UL 9540A:2025 Standard Update for Battery Energy Storage Safety

The document details the revised ANSI/CAN/UL 9540A:2025 standard for evaluating thermal runaway fire propagation in battery energy storage systems (BESS). It updates terminology, including definitions for non-residential and residential use, open parking garages, and thermal runaway propagation. New requirements mandate continuous temperature measurement with 60-second averaging for surfaces and heat flux, while cell surface temperatures are recorded continuously without averaging. Cell-level testing now requires charging to 100% SOC with a minimum 1-hour rest, and heating rates of 4-7°C per minute. If heating fails to trigger thermal runaway, alternative methods like nail penetration or overcharging are allowed. A hold point at 5-15°C above separator melting temperature is specified, with a 4-hour maximum hold, unless separator data is unavailable. Thermocouple placement is refined, requiring sensors under the heater, near the positive terminal, and at the vent area. The standard covers cell, module, unit, and installation level testing, enhancing safety assessment for energy storage products.