Considerations for the Thermal Modeling of Lithium-Ion Cells for Battery Analysis

Recent well-publicized events involving lithium-ion batteries in laptops, electric cars, commercial aircraft and even hover boards have raised concerns regarding thermal runaway -- a phenomenon in which stored energy in a cell is rapidly released as heat along with vented effluents. If not properly managed, testing has shown that thermal runaway in a single cell can propagate to other cells in a battery and may lead to a potentially catastrophic event. Lithium-ion batteries are becoming more widely used in a number of human-rated extravehicular activity (EVA) space applications on the International Space Station. Thermal modeling in support of thermal runaway propagation mitigation in the Lithium-ion Rechargeable EVA Battery Assembly (LREBA) and the Lithium-on Pistol Grip Tool (LPGT) was pursued to inform design decisions and to understand the results of extensive development testing with the goal of enhancing safety. A correct representation of thermal runaway in battery-level thermal models requires an understanding of internal cell triggering mechanisms, heat transfer mechanisms through the cell wall, an accounting of energy transport through vented gases and effluents and proper consideration of heat transfer mechanisms within the battery. Development and refinement of internal cell multi-physics models provided heating profiles used for a simplified cell thermal network representation. A collection of simplified cells was used to formulate battery-level models of, both, the LREBA and LPGT battery configurations. Limited correlation of these models was performed using test data. An assessment of heat transport via vented gases was performed using computational fluid dynamics (CFD). Use of the models in conjunction with testing led to design enhancements for, both, the LREBA and LPGT configurations. These thermal-runaway severity-reduction measures are also being applied to other lithium-ion batteries being developed for the International Space Station, Extravehicular Mobility Unit and other programs. Modeling guidance and future efforts are discussed.