Streamlining of cell level requirements for stationary applications
Summary
- Cell level proposed requirements keep the current Appendix E option and add a second that is based on UL 1642
- Cell construction analysis proposed to be added
- Cell specifications with specific information would be required
- Comments have been received by the Standards Technical Panel and are under review
Proposed revisions to the UL 1973 standard were first released on November 6th, 2020, and included over thirty updates. UL 1973 is specific to Batteries for Use in Light Electric Rail (LER) Applications, Stationary Applications, and Vehicle Auxiliary Power. It covers requirements for the battery cell, battery modules, and battery pack/assembly.
Proposed Revisions
Of the proposals, simple ones include editorial changes and clarifications. Beyond these follow extensive proposals such as additional requirements for vehicle auxiliary power systems, inclusion of EMC (Electromagnetic Compatibility) testing for electronic safety controls, the addition of requirements for repurposing batteries and the transfer of all lithium cell requirements to UL 1973. Visit the UL Standards Twitter account for the most up to date information on UL standards.
This summary will focus only on the proposal to embed the lithium cell requirements into UL 1973 and remove references to other cell safety standards. Currently, UL 1973 requires component cells to comply with either UL 1642 or Appendix E of UL 1973. It’s not uncommon for Lithium-Ion cells to be recognized to UL 1642.
Further, this proposal moves many of the requirements for cell level testing from UL 1642 into Appendix E and additional items into section 7 of UL 1973. There will also be changes to the reference documents, definitions, and marking requirements in sections 5, 6, and 41 to support the move of requirements.
The proposal is intended to allow for the use of a single standard for the evaluation of the entire device/battery assembly and allow for more flexibility in the future as requirements can be changed to address concerns specific to the intended use of the cells. It removes the possibility to use a component cell that is only evaluated to UL 1642 as it currently allows without making a thorough comparison of the completed testing. It’ll also provide more flexibility in the future as requirements may change to address concerns specific to the intended use of the cells.
Section 7
Section 7 of the standard has always contained cell requirements but updates will expand this section and remove the allowance for use of cells compliant with UL 1642 without further testing and evaluation as was previously allowed in 7.11.2. This paragraph continues its reference to Appendix E of UL 1973 and does not reference UL 1642 any further.
Additional proposed requirements in this section include providing a technical specification for cells to include charging and discharging instructions, installation, storage, and disposal requirements for safe use.
A new paragraph, 7.11.2A, has also been proposed and would include requirements for the teardown of cells to verify construction elements that mitigate the risk of internal shorts. Internal shorts can lead to catastrophic events within lithium-ion cells. Cell teardowns focus on the placement of insulators, sizing of negative electrode active materials, proper separation of internal and external parts of opposite polarity, use of proper protection mechanisms (separator shutdown, protective coatings, and electrolyte additives), and separator strength.
Appendix E
Appendix E provides streamlined evaluations and testing paths for cells to be directly evaluated to UL 1973. It also now will give a second option that mirrors some of the criteria in UL 1642. This can help further streamline evaluation of any cell that is UL 1642 compliant by comparing the testing completed to the criteria added in Clause E9 and Table E.2.
Appendix E initially included a cell level test program that could be completed as an alternative to UL 1642. The new proposal adds a second optional test program in the appendix and would mandate compliance to one of the two test programs. The testing that is being added to the Appendix as the second option, is very closely based on current UL 1642 which may limit the need for a file review for devices that used the UL 1642 option that is currently available. Some of the proposed new requirements for the appendix are based on requirements in other UL or global standards for cells and batteries. This includes use of cells for testing that are less than 6 months old as well as verification that the stated capacity in the specification is met.
Table E.1 outlines the first test program and is the same as what is currently in UL 1973. This includes a reduced number of samples compared to UL 1642, but it does include conditioning of cells using their upper and lower limit temperatures, similar to IEC 62133-2. Testing includes two samples for each of the seven tests: short circuit, cell impact, drop impact, heating, overcharge, forced discharge, and projectile. This section is not proposed to change from what is currently in the standard.
Sections E1A to section E11 contain all new requirements based on current UL 1642 requirements and is summarized in Table E.2. Because these new requirements are based on the current UL 1642 test program, it should allow for a seamless transition to UL 1973 cell certification for those manufacturers who may already be recognized to UL 1642. It includes two samples for each of the ten tests: short circuit, abnormal charge, crush, impact, shock, vibration, heating, temperature cycling, low pressure, and projectile.
If approved, all cells will have to complete a capacity check prior to testing verifying they supply the capacity stated under manufacturer defined maximum charge and discharge parameters. Cells not meeting the stated capacity will not be used for testing.
The remainder of the proposed revisions in Appendix E define the testing parameters to be applied to the cells.
Marking Requirements
New marking requirements in Section 41 are standard for all UL Recognized products and are intended to allow for good traceability in case of an incident in the field. There is a new requirement for cells to be provided with specific instructions that’ll include operating regions (voltage, current, and temperature) for charging, discharging, storage, installation, and disposal. This is likely to be the same document now required in Section 7 as a cell specification.
All comments to the proposed revisions to the UL 1973 standard were due December 21, 2020.
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