Long-duration energy storage (LDES) solution manufacturer Eos Energy Enterprises has announced the positive results of independent destructive testing on its Z3 battery modules. Alongside the positive testing results, the modules received ISO 14001 certification.

Conducted by energy storage safety and fire testing organization Energy Safety Response Group (ESRG), the testing run was equivalent to industry standard large-scale fire testing, Eos representatives say. The company adds that under the conditions tested, the batteries “exhibited no thermal runaway, no sustained fire, and no propagation to adjacent live modules.”

“The inherent fire safety of our zinc aqueous chemistry has been foundational to Eos from the beginning,” says Francis Richey, CTO of Eos. “Independent testing using LSFT-equivalent methods turns that design into validated performance. Under direct flame and overcharge, the Z3 modules did not experience thermal runaway, sustain fire, or propagate failure. These are the outcomes that matter most to customers, regulators, first responders, and the communities that host our systems.”

The company’s unique battery architecture gives LDES modules a “fundamentally different” fire risk profile from more conventional chemistries, and in turn, ESRG evaluated the firm’s modules under more rigorous conditions than typically expected in real-world applications.

Meeting the modern standard

In the 2026 edition of National Fire Protection Association (NFPA) 855, battery systems must test to confirm that heat-related system failures do not spread between units. In order to address those concerns, Eos subjected its modules to both direct flame impingement and overcharge.

The company says that even with active flame applied to the Z3 battery modules, none of them entered thermal runaway, sustained fire, or propagated to adjacent modules. Additionally, the tests showed no ignition of off-gas.

“As energy storage becomes a larger part of our nation’s energy infrastructure, communities need confidence that safety is being prioritized,” says Matthew Brown, chief of the Allegheny County Department of Emergency Services. “In November 2025, my department, along with several municipal departments, responded to an incident involving overcharged battery modules at Eos’ Turtle Creek facility.

“Even under that real-world stress, the system did not enter thermal runaway or escalate into a fire event. Eos’ recent independent fire test further validates what we witnessed firsthand in the field.”

During the testing sequence, the Z3 batteries showed a distinctly high safety profile, according to Eos CEO Joe Mastrangelo. As energy storage continues to become paramount to American energy infrastructure, it must be safe enough to sit close to where people live and work, he adds.

“As data centers, utilities, and industrial users add storage inside dense urban environments, the market needs systems that scale without adding fire risk,” Mastrangelo says. “ESRG’s independent testing validates that Eos technology is built to do exactly that, and it speaks directly to the siting, permitting, and insurance questions that shape where storage can go.”

Tags: batteries, BESS, Eos Energy Storage LLC, ESS, products, storage