Interview with Antonio García, Associate Professor at the Thermal Engine Institute (Instituto de Motores Térmicos) of Universitat Politècnica de València
What is the process of cell testing to evaluate safety?
The process of cell testing to evaluate safety consists of understanding how the battery behaves under conditions that can lead to a dangerous situation known as thermal runaway.
Various mechanisms can trigger uncontrollable chain reactions, including external heating, overcharging, or over-discharging, as well as physical abuse. However, before conducting destructive tests, it is crucial to assess the battery’s performance under normal operating conditions, characterise important parameters for modeling purposes, ensure high manufacturing standards, and prevent erratic behavior during experimental tests. Therefore, it is essential to measure parameters such as capacity, voltage, current, impedance, and energy density to conduct cell safety tests.
Additionally, understanding the composition of different battery components and how they influence the main chain reactions is important for interpreting the measured parameters during thermal abuse tests. To determine the temperature at which exothermic reactions leading to critical temperature and unstoppable processes occur, various safety-related triggers, ambient conditions (initial temperature, pressure, humidity), and battery aging conditions (such as solid electrolyte interphase -SEI- growth and plating) can be experimentally simulated. Knowing this temperature serves as an alarm for the battery management system, providing an opportunity to intervene and halt the process.
Measuring the heat released from the battery surface is also important, as it enables the design of cooling systems to absorb the heat and prevent the propagation of thermal runaway in a battery pack. Moreover, measuring gases emitted during thermal runaway, which depend on the battery’s material composition, is crucial for mitigating combustion risks. Therefore, a comprehensive battery testing methodology must address these different aspects to enhance battery safety and drive its development.
How is UPV conducting these tests?
UPV has developed a battery test facility over the past few years, equipped with climatic chambers and bidirectional sources to accommodate a wide range of battery sizes and modules. This setup allows for the replication of vehicle-level tests, such as driving cycles. To study thermal runaway, UPV employs calorimeters capable of analysing thermal runaway at the cell and module levels.
Collecting data to support thermal runaway models and verifying compliance with safety standards are crucial aspects of the testing process. Additionally, UPV has the capability to measure various gas emissions from batteries using different gas analysers based on different principles of operation. Establishing a chemical kinetic reaction mechanism and predicting gas emissions based on battery components play fundamental roles in their evaluations.
Furthermore, UPV utilises optical techniques to characterise the venting process under different conditions and employs a thermographic camera with temperature correction for different optical setups to measure temperature distribution on the cell surface. UPV has also developed an in-house method using laser heating to induce thermal runaway, providing better control over the process. The modeling efforts at UPV involve the use of P2D models, which offer a good balance between cost and quality in terms of results. For more detailed studies, computational fluid dynamics (CFD) simulations are utilised to simulate battery safety in different contexts, including global environments and specific parts of the battery, such as venting cap hole design.