While renewable energy sources are critical in the movement to lower CO₂ emissions, a challenge of incorporating more renewable energy sources is that many are not consistently available to meet energy demands. While plentiful as sources of energy, the sun does not always shine, the wind does not always blow, and rivers may only flow seasonally. Likewise, people tend to have greater energy requirements at different times of the day and throughout the year. To address these inconsistencies between supply and demand, there is a growing need for technologies that can capture renewable energy when it is available and store it for times when it is needed.

Energy storage systems (ESSs) are one such technology that work by capturing renewable energy when it is available and releasing it as electricity to the grid when it is needed. A battery energy storage system (BESS) is a specific type of ESS that uses an arrangement of rechargeable batteries and other electrical equipment to store electrical energy. BESSs that run on lithium-ion batteries (LIBs) have become increasingly popular due to their high-power density relative to other types of batteries ².

A LIB is a type of rechargeable electrochemical battery that charges and discharges when lithium ions move back and forth through an electrolyte between a positively charged electrode (the cathode) and a negatively charged electrode (the anode). The construction of a BESS starts at the simplest level as a single battery or cell. To increase a BESS’s capacity, multiple cells are connected to create a module. Multiple modules are then connected to create a pack that is controlled by a battery management system (BMS), and finally, multiple packs are combined to store and deliver an amount of energy suitable to the intended use of the BESS (e.g., a residential or commercial use) ³. The BMS monitors and maintains state-of-health parameters such as temperature, current, voltage, and charging and discharging behavior of the cells, modules, and packs ³.

Any of these hazards can disrupt the enclosed electrolyte, causing an internal short circuit and the stored chemical energy in the battery to be converted to thermal energy, which may trigger a thermal runaway event and the release of flammable and toxic gases, leading to fire and explosion ². This phenomenon occurs when LIBs experience a “runaway state” exothermic chemical reaction in which the heat released will continue to fuel the exothermic reaction, which releases more heat, perpetuating the cycle and producing a rate of heat generation greater than the rate at which heat can dissipate from the battery ⁵. This runaway state will continue until the thermal and electrochemical energy contained in the battery is entirely consumed.

LIB thermal runaway is a serious hazard for any LIB-containing device. The additional concern for LIBs in BESSs is the potential for thermal runaway propagation from cell to cell, module to module, and unit to unit in larger setups ⁵. BESS failure events are tracked in the publicly available BESS Failure Incident Database, including one notable event, the APS McMicken Battery Energy Storage System explosion, which demonstrated the extent to which thermal runaway can propagate in a BESS where appropriate safety measures, such as a thermal suppression system, thermal barriers between cells, and sufficient ventilation for off-gassing have not been integrated into the design of the BESS ⁶. When a HAZMAT team entered the BESS, an explosion occurred, seriously injuring firefighters ⁷.

Incidents such as the McMicken event highlight the challenges local planning and zoning officials face when deciding where to install BESSs and how to mitigate potential safety issues in their community. This challenge is compounded by the fact that residential BESS units are small, modular, and typically have limited infrastructure needs, meaning a BESS can be built nearly anywhere [8]. While installation flexibility may sound positive, many jurisdictions currently lack knowledge on how to respond to the risks associated with BESSs, leading to energy system projects being withdrawn⁹¹⁰ or even banned.¹² 

Over time, the UL 9540 TC has grown to become the largest TC of any UL standard with nearly 150 members from 15 different countries as of 2025. Such growth in TC membership reflects the rising interest in energy storage systems (ESSs) across many sectors. This includes commercial/industrial users, producers, supply chain representatives, testing and standards experts, state government agencies, authorities having jurisdiction (e.g., municipalities), and other interest groups. It also shows that many stakeholders want to see improved safety guidelines for ESSs and view participation in the TC as a way to share their expertise and contribute to shaping ESS safety standards. TC members worked together to improve the first edition and make proposals and recommendations reflective of the most up-to-date industry knowledge on ESSs, to ultimately support creating the best safety standard possible to protect people and their communities. The continued efforts of the TC would lead to publication of the second edition of UL 9540 in February 2020 and the third edition in April 2023.

The UL 9540 standard addresses the safety of energy storage systems by describing the construction requirements for an ESS, as well as performance requirements during various test procedures. These include:

• Electrical tests (e.g., charge and discharge processes; testing under abnormal temperature, fluid, and pressure conditions to push the ESS into failure while noting how well internal protective controls work to prevent fire, explosion, and other hazards)
• Mechanical tests (e.g., testing how well moving parts function; how well internal controls prevent parts from going into overspeed)
• Environmental tests (e.g., moisture exposure tests for outdoor installations; special environmental installations such as exposure to salt fog in marine environments, seismic events, high altitudes)

UL 9540 is a system-level standard, which requires that all components of an ESS must be tested together to ensure their safer function, as opposed to testing their parts individually and assuming they will work safely when integrated as a system. As such, all these tests evaluate the risk for the overall ESS to experience safety issues like fire, explosion, electrolyte leakage, electric shock, and loss of overall protection controls.

Community safety through standards and codes

In energy storage system projects, both standards and codes have an important role in ensuring the safety, reliability, and proper functioning of the system. Since the initial publication of UL 9540, the International Fire Code, International Building Code, and National Fire Protection Association 1 and 855 fire codes have all required that electrochemical ESSs be listed to UL 9540. This relationship highlights that standards complement codes in that “codes tell you what needs to be done, while standards tell you how to do it” [13].

As installation code requirements are updated to reflect new industry advancements through research and testing, UL 9540 also evolves to better meet the safety needs of industry and the regulatory community. For example, the energy storage capacity of BESSs and the separation requirements between units of batteries in the systems were updated in the second edition of UL 9540. At the same time, UL 9540 also added a requirement for BESSs to be subjected to large scale fire testing in accordance with UL 9540A, the Standard for Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems, to determine compliance with limitations for energy capacity and separation distances for electrochemical ESSs in accordance with the fire and installations codes. The UL 9540A test method addresses the safety concerns of potential fire and explosion hazards associated with residential and non-residential use BESSs. 

The UL 9540A test method intentionally attempts to push a BESS into thermal runaway to evaluate how the battery technology performs under failure and to observe if and how fire spreads through the unit and if the fire propagates to adjacent units. The test method includes a series of progressively larger fire tests, beginning at the cell level, and if thermal runaway is achieved, the test progresses to the module level, unit level, and finally, the installation level [3]. Data obtained during the tests include gas composition, heat and gas release rates, deflagration hazards, reignition hazards, and effectiveness of fire protection systems [5]. Collected data are then used by manufacturers, system designers, safety professionals, and permitting authorities to determine the required ventilation, fire and explosion protection equipment, separation distances between individual BESS, manufacturer’s installation instructions, and what fire strategy and tactics should be considered if a failure event should occur for an ESS installation [2].

While risk can never be entirely removed from BESS installations, applying guidelines from standards like UL 9540 and collecting data from tests like UL 9540A allows an energy storage system to be designed, built, and operated in such a way that if it does fail, it can fail relatively safely. This was illustrated during a fire caused by a cooling system leak at an Australian commercial-scale Tesla BESS facility in July 2021. When one of the project’s 212 units caught fire, proper spacing contained the failure to the originating unit. However, the fire caused minor damage to a neighboring unit, which was attributed to high wind speeds that reached up to three times the wind speeds used in the UL 9540A tests [14]. While this was considered a safe failure, it also demonstrated the ongoing need for iterative evaluation and improvements to standards and testing procedures as failure incidents teach us how to navigate gaps in our knowledge and integrate complexities, such as local climate variables in the Australian example.