Underwriters Laboratory has developed a method of spotting internal short circuits in lithium-ion cells to improve standards relating to the batteries. Such a condition may have caused the notorious Boeing battery Dreamliner fires.
The Indentation Induced test is, according to Underwriter Laboratory (UL), a simple and repeatable way to induce internal short circuits to study battery behaviors when an internal short occurs.
UL says this approach is more rigorous than previous short circuit testing such as by nail penetration and will test cell’s reactions to a mechanical failure. While this is appropriate for impacts on the battery, such as vehicle crashes, it will still not be able to detect or protect cells against internal short circuits that occur due to causes other than mechanical means. It does test a cell for cathode dendritic growth, which many internal short circuits are attributed to.
In the Sustainable Energy Journal from the UL, the report titled ‘Lithium-ion Batteries. stated: “The Indentation Induced internal short circuit test was developed based on best-practice principles to provide a practical and simple method that is very suitable for battery safety standards. This test gives UL the ability to simulate how a lithium-ion cell behaves when subjected to an internal short circuit condition, which will help mitigate the hazards of ISCs and support the safe commercialisation of lithium-ion batteries.”
For the test, a cell is placed in a holder to prevent movement, an indenter presses the cell casing from above at a constant speed of 0.01 – 0.001mm/s. The test is suitable for cylindrical, pouch and prismatic cells at different states of charge or stages of aging and takes place in a temperature controlled chamber. As the indenter presses against the casing, the layers of anode, cathode and separators are deformed below the point of indentation. This heavy strain leads to a mechanical failure of the separator, which causes the electrodes to touch. This causes a drop in the open circuit voltage followed by a rapid increase in the cell’s surface temperature – up to 700oC –, which results in an explosive release of gases and fire.
The criteria for the test were: it must be able to generate a localised internal short circuit within a closed cell that would simulate the conditions similar to those found in the field failures of lithium-ion batteries; secondly it must be acceptable for battery safety standards. UL has partnered with NASA and Oak Ridge National Laboratories to develop test approaches.