White Paper on Case Study of Safe Installation of Second-Life Energy Storage System

This technical report provides for a case study for the safe installation of a second-life, or repurposed, battery, that has been reconfigured for use as a stationary energy storage systems (ESS). Driven by legislative requirements such as California Senate Bill 615 and projections that retired EV batteries could meet a substantial portion of U.S. grid ESS needs beginning in 2035, the repurposing of EV batteries is anticipated to grow significantly. However, safety concerns arise from the effects of aging, unknown prior usage history, and changes in thermal runaway behavior, which may increase failure risks compared to new batteries. NFPA 855, the predominant U.S. standard for ESS installation, mandates that second-life batteries meet all requirements for new batteries, with repurposers complying with UL 1974 in addition to obtaining UL 9540 and UL 1973 listings. These are certifications that few repurposers have achieved and represent a regulatory barrier to entry for the market as a whole. Key technical challenges include limited access to original equipment manufacturer battery management system data, inability to modify physical module designs, and EV-specific design features that may not optimize performance for stationary applications. Within the framework of existing codes and standards, approval of unlisted second-life batteries can be pursued through NFPA 855's equivalency process, which requires collaboration with Authorities Having Jurisdiction. This report presents a research and development installation case study in which multiple analytical tools - including blast-overpressure modeling, computational fluid dynamics modeling for thermal runaway gas dispersion, toxic gas plume modeling, quantitative risk assessment, and hazard mitigation analysis - were applied to design a system that met an acceptable level of risk as determined by the authority having jurisdiction. Additional safety measures implemented beyond code requirements included enhanced deflagration venting systems, increased separation distances, additional firefighter signage, early-trigger gas detection thresholds, and high ventilation rates. While these strategies do not substitute for compliance with applicable codes and standards, they provide one potential framework for mitigating hazards associated with second-life batteries and offer insight into paths forward for safe deployment in ESS applications. This content was generated using AI and reviewed by the authors for accuracy.