Inside a BESS: Cells to Containers, Inverters to Fire Systems

A utility battery looks like rows of white boxes behind a fence. Each box is a small industrial plant with its own electrical system, climate control, gas detection, fire suppression and computer network. When something goes wrong, the investigation report usually names one of those systems, and often it is not the cells.

The hierarchy

Part by part

Containers, cabinets and buildings

Sandia's handbook describes two common outdoor formats: walk-in shipping containers with racks along the walls, and smaller cabinets shipped fully assembled and repeated as many times as needed. Sandia stresses that spacing between racks and modules is critical to stopping fire spread when a few cells run away. The 2018 IFC that Idaho enforces treats an outdoor container people can walk into for service as a battery storage room, requires walk-in units to be entered only for inspection and repair, and requires outdoor battery areas to be secured against unauthorized entry.

The other format is a building. Moss Landing 300 and the Gateway plant in San Diego, both in EPRI's table as indoor, held hundreds of MWh under one roof. EPRI's October 2026 report found indoor facilities overrepresented among failures and sustaining more damage, and recommended fire walls or other barriers to divide large indoor sites.

Residents opposing projects point out that the outdoor container layout assumes a unit may burn and is designed to keep the fire to one box. That is accurate, and it is also how the codes are written: the 2026 NFPA 855 requires large-scale fire testing showing whether fire in one unit spreads to the next. The argument is over whether "one box burns for hours next to my road" is an acceptable design outcome. That is a siting question as much as an engineering one.

Cooling is a safety system

Sandia's handbook puts it bluntly: without proper thermal management, overheating cells degrade, malfunction or go into thermal runaway, and the main heat source is the batteries themselves. Cooling also has to keep cells close to the same temperature, because uneven temperatures make cells age unevenly, which the BMS then has to manage. That is why HVAC or liquid cooling failure shows up in incident reports, and why cooling equipment runs constantly and makes noise.

Energy bought to deliver 400 MWh: 400 / 0.85 = about 471 MWh
Energy lost:                      471 - 400  = about 71 MWh per day

If charging and discharging take 8 hours combined:
  71 MWh / 8 h = about 8.9 MW of heat, on average,
  during those hours

Detection, suppression and explosion control

Sandia lists the ways systems detect thermal runaway: cell voltage changes seen by the BMS, swelling detected by deformation sensors, temperature from thermocouples or fiber optics, and gas sensors. The most common suppression medium has been a gaseous clean agent such as Novec, which works mainly by cooling. Sandia notes two limits: as energy density has risen, clean agents have struggled to fully stop propagation, and any suppression that stops flames can let combustible gas build up instead of burning off. That is why makers added carbon monoxide and other gas sensors, and why designs include deflagration vent panels that channel a blast upward.

The Surprise, Arizona explosion on April 19, 2019 is the textbook case. UL FSRI's report lays out the timeline: smoke alarm and clean agent discharge at about 4:55 p.m.; hazmat team entries finding dangerous hydrogen cyanide and carbon monoxide; the door opened at about 8:01 p.m.; deflagration at about 8:04 p.m. Four firefighters were seriously injured. FSRI noted the system was commissioned before the first draft of NFPA 855 and complied with the codes in force then. Its recommendations included remotely accessible gas monitoring and explosion prevention.

The 2021 and later IFC editions require explosion control for walk-in units and rooms under Section 911, which points to deflagration venting (NFPA 68) or explosion prevention by ventilation and gas detection (NFPA 69). Idaho's statewide 2018 edition has no such explosion control section for batteries. It handles gas through room ventilation sized to keep flammable gas under 25 percent of its lower flammability limit (or hydrogen under 1 percent of room volume), and, where a hazard mitigation analysis is required, a test that released gas not create an explosion hazard for occupants or responders. Lesson 8 covers what that means locally.