Meaning
Subsonic combustion of flammable vapors released from a battery during thermal runaway creates a pressure wave that travels slower than the speed of sound. This offgas deflagration involves the rapid oxidation of hydrogen and carbon monoxide in the presence of an ignition source.
Ignition Condition
Presence of a spark or a hot surface within the battery compartment provides the energy needed to start the reaction. During an offgas deflagration, the temperature of the gas cloud must exceed its auto-ignition point or encounter an external flame. These conditions often occur when the internal cell temperature reaches the point where the separator fails.
Mitigation Strategy
Installation of flame arrestors and explosion vents prevents the internal pressure from reaching the burst point of the container. If the offgas deflagration is caught early by a suppression system, the risk to the surrounding infrastructure is lowered. Proper design ensures that the flame cannot propagate back into the main cell storage area.
Structural Loading
Design of the battery room must account for the peak force exerted by the expanding gas cloud during a fire. Because offgas deflagration produces a predictable pressure-time curve, engineers can calculate the required strength of the walls and doors. This assessment ensures that the building remains safe for occupancy and that the failure of one battery rack does not lead to a total loss of the facility through structural collapse or door failure.
