Thermal Runaway and Venting
PRV-Sizing starts one step after thermal runaway has already begun inside a battery pack. To use it well, it helps to know what came before that step and what comes after it.
Thermal Runaway in One Paragraph
A lithium-ion cell driven past its stability limit, by overcharge, internal short, mechanical damage, or external heating, enters a self-sustaining chain of exothermic decomposition reactions. The passivation layer breaks down, the separator softens and fails, the electrolyte vaporises and decomposes, and the cathode releases oxygen. Each reaction heats the cell further, which accelerates the next one. The cell can no longer be cooled back into a stable state, and the process runs to completion on its own.
For our purposes, the important consequence is not the heat. It is the gas.
What Comes Out
A cell in thermal runaway produces a large volume of hot gas in a few seconds. The mixture is dominated by hydrogen, carbon monoxide, carbon dioxide, methane, and ethylene, together with vaporised electrolyte solvents and entrained particulates. The exact composition depends on chemistry, state of charge, and what triggered the event.
PRV-Sizing does not model this chemistry. It takes the result of it, as a curve of cumulative gas moles released per cell against time, and treats that as a known input. Where that curve comes from is covered in Input Data Sources.
Two Different Valves
The word “vent” gets used for two devices that behave in opposite ways. Mixing them up is the most common misreading of this kind of analysis.
| Cell safety vent | Pack pressure relief valve | |
|---|---|---|
| Where | On the cell casing | On the enclosure wall |
| Purpose | Stops the cell itself from bursting | Stops the enclosure from bursting |
| Behaviour | Ruptures once, stays open | Opens above a setpoint, and a spring type recloses below it |
| Reusable | No | Spring types, yes |
| Modelled here | No, it is upstream of our control volume | Yes, this is the device you are sizing |
PRV-Sizing models the second one. The first one is what puts gas into the pack in the first place, and its behaviour is already baked into the venting curve you supply.
Why a Sealed Pack Is the Problem
A battery pack enclosure, or housing, is sealed to a high ingress protection rating to keep water, dust, and salt away from cells and busbars. That same seal means the gas from a venting cell has nowhere to go. Pressure rises across the whole enclosure within seconds, and a pack designed to keep water out will happily hold enough pressure to deform its lid or split a seam.
The relief valve resolves the contradiction. It keeps the enclosure sealed during normal operation and opens a large flow path once internal pressure crosses its setpoint, so the gas leaves through a designed opening instead of through whatever fails first.
One Cell, or Many
A cell in thermal runaway heats its neighbours. If the pack does not stop that, cells go one after another, and the gas load on the enclosure is several cells rather than one.
The tool handles this with Number of Venting Cells, which scales the single cell venting curve. That scaling assumes every cell follows the same curve at the same time, which is the conservative reading of a propagation scenario and is discussed in Assumptions and Limits.
Choosing that number is a scenario decision, not a tuning knob. It should come from your propagation testing or from the safety case you are required to defend, for example “the enclosure must survive N cells venting”.
Where the Chain Stops Here
Vented gas is flammable, and once outside the pack it disperses, accumulates, and can ignite. That is a real and well studied hazard, and it is entirely outside this tool. PRV-Sizing ends at the valve outlet. If you need the downstream picture, the studies in Further Reading cover the full chain from cell venting to enclosure and container level consequences.