Redwood's energy storage systems meet (and exceed) new safety benchmark

Battery energy storage is one of the fastest-growing technologies of the energy industry, and fire safety remains its most critical challenge. Earlier this month, UL Standards and Engagement, the global safety science organization that develops the product safety standards used across industries, released the 6th Edition of UL 9540A, the Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems. This update introduces the most comprehensive fire safety test method for battery energy storage systems (BESS) to date, the Installation Level – Large Scale Fire Test, and includes new test methods to evaluate explosion (deflagration) risk. 

Redwood’s BESS passed every applicable test, including the voluntary tests identified in the informative Annex C of UL 9540A, that are designed to address deflagration risks. This test annex was designed to align with the requirements in the recently published 2026 edition of NFPA 855, the Standard for the Installation of Stationary Energy Storage Systems, which governs the safe installation of ESS. 

Redwood Energy systems utilize both new and repurposed EV battery packs, engineered for one of the most demanding applications imaginable.  Inside electric vehicles, these batteries operate at high speeds, under extreme temperatures, constant vibration, intense mechanical stress, and aggressive charge-discharge cycles. At Redwood, we take those packs, still highly capable and often with 50-80% of their usable life remaining and give them a less demanding life storing energy for the grid. 

The results of this testing completed by UL Solutions matter given the level of safety demonstrated by the batteries inside our system and because of how Redwood designed the architecture around them.  

Traditional BESS configurations enclose batteries in large but confined spaces. In failure scenarios, heat and gases can accumulate, turning a single issue into a cascading event with the potential for fire propagation or deflagration. The industry’s response has largely focused on detecting and mitigating these risks through sensors, suppression systems, and complex ventilation systems. 

Redwood has taken a different approach: eliminating the conditions that allow those risks to develop in the first place. 

Our open architecture prevents gas accumulation by design, with no enclosed space for heat or pressure to build. In our UL 9540A testing, there was no propagation to adjacent units and the deflagration risk was fully mitigated, even under the most demanding and extreme scenarios. In other words, Redwood’s system behaved exactly as it should: controlled, predictable, and contained. 

"The release of UL 9540A 6th Edition represents the most comprehensive fire safety requirements ever applied to battery energy storage systems," said LaTanya Schwalb, principal engineer in the Energy and Industrial Automation group at UL Solutions. "This first large-scale fire test and deflagration test completed for repurposed electric vehicle batteries under these standards demonstrates that second-life batteries can meet the same demanding fire safety testing benchmarks as new batteries, an important data point for the industry and for the communities where large-scale battery storage is deployed." 

Our results make it clear that Redwood’s battery systems meet the highest industry standards, now including the latest UL standard for fire and explosion testing of BESS. When you design a system from the ground up for safety, using batteries engineered for one of the most demanding applications, you're setting a new standard.