The future of mass transportation is clearly moving towards increased use of hybrid and electric vehicles. With the introduction of high-power/high-energy storage devices such as lithium ion battery systems serving as a key element in the system, valid safety and security concerns emerge. This is especially true when the attractive high-specific-energy and powerchemistry lithium nickel cobalt aluminum oxide (NCA) is used.
NCA chemistry performs well but presents a safety and security risk when used in large quantities, such as for a passenger bus. If triggered, the cell can completely fuel its own fire, and this triggering event occurs more easily than one may think. Researchers at the Mineta National Transit Research Consortium, led by principal investigator Timothy Cleary, partnered with advanced chemistry battery and material manufacturers to study the safety concerns of NCA batteries for use in transit buses.
The research team ran various experiments and triggering events were tested at both the battery cell and module level, including overcharge, short-circuit, nail puncture, and crash scenarios. An investigation into packaging materials was also performed during these tests to help determine the response of a full battery system. Results are documented in the Safety of Lithium Nickel Cobalt Aluminum Oxide Battery Packs in Transit Bus Applications report.
According to lead author, Cleary, the “findings indicate that when considering the use of an NCA battery system in a transit bus application, material and structural design is critical to keeping passengers safe. Common plastics such as acetal and PET have acceptable structural properties, but their swift and sustained combustibility under the high temperatures of a nearby thermal event poses a risk of rapid and severe events.” Ultimately, said the researchers, “the greatest safety concern when using high-energy, NCA batteries is ensuring passenger safety when a cell’s electrolyte boils and causes the ventilation of high-temperature toxic material.”
Though no system is perfect, a properly functioning and intelligent battery management system with redundant voltage measurement and real-time internal temperature modeling or a virtual sensor should be mandatory to avoid cell-venting events. In addition, following a venting event, the large format battery cells can remain at extremely high temperatures for long periods so appropriate cooling mechanisms are essential as part of crash or incident recovery and system design.
