A little after 8:00 p.m. on April 19, 2019, a captain with the Peoria, Arizona, fire department’s Hazmat unit, opened the door of a container filled with more than 10,000 energized lithium-ion battery cells, part of a utility-scale storage system that had been deployed two years earlier by the local utility, Arizona Public Service.
Earlier that evening, at around 5:41 p.m., dispatchers had received a call alerting them to smoke and a “bad smell” in the area around the McMicken Battery Energy Storage System (BESS) site in suburban Phoenix.
Sirens blaring, three fire engines arrived at the scene within 10 minutes. Shortly after their arrival, first responders realized that energized batteries were involved and elevated the call to a Hazmat response. After consulting with utility personnel and deciding on a plan of action, a fire captain and three firefighters approached the container door shortly before 8:00 p.m., preparing to open it. The captain, identified in a later investigation as “Captain E193,” opened the door and stepped inside. The other three stood nearby.
The BESS was housed in a container arranged to hold 36 vertical racks separated into two rows on either side of a 3-ft-wide hallway. Twenty-seven racks held 14 battery modules manufactured by LG Chem, an 80 kW inverter manufactured by Parker, an AES Advancion node controller used for data collection and communication, and a Battery Protection Unit (BPU) manufactured by LG Chem.
The battery modules in turn contained 28 lithium-ion battery cells of Nickel Manganese Cobalt (NMC) chemistry. These modules were connected in series, providing a per-rack nominal voltage of 721 V. The entire system had a nameplate capacity to supply 2 MW of power over one hour for a lifetime energy rating of 2 MWh. With 27 full racks, there were 10,584 cells in the container. After a full day of charging, the batteries were around 90 percent of capacity.
With the door to the BESS container open and Captain E193 at its threshold, combustible gases that had built up inside since the incident began several hours before received a breath of oxygen and found an ignition source.
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