Investigators looking into an April 2019 fire that destroyed a 2-MW grid-connected lithium ion battery storage array owned by utility Arizona Public Service found that it was an extensive “cascading thermal runaway” and not a fire that caused the unit to explode.
The thermal runaway began when an internal battery cell failed because of a defect, specifically, an abnormal lithium metal deposit and dendritic crystal growth in the cell. The internal fire suppression system operated as designed, but the system was not capable of preventing or stopping thermal runaways.