SK On demonstrates ESS fire safety in UL 9540A large-scale test

Test results

SK On’s energy storage system met UL 9540A evaluation criteria in a large-scale fire test. During the test no thermal runaway was observed in the battery cells of the adjacent container, and temperature measurements inside the container met the applicable evaluation criteria. The large-scale fire test simulates real-world ESS fire scenarios, and in March 2026 the testing requirements were strengthened with more clearly defined criteria for evaluating fire propagation.

Safety design

SK On attributed the outcome to its ESS design, which allows heat to dissipate gradually over an extended period. That design reduces sharp increases in heat release rates and lowers the risk of fire spreading to neighboring containers.

Commercial implications

Following the successful test, SK On plans to accelerate its global ESS expansion. Recent commercial moves include an LFP supply agreement with NeoVolta (signed in August) and a 1 GWh ESS supply agreement with Flatiron signed in September 2025. In February the company secured 284 MW of ESS capacity in South Korea’s competitive bidding, and it is converting 3 GWh of its 7 GWh Seosan capacity to dedicated LFP lines for ESS applications.

Company statement

“This test confirmed the fire safety of SK On’s ESS, demonstrating its ability to limit fire propagation and maintain key safety performance under simulated fire conditions,” an SK On spokesperson said. “We will continue to enhance our safety technologies and product competitiveness to provide customized ESS solutions and expand our presence in the global market.”