Camelot Unpacks UL 9540 – Part 1

At Camelot, reviewing the UL Listing status of battery energy storage systems (BESS) for the projects we are overseeing as an Owner’s Engineer (OE) or Independent Engineer (IE) is something our team considers a good starting place in the due diligence process. This Listing is so foundational to a successful and code-compliant BESS project that we often take it for granted that everyone understands what this important Standard entails. 

Unfortunately, there is a great deal of misunderstanding about the UL 9540 Listing process, even among some engineers who are otherwise pretty familiar with BESS technologies. Missing a step in verifying the proper UL listing of the BESS on a project can have large implications. For instance, an astute authority having jurisdiction (AHJ) that notices your BESS is not properly Listed may find it is not code-compliant, causing significant delays in permitting and significant costs in addressing deficiencies with the BESS manufacturer. Moreover, a UL 9540 Listing represents the successful completion of a battery (we could not resist, of course) of tests related to safety, reliability, and performance. 

Understanding Standards 

Most folks involved in BESS projects think they know what a Standard is, as it seems pretty self-explanatory, right? Perhaps, but once you move beyond the surface level and try to parse the difference between a “Listed”, “Certified”, and “Recognized” product, it can quickly get confusing. So, let’s address a few common misconceptions. 

Misconception 1: Projects Have to Comply with Standards 

The rollout of new standards, like NFPA 855 and UL 9540, have undoubtedly made BESS projects safer. However, complying with these Standards is not required. Organizations like NFPA or UL have no legal authority to provide, or deny, any project a permit. Permits are issued, rather, based on Codes (e.g., Electrical Code, Building Code, Fire Code) and if the Code for your project’s jurisdiction does not incorporate one of these Standards, then the AHJ may not be able to enforce the requirement. This can happen, for instance, when a local Code has not been updated recently enough to incorporate the latest versions of relevant Standards. So, unless the Code references a particular Standard, the project does not have to comply with the Standard, at least from a permitting perspective. Fortunately, many savvy asset owners have developed their own BESS technical criteria. While these criteria are unrelated to permitting, they can be used as a condition of financing. In this way, the investment community can drive better and safer installations by holding developers to the highest current Standards (literally). 

Misconception 2: Standards Represent the Gold Standard of Safety and Quality 

Given all the time taken, and the expertise of the dozens of industry experts applied, in crafting Standards it is natural to assume that each one represents the pinnacle of current thinking in design, safety, and quality. Not so. It is best to think of a Standard as the lowest common denominator that a bunch of technical folks with often-competing priorities can agree on. Anyone that has ever got more than one engineer in a room to talk about BESS likely knows that we can be an opinionated bunch, so imagine what a room with fifty engineers is like when coming up with a new technical Standard. The results are incredible acts of service to the industry, but they are only a starting place. Complying with Standards should be a bare minimum, not a stretch goal. 

Misconception 3: A BESS can “Pass” or be Listed to UL 9540A 

Most folks understand a Standard as something that can be “passed” or “failed”. This is an understandable interpretation, as it applies to everything from everyday household appliances to BESS equipment. Unfortunately, UL 9540A is a little different. UL 9540A is actually a testing Standard that describes how a testing laboratory is to initiate and measure the impacts of thermal runaway. In completing the tests, it is literally impossible to not destroy the BESS (/ the BESS is intentionally destroyed). If thermal runaway is not initiated through one initiation method (e.g., heating), then the test continues using other methods until thermal runaway occurs (e.g., nail penetration, overcharging). There are non-lithium-ion BESS that are not subject to thermal runaway but even these do not “pass”. Instead, at each level of testing, a higher level of testing is required unless the test results fall within a particular range. For example, if a cell is tested and does not exhibit thermal runaway, it is not required to test at the module or unit level. 

Misconception 4: UL 9540 Replaces Other Battery Standards 

In fact, UL 9540 is carefully crafted to build on other key standards, not replace them. Though many spec sheets will list UL 9540 alongside UL 1973 or UL 1741, compliance with UL 9540 already includes many of these relevant equipment-specific Standards, such as: 

• UL 1973 for battery cells and modules 

• UL 1741 for inverters (such as in AC block BESS products) 

• UL 9540A for testing thermal runaway propagation risks 

Wrapping Up Part 1 

Misunderstandings about UL 9540 aren’t just academic - they can cause costly delays, strained relationships with AHJs, and headaches during financing or commissioning. Clearing up the myths is the first step, but knowing exactly what UL 9540 covers, when it’s required, and how to navigate the Listing or Field Listing process is where the real project-saving insight comes in. 

In Part 2, we’ll take that next step: unpacking the key requirements baked into UL 9540, explaining how they connect to other Codes and Standards, and clarifying the often-misunderstood Field Listing process. If Part 1 was about avoiding the traps, Part 2 is about charting the course to a compliant, bankable BESS installation.