The safety of an energy storage system doesn’t have to be a guessing game. Both customers and installers can take comfort by choosing UL-rated systems and installing to National Fire Protection Association (NFPA) standards. Although energy storage standards from both organizations are relatively young (UL 9540 began in 2016; NFPA 855 in 2020), they received input from hundreds of stakeholders, including engineers, manufacturers, first-responders and safety policymakers — all in an effort to prevent loss of life and property.
Many safety concerns, especially with lithium-based batteries, relate to thermal runaway — when a battery experiences an increase in temperature that eventually leads to cell short-circuiting or disintegration that can spark a fire. Batteries can go into thermal runaway through physical damage, thermal neglect and electrical abuse, but the chances of this are slim when energy storage systems are tested and installed to the industry standards explained below.
UL 9540 – Standard for Safety of Energy Storage Systems and Equipment
In order to have a UL 9540-listed energy storage system (ESS), the system must use a UL 1741-certified inverter and UL 1973-certified battery packs that have been tested using UL 9540A safety methods. It’s quite a UL-mouthful, but basically, the batteries and inverter inside a UL 9540-certified ESS have all met product safety standards.
When an ESS provider says it has completed UL 9540A test methods, that doesn’t mean it’s fully certified and ready to install, said Maurice Johnson, business development engineer with UL’s energy systems and e-mobility group, in a press release about the tests.
“As a test method, UL 9540A testing does not provide a certification, UL Mark or pass/fail results,” he said. “The information from UL 9540A testing supports important safety decisions about how the battery ESS will be installed and used.”
UL takes the results of the 9540A tests, analyzes the ESS in a few more categories and allows the system to be presentable to installers as a UL 9540-certified ESS. UL 9540 covers any technology that stores energy in any size-rating — not just lithium batteries in predetermined cabinets.
“UL 9540 has requirements for what we want to see going on inside the energy storage system,” said Ken Boyce, UL’s senior director for principal engineering, industrial. “We look at making sure the cells are appropriately proven for safety and that they’re being integrated into the system the right way and have the right software and hardware controls to govern the functional safety of the unit.”
An ESS without UL 9540 certification does not inherently mean it is not safe, but many jurisdictions now require that only certified batteries be installed.
“It’s certainly possible to develop a safe lithium battery energy storage system, but you have to pay attention to those safety requirements that are codified in UL 9540 because lithium-ion batteries can be susceptible to thermal runaway. It’s important to manage that potential hazard in a safe manner,” Boyce said.
UL 9540A – Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems
Although it doesn’t provide certification, the UL 9540A test method does bring some clarity to how batteries perform in fire conditions. If thermal runaway should occur, this test shows how the ESS would react. A battery can’t pass or fail these tests, but the results help installers design systems for the most successful performance.
“UL 9540A gives us a very scientifically focused way to demonstrate what the consequence of a thermal runaway would be,” Boyce said. “We’ll simulate a thermal runaway in a single cell and see if that spreads. Sometimes that’s the end right there — there’s no significant effect of that cascading. But sometimes you see it happen, so then we’ll go to the next level.”
UL 9540A tests look at the battery cell, battery module and the battery unit. The test tries to initiate thermal runaway at the cell level, then notes how it happened and how the module reacted — for example, if any off-gassing occurred. Next, researchers will consider how fire spreads from unit to unit to determine the potential for an explosion. Finally, a closed room test is performed to show how a unit-to-unit fire spreads and reacts to fire mitigation equipment.
The results of these extreme tests aid fire departments and other safety organizations in both installing energy storage systems and successfully suppressing any fires or other negative outcomes that may occur during their use.
Many other outside organizations, like the International Fire Code (IFC) and the NFPA, also refer to UL 9540A testing methods for their own standards.
NFPA 855 – Standard for the Installation of Stationary Energy Storage Systems
This standard from the National Fire Protection Association specifically focuses on how to prevent and extinguish ESS fires by installing systems correctly and providing accurate safety labeling for worst-case scenarios. NFPA’s installation standards aren’t enforceable unless adopted by the local jurisdiction.
NFPA 855 is largely associated with nonresidential systems; the group defaults to common-sense installation practices for smaller batteries. For example, if a residential ESS is installed in a home’s garage, it should be positioned where the risk of accidental impact from a vehicle is limited. There is more of a concern for large-scale storage systems installed both indoors and outdoors because of the larger risk of property and life loss.
NFPA 855 requires 3 ft of space between every 50 kWh of energy storage, but the AHJ can approve closer proximities for larger storage systems based on thermal runaway test results from UL 9540A. The NFPA installation standard also uses results of UL 9540A testing methods to determine what safety labels and fire suppression systems are necessary. Labels will explain what type of battery is installed and what safety measures are already in place, so first responders aren’t surprised when they arrive at an emergency.
Different safety installation codes and standards are used in the case of huge, utility-owned energy storage sites where the inverters and batteries are housed in separate locations and the entire project is often far from other buildings — like the 1,600-MWh setup at Moss Landing in California.
Although ESS fires and explosions are rare, it’s important to at least acknowledge the possibility and prepare for the worst. Installing UL-certified systems to NFPA standards ensures that energy storage adoption is a safe option for everyday power needs.