On April 19, 2019, an explosion at the McMicken Battery Energy Storage System in Surprise, Ariz., severely injured a fire captain, fire engineer and two firefighters. Preceding the blast, a 2.16-megawatt-hour lithium-ion battery energy storage system (BESS) serving a solar farm system experienced thermal runaway.
What is thermal runaway?
Thermal runaway involves the rapid, uncontrolled release of heat energy from a battery cell, according to the National Fire Protection Association (NFPA), Quincy, Mass. This occurs when a cell creates more heat than it can effectively dissipate. It can progress when that single cell heats up neighboring cells, causing a chain reaction that propagates thermal runaway. The result is usually a battery fire that is difficult to extinguish because it produces its own oxygen.
Even when initially extinguished by sprinkler systems or fire departments, fires caused by thermal runaway can reignite because of stranded energy, latent heat and the self-production of oxygen. The stranded energy is often difficult to discharge due to damaged terminals, requiring immersion in a large salt-water bath. Reignition can happen hours, days or even weeks later. Because of the danger to firefighters due to the issues with stranded energy and off-gassing, unless the lithium-ion fire is endangering personnel, the generally accepted firefighting practice is now to let the fire burn itself out and possibly spray water on nearby objects to prevent the blaze from spreading, according to Curtis Ashton, principal member of NFPA 800 and 855 and an IEEE fellow.
The McMicken incident, referenced in the NFPA’s Energy Storage Systems Safety Fact Sheet, sparked improvement of energy storage design and related safety practices for the electrical industry, first responders and building planners. Ashton said that Electric Power Research Institute statistics show that battery energy storage systems are now about 30 times less likely to experience propagating thermal runaway than they were in 2018.
Adhering to strides already made is more important than ever amid increasing demand for power generation, energy storage and increased use of renewable forms of energy.
Education efforts
For that reason, NFPA has taken on a nationwide effort to educate about the hazards associated with battery energy storage, renewables and thermal runaway, offering courses and a tipsheet for handling the risks. You can view the tipsheet here.
“Installations of battery energy storage systems are definitely growing when paired with solar and wind, and they can also provide utility grid support,” said Robert Hattier, director of renewable energy for IBEW 134. “Renewables have been the focus of my career, and these installations are a great driver of BESS. But with that comes the potential of thermal runaway when not installed or maintained correctly.”
BESS gets big
BESS installations have become prolific across the country as wind and solar replace coal-fired plants. Illinois’ Clean and Reliable Grid Affordability Act took effect in June 2026 and aims to enhance energy reliability, affordability and sustainability. This entails facilitating the development of new BESS projects with a goal of 3,000 megawatts of cumulative nameplate capacity for commercial operation by the end of 2030.
BESS equipment supports backup power and generation for industry and data centers. They’re a growing presence in manufacturing operations, commercial buildings and some homes, as they address needs to supplement power, level loads and provide uninterrupted power.
While BESS technology is improving, it remains vulnerable to failures of climate control systems and fire detection and suppression systems, as well as exposure to water and seismic activity—all of which can trigger thermal runaway.
The role of codes
Current code adoption and enforcement support thermal runaway prevention, Hattier said, who serves on code technical committees for NFPA 800 pertaining to battery safety and NFPA 855 for installation of stationery energy systems. He promotes enforcement of these codes along with NFPA 70B for regularly scheduled maintenance, the International Fire Code, the International Building Code and UL 9450, and listing of battery energy storage systems to UL 9450.
“Codes take into account previous failures [like the McMicken incident],” Hattier said, adding that, “NFPA 855 has expanded with each edition as more data has become available. UL 9540 as well is undergoing large-scale fire propagation testing [UL 9540A, or equivalent].”
Beyond that, training is critical for electrical workers to learn to recognize and mitigate hazards.
“We built an addition to our school [Local 134/ECA Chicago’s JATC] that includes a BESS lab with equipment both donated by companies and purchased,” Hattier said. “We try to show apprentices a variety of components and configurations. With on-the-job training, exposure to BESS depends more on their employer and who may be working on a BESS project.”
Beyond that, “we get them involved in codes, standards and best practices,” Hattier said, adding that, “Potential for thermal runaway is a topic we cover in training for proper operations.”
Newer BESSs may be less problematic due to less energetic chemistry choices and better internal design of the modules to help mitigate cell-to-cell propagation, but attention is still needed.
“There are lots of automatic checks in the newer systems, but regular physical tests are still required with 70B periodic inspections,” Hattier said. “Apprentices must have knowledge of all related codes as they are called to adapt to new changes.”
In Illinois, a home rule state, NEC editions are adopted at the municipal level and play out differently across different areas.
“Energy storage regulations were not adopted into NEC until 2017,” Hattier said “We have many municipalities enforcing codes earlier than 2017. So, some municipalities are enforcing older code standards that don’t even recognize the energy storage systems we have today.”
Educating community planners, inspectors and first responders about BESS codes, standards and the potential for thermal runaway is also necessary to curb related problems, Hattier said.
“The way we cope with risk of thermal runaway is by proper design, municipal permitting and inspection. And that all ties into our current electrical, build and fire codes,” Hattier said. “So further understanding of codes must inform our industry as a whole.”
Local 134 has a codes and standards committee that works with municipalities on code adoption and enforcement.
“While this work benefits new projects, there’s always a complication with older systems grandfathered in, in that they don’t have to be brought up to code,” Hattier said. “Any existing system is just as vulnerable today as the day it was installed.”
A statewide effort
Hattier has launched and maintained a statewide education effort through the Illinois IBEW Renewable Energy Fund, which offers free classes for allied professionals. The courses developed for fire building officials and fire department personnel are based on industry best practices. They also cover emerging electrical technologies and related hazards; current systems equipment; and relevant building, fire and electrical codes.
Besides identifying thermal runaway scenarios in buildings and commercial operations, the training also covers identifying thermal runaway hazards associated with electric vehicles, e-bikes and scooters, laptops and lithium-ion battery household appliances such as lawn mowers.
‘A serious issue’
For fire inspectors who weigh in on preconstruction permitting, inspection and fire investigation, and first responders who fight fires, thermal runaway demands increasing attention.
“Thermal runaway is a serious issue,” said Daniel Medina, fire marshal for the Addison Fire Protection District, which serves the village of Addison, Ill., and surrounding unincorporated areas. “In the last year alone, in Addison, there were roughly six to a dozen fire incidents related to thermal runaway or had battery-type involvement to contribute to the fire.”
The majority of those fires occurred in Addison’s industrial area and not in residential settings, Medina said.
“We’re seeing more reliance by industry on battery backup, lithium-ion batteries and solar panel integration,” he said.
Medina, who also serves as president for the Illinois Fire Inspectors Association, has identified a statewide need for education about thermal runaway in commercial and industrial settings.
What ECs Need to Know about Thermal Runaway
Thermal runaway is a chain reaction you can’t simply switch off.
When a lithium-ion cell overheats past a critical point, it produces its own heat. That heats the cells next to it, and the failure spreads through the pack. The trigger can be overcharging, physical damage, an internal short or high outside temperatures. Opening a disconnect won’t stop the reaction once it starts because the energy is stored chemically inside the cells. So prevention is what matters: proper equipment selection, installation and commissioning.
Only install listed equipment, and know what each listing covers.
An energy storage system (ESS) should be listed to UL 9540 (UL 1973 for the battery modules and UL 1741 for the inverter). The large-scale fire test data from UL 9540A is what building and fire officials use to decide on spacing and fire protection. Electric vehicle supply equipment should be listed to UL 2202 (chargers) or UL 2594 (EVSE), or UL 9741 (EV power export equipment, or EVPE). Construction tools and other equipment containing Li-ion batteries should be listed to UL 2054, UL 2595 or IEC/UL 62841. Don’t mix batteries, inverters or controls from different manufacturers unless the listing covers that combination. A mismatch can void the listing and disable the battery management system’s protections.
Placement and spacing are code issues, not preferences.
NEC Article 706 (ESS), Article 480 (batteries), NFPA 855 and IFC Section 1207 limit:
- Where systems can go (garages, basements, living spaces, exterior walls)
- Energy capacity per unit and in total
- Distance between units, and from doors and windows
- Protection from vehicle impact
For all equipment involving Li-ion batteries, protect equipment and cords from damage and avoid indoor charging, if possible. Check the local adopted code edition and any amendments. Your AHJ may be stricter than the model codes.
Commissioning and communications keep the safety systems working.
The battery management system is the main defense against overcharge and overheating. Make sure communication is working between the batteries, inverter and charger before you leave the site. Confirm that firmware is current and that alarms and remote monitoring are active. With bidirectional EVs—vehicle-to-home or vehicle-to-grid (V2H, V2G or V2x)—the car becomes an energy source for the building. That brings in additional NEC 625 and 705 requirements and adds more points where communication has to work.
Plan for emergencies and protect your crew.
Warning signs of thermal runaway include venting, hissing, swelling and smoke. Evacuate right away. Li-ion battery off-gas is toxic and flammable. In addition:
- Install required disconnects, labels and emergency shutdown locations exactly where first responders expect them.
- Coordinate with the AHJ and fire department on large systems.
- Treat damaged batteries as hazardous. Do not reuse them.
- Include these hazards in your safety training and job hazard analyses.
Batteries fail in a different way than the rest of an electrical system. Listed equipment, correct placement, careful workmanship and verified commissioning are the contractor’s best defense against thermal runaway and liability.
—Kyle Krueger, with input from Curtis Ashton
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About The Author
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DeGrane is a Chicago-based freelance writer. She has covered electrical contracting, renewable energy, senior living and other industries with articles published in the Chicago Tribune, New York Times and trade publications. Reach her at [email protected].
