Lithium-ion UPS batteries

Lithium-ion UPS batteries are the lithium-ion cells that replace lead-acid batteries in data center UPS (uninterruptible power supply) systems: smaller and longer-lived, they bring thermal runaway and hard-to-extinguish fires into the data hall. For twenty years, a data center UPS relied on sealed lead-acid (VRLA, valve-regulated lead-acid) batteries, heavy and due for replacement every three to six years. Lithium-ion has been pushing them out since the mid-2010s: two to three times the energy density, a stated service life of ten to fifteen years, cell-by-cell monitoring through the BMS (battery management system). The property underwriter looks at the chemistry. A faulty cell (internal short, overcharge, impact during handling) can go into thermal runaway, release flammable and toxic gases and spread fire to neighboring cells; an inert gas puts out the flames without cooling the cells, which reignite. NFPA 855, the National Fire Protection Association standard first published in 2020, sets energy thresholds, separation distances, ventilation and propagation testing under UL 9540A, the Underwriters Laboratories test method, for stationary installations, and carriers refer to it in their surveys. Everything turns on compartmentation: a separate battery room behind fire-rated walls, off-gas detection, distance from the white space. A battery cabinet placed in the row, right next to the racks, turns a localized ignition into a whole-room loss, with smoke and soot on servers the flames never reached. The Pangyo fire in 2022, the Singapore fire in 2024 and the Daejeon fire in 2025 all started in lithium-ion UPS batteries. The question has accordingly moved up the questionnaires, right after installed capacity.