Battery Survivability and Safety Characterization for Army Microgrid and Electric Aircraft Applications

Passive protection from battery thermal runaway is a critical problem to solve to enable future air mobility markets, including those intended to be served by ElectraÆs eSTOL aircraft as well as all other eVTOL and eCTOL configurations for urban and regional air mobility. Many of the leading battery manufacturers today use cylindrical cells and thermal runaway protection methods that entail milling a block of aluminum and nesting each cell within the gaps to provide thermal mass for cooling. This is fine on takeoff when excess heat is generated but it results in an unnecessarily inefficient and heavy system, especially when every pound counts on an aircraft, even more so for electric aircraft coping with the lower energy densities of batteries. ElectraÆs battery design involves rectangular cells with ultra-lightweight Titanium dividers which are strategically placed between cell stacks with air gaps and flow channels in between to provide a designated route for gasses to escape the battery through side vents in the event of thermal runaway within an individual cell. Further, ElectraÆs battery is liquid cooled with a water-glycol mixture through aluminum cold plates between alternating horizontal layers of battery cells which ensure that heat can be efficiently transported away from the cells to create a thermal barrier between adjacent cells. This lowers the volumetric efficiency of the battery by ~5 to 10% but ultimately enables a lighter, higher performance battery. Electra proposes to utilize one of its existing flight batteries, instrument it with thermocouples, and conduct a full-up thermal runaway test to validate its ability to passively prevent cell-to-cell thermal runaway propagation. Additionally, Electra will conduct a shock and vibration test. The resulting data will provide a comprehensive characterization of the survivability and safety considerations for aviation-grade batteries used in Army applications. Further study in a follow-on Phase II effort would likely be required, but this will be sufficient for a feasibility study.