UL 1973: Stationary Battery Safety Standard Explained

UL 1973 is the safety standard for battery systems used in stationary applications, vehicle auxiliary power and light electric rail. A laboratory, test house or facility reads it as the set of construction, testing and marking requirements that a battery system is evaluated against, not as a performance or lifetime specification. The current edition, ANSI/CAN/UL 1973:2022 (Ed. 3), replaced ANSI/CAN/UL 1973:2018 (Ed. 2) and added lithium-cell requirements, a system safety analysis, electromagnetic immunity testing and sodium-ion coverage.

Short answer

UL 1973 evaluates whether a battery system, for example the kind installed as stationary energy storage, is safe under electrical, mechanical, environmental and fault conditions. The testing laboratory Intertek describes it as covering batteries for stationary and light electric rail (LER) applications, including lithium-based, valve-regulated lead-acid (VRLA) and nickel-metal hydride chemistries, with electrical, mechanical and environmental tests and an evaluation of the battery management system (BMS). Tesla, a manufacturer, states that energy storage system (ESS) units listed to UL 9540 must meet the requirements in UL 1973; that statement is the manufacturer’s, not a quotation from UL.

Definition and scope

The published title of the 2022 edition, as carried in Intertek’s standards update notice dated April 5, 2023, is Batteries for Use in Stationary, Vehicle Auxiliary Power and Light Electric Rail (LER) Applications, with the identifier ANSI/CAN/UL 1973:2022 Ed.3. The previous edition, ANSI/CAN/UL 1973:2018 Ed.2, carried the same title. Some retail listings of the standard use the wording “Stationary and Motive Auxiliary Power Applications”, so the same document may appear under slightly different titles depending on where it is catalogued.

Tesla’s public energy library summarises the scope in practical terms: UL 1973 is the safety standard for battery systems in stationary applications, and the testing covers electrical, electromagnetic, mechanical and environmental conditions as well as failure tolerance. Results that count as failures include explosion, fire, combustible vapor, shock hazard, leakage, rupture and loss of protection controls. For lithium-ion systems the same source describes a single-cell internal failure test whose pass condition is that the failure does not spread by fire and does not cause an explosion.

Two boundaries are worth keeping in mind. First, UL 1973 is a standard for the battery system, not a statement about any one product’s capacity, cycle life or efficiency; those are manufacturer specifications. Second, it is one document in a family. It sits alongside UL 1974 (the process standard for repurposing cells and batteries), UL 2054 (cell tests for sealed nickel-metal hydride cells) and UL 9540, the standard for ESS units that Tesla refers to. A compliance reader therefore needs to know which of these a report or label actually cites.

How UL 1973 is used in practice

Testing and listing

A manufacturer submits a battery system to an accredited testing laboratory, which evaluates it against the clauses of the standard and issues a report. Intertek’s standards update notice shows how this works when an edition changes: because of its accreditation, Intertek states that it must review reports against the revised standard, update the standard reference in the report once compliance is confirmed, and withdraw reports that are not updated by its effective date. For the 2022 edition that date is February 25, 2024. That date and the withdrawal rule are Intertek’s own policy for its own reports; other certification bodies set their own transition rules.

The system safety analysis (clause 7.8)

The 2022 edition added a requirement for a documented safety analysis consisting of hazard identification, risk analysis and risk evaluation. It must determine which parts of the system are safety related, identify the safety circuits or software addressing each hazardous condition, and consider their performance. At a minimum the analysis must consider cell over-voltage and under-voltage, battery over-temperature and under-temperature, and over-current during charge and discharge, unless the manufacturer justifies that a condition is not hazardous. Clause 7.8.3 adds that the analysis must consider the reliability of monitoring components and communication systems that feed the controls, and single-fault conditions in the protection circuit.

Protective circuits and electromagnetic immunity (7.9 and section 27)

Clause 7.9 states that active protective devices shall not be relied upon for critical safety and must comply with one of three listed approaches. Where a BMS, solid-state circuit or software control is relied upon as the primary safety protection, the new section 27 requires it to show sufficient immunity to electromagnetic interference through the tests in 27.2 to 27.7; alternate procedures from other standards are acceptable only if equivalent or more severe. In plain terms, a BMS that is claimed to keep a system safe has to be shown to keep working in an electrically noisy environment, and the analysis has to assume that part of the protection can itself fail.

Fault and abuse tests introduced or revised in the 2022 edition

• High Rate Charge (section 16): evaluates the system’s ability to protect against charging at currents above the specified maximum charging current.
• Short circuit (17.3): the direct short circuit test is also run on a battery module that is intended to be installed or replaced in the field, using a shorting device with a maximum total resistance of 20 mΩ.
• Overload Under Discharge (section 18): run on a fully charged unit with parallel-connected cells or modules; series-only units may be tested at cell or module level if that is judged equivalent.
• External Fire Exposure for Projectile Hazards (section 41): a 20-minute fire test in a controlled setting at 0 °C to 46 °C ambient, with a water layer of about 15.24 cm (6 in) under the fuel in the pan, followed by a low-impact hose stream through a 38 mm fog nozzle at 30°, 517 kPa and at least 4.7 L/s, with the test enclosed in a protective chamber or by walls at least 1.5 m from the inner perimeter marking.

Chemistries and related standards covered

The table below maps each chemistry or technology to the clause or annex of the 2022 edition that addresses it, as listed in Intertek’s update notice. It is a reading aid for test reports; the standard itself governs the details.

Technology	Where the 2022 edition addresses it	Cross-reference
Secondary lithium cells	7.12.2 and Annex E (cell test program, completely rewritten); cell marking in 44.15 and 44.16	Cell design measures in 7.12.3 point to IEEE 1625 and IEEE 1725 guidance
Sealed nickel-metal hydride cells	7.12.1	Cell tests of UL 2054 apply in addition to UL 1973
Sodium-beta cells and batteries	7.12.5	Cell tests in Annex B
Sodium-ion cells (for example Prussian blue or transition metal layered oxide)	7.12.10 (new in 2022)	Annex E and the lithium-style marking in 44.15 and 44.16
Flowing electrolyte batteries	7.11.8 leak detection; Annex C test program, including insulation resistance in C.5.5	Insulation at least 1 MΩ when tested at 500 Vdc for 1 min
Vented and VRLA lead-acid, nickel-cadmium	Marking in 44.17	Includes a fire, explosion and burn warning statement, which may be placed in the instructions instead
Mechanically rechargeable Al-air and Zn-air batteries	Annex I (new in 2022)	Must also meet the applicable construction and test requirements of the main standard
Repurposed cells and batteries	7.13.1 and marking in 44.3	Repurposing process per UL 1974; marked “Repurposed” or “Second Life” and “UL 1974”

For readers following flow battery technology, the 2022 additions for flowing electrolyte systems are the most relevant clauses. The CASRAI guides on flow batteries and the vanadium redox flow battery explain the chemistry that makes electrolyte leak detection and insulation resistance matter. Lithium-sulfur cells are a different chemistry again; see how lithium-sulfur batteries work and fade for why a standard needs chemistry-specific cell tests.

What changed between the 2018 and 2022 editions

Intertek’s notice lists ten headline changes: module testing during the short circuit test; all lithium cell requirements moved into UL 1973; requirements for repurposing batteries; vehicle auxiliary power system requirements; a revised external fire test; cell voltage measurement during overcharge and overdischarge tests; functional safety updates; EMC testing for electronic safety controls; inclusion of sodium-ion batteries; and an evaluation proposal for galvanic corrosion. New clause 7.7 also requires transformers to be evaluated to UL 1562, UL 1310 or an equivalent standard for overload conditions, and 26.6 adds a low-voltage transformer evaluation for circuits at or below 60 Vdc.

The practical consequence is that a report issued against the 2018 edition does not demonstrate the system safety analysis, EMC immunity, overload-under-discharge or revised fire-exposure results above. Anyone comparing two reports should check the edition (2018 Ed.2 or 2022 Ed.3) before comparing their results. The data table below lists the clause-level changes with their subject area.

Where it matters for labs, facilities and procurement

Research facilities that install stationary storage, run uninterruptible power for instruments or specify battery systems for a new building normally meet UL 1973 through documentation: a listing or test report, the edition cited and a nameplate. Three reading checks follow directly from the standard text. Confirm which edition the report cites. Confirm whether repurposed cells are involved, because 44.3 requires the “Repurposed” or “Second Life” marking together with “UL 1974”. And confirm that the lithium cells carry the manufacturer, model and date-code marking required by 44.15, with a distinct factory marking where a manufacturer makes the cell at more than one factory (44.16).

Shipping and handling of the cells themselves fall under different rules from the installed system standard; for transport see the CASRAI guide to lithium battery shipping regulations.

Common misunderstandings

• “UL 1973 is a cell standard.” It evaluates battery systems; cell requirements are included in it from the 2022 edition, but the system safety analysis, protective circuits and fire exposure test are system-level.
• “UL 1973 and UL 9540 are the same thing.” They are different documents. The only relationship stated in the sources read here is Tesla’s statement that ESS units listed to UL 9540 must meet UL 1973.
• “The effective date is when the standard changed.” February 25, 2024 is the date by which Intertek requires its reports to be updated, not a date set by the standard itself.
• “A protective BMS makes a system safe by itself.” Clause 7.9 says active protective devices shall not be relied upon for critical safety without meeting its conditions, and section 27 requires electromagnetic immunity where they are the primary protection.
• “It only covers lithium-ion.” The 2022 edition addresses lithium, nickel-metal hydride, sodium-beta, sodium-ion, flowing electrolyte, lead-acid, nickel-cadmium and metal-air systems.

Frequently asked questions

What is UL 1973?

UL 1973 is a safety standard for batteries used in stationary, vehicle auxiliary power and light electric rail applications. Its current edition is ANSI/CAN/UL 1973:2022 (Ed. 3).

What is the difference between UL 1973 2018 and 2022?

The 2022 edition adds module short circuit testing, all lithium cell requirements, repurposing requirements, vehicle auxiliary power requirements, a revised external fire test, EMC testing of electronic safety controls, sodium-ion coverage, a system safety analysis and new high-rate charge and overload-under-discharge tests.

Does UL 1973 cover sodium-ion batteries?

Yes. Clause 7.12.10 of the 2022 edition requires sodium-ion cells, such as Prussian blue or transition metal layered oxide cells, to comply with Annex E and the marking in 44.15 and 44.16.

Is a battery listed to UL 9540 also tested to UL 1973?

Tesla states that ESS units listed to UL 9540 must meet the requirements in UL 1973. Check the specific certificate and report for the scope the certifying body evaluated.

What does the repurposed marking mean?

Under clause 44.3, batteries and systems that use repurposed cells or batteries evaluated through the UL 1974 process are marked “Repurposed” or “Second Life” and “UL 1974”.

Sources

• Intertek, Standards Update Notice for UL 1973 (ANSI/CAN/UL 1973:2022 Ed.3), issued April 5, 2023: cdn.intertek.com. Read 2026-11-14.
• Intertek, UL 1973 testing page: intertek.com/batteries/ul-1973-testing. Read 2026-11-14.
• Tesla Energy Library, UL 1973: Batteries for Use in Stationary and Motive Auxiliary Power Applications: energylibrary.tesla.com. Read 2026-11-14.