Abstract
Modern aircraft designs for “more electric” and “fully electric” aircraft have large battery packs ranging from tens of kWh for urban aviation to hundreds or thousands of kWh for commercial aviation. Such large battery packs require careful consideration of the safety concerns unique to aviation. The most pertinent safety concerns related to batteries can be categorized into two broad areas: exothermic heat related events (thermal issues) and partial or complete loss of safety–critical power supply (functional issues). Degradation during operation of a battery can contribute to capacity fade, increased internal resistance, power fade, and internal short circuits, which lead to the loss of or decrease in propulsive power. When batteries are the primary source of onboard power and energy, it is crucial to be able to estimate their state-of-health in terms of capacity and power capability. Internal short circuits and other sources of excessive heat generation can lead to high temperatures within the cells of a battery pack leading to safety concerns and thermal events. One of the biggest risk factors for batteries used in aviation is the potential for thermal runaway where temperatures reach the flashpoint of one of the cell components, eventually cascading over multiple cells leading to system-wide battery pack failure and a fire hazard. This article reviews the current understanding of the safety concerns related to batteries in the context of urban and regional electric aviation.
Highlights
As rechargeable Li-ion batteries have reached technological maturity, with an increase in performance metrics (Wh/kg, Wh/L, W/kg, and W/L) and a drop in price ($/kWh), they have enabled the electrification of multiple modes of transportation, recently including electric aircraft.[1,2,3]
While battery performance metrics and adoption have increased, incidence of safety related issues has increased.[5]. The majority of these reported incidents are related to the failure mode of thermal runaway, either with[6] or without internal shorts,[7] where an exothermic reaction and ignition in one cell cascades into similar exothermic reactions in neighboring cells and eventually a critical portion of the battery pack itself.[8,9]
urban air mobility (UAM) development is primarily focused on various diverse designs for small battery-powered electric vertical takeoff and landing aircraft, with some interest in short takeoff and landing aircraft.[20,21]
Summary
As rechargeable Li-ion batteries have reached technological maturity, with an increase in performance metrics (Wh/kg, Wh/L, W/kg, and W/L) and a drop in price ($/kWh), they have enabled the electrification of multiple modes of transportation, recently including electric aircraft.[1,2,3] The specific energy of commercially available Li-ion cells has increased from about 100 Wh/kg in 1990 when they were first commercialized to greater than 250 Wh/kg by 2020,4 with prototype cells currently achieving between 300 and 350 Wh/kg. Along with an increase in the adoption of EVs, the number of reported EV battery fires has increased.[10] Battery fires have occurred in a variety of scenarios, including in parked, driving and charging vehicles Most of these incidents have been due to one or more faulty cells reaching operating conditions beyond the safety limits, leading to thermal runaway. The operating conditions in an airborne environment are different from terrestrial conditions, given the changes in pressure, temperature and unique power demands of aviation We evaluate these risks with the help of existing literature on thermal and safety modeling of Li-ion batteries in the context of an airborne environment, along with mitigation strategies for the same. We conduct an evaluation of the regulatory literature on rechargeable Li-ion batteries used in aircraft to identify key areas where current and upcoming work from the battery research community needs to be incorporated in the said aviation regulatory literature
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