Mine are built-in to the batteries. There is a UL standard for it (UL 9540A). It stops the thermal runaway that causes the battery to continually burn. And, when a certain temperature is detected by the built-in battery management system (BMS) the battery shuts down. Without the thermal runaway between cells, they don't burn. And, the battery itself is designed not to burn in the first place. In addition to this, they may also have a built-in aerosol fire extinguishing system.
“Mine are built-in to the batteries.”
Thanks, that’s the BMS (battery management system), and it is good at stopping external factors from starting a fire, like shorts, overloads, overcharging, extreme temperatures, etc - by shutting down the battery. It’s actually a great improvement, as the original batteries depended on people who didn’t even understand battery limitations to prevent conditions that caused fires to start. I have no doubt that all of the batteries used in the huge battery farms have BMS, probably at several levels.
“It stops the thermal runaway that causes the battery to continually burn.”
By definition thermal runaway cannot be stopped, or it wouldn’t be called “runaway”. But there certainly are design measures that can minimize the odds of it spreading. But, since the constituents of the cells are what actually is burning, at least initially, there is no way to extinguish it. Later on, as the fire spreads outside of the cells, then conventional methods (oxygen removal, water saturation, etc. can work) can work, providing they stop the runaway, meaning they can keep adjacent cells from reaching their self-ignition temperature, around 400F.
Yet the fires keep happening...so there’s more to it. As to what that is, I can only speculate, but my guess is that when you have, literally, many millions of small cells comprising a battery farm, some might be defective, and set off a runaway condition. The fact that these battery farms, like the original nuclear plants, are basically exempt from liability in these types of failures (I assume, as they wouldn’t be built otherwise) leads me to believe that the builders of these farms don’t mind taking ‘shortcuts’ once in a while.