Immunity

Thermal runaway prevention has emerged as a key safety concern as the application of lithium secondary batteries continues to increase. Although extensive research has been conducted on cell-level countermeasures and early detection of thermal runaway, recently reported incidents in military operations underscore the importance of considering factors beyond these aspects. This study investigated a thermal runaway incident involving a military lithium secondary battery by using the NFPA 921 investigation methodology to identify the probable cause. A malfunctioning peripheral device was identified as the most probable cause, and a circuit with improved surge resistance was proposed as a countermeasure. Given that the existing circuit relied on a single varistor, a transient voltage suppressor (TVS) diode was connected in parallel to improve surge immunity and protect critical components. Tests based on the KS C 9610-6-2 demonstrated an increase in surge immunity from 2 kV to 4 kV, and showed that the Solid State Relay, identified as the most probable failure mode associated with the thermal runaway incidents, was successfully protected. These findings demonstrate that ensuring the safety of peripheral devices is essential for preventing thermal runaway in military lithium secondary batteries, and that the parallel use of a varistor and a TVS diode provides a design guideline for improving surge immunity.