Characteristics of Spring and Membrane Valves for Battery Packs
The relief valve is the only part of this problem you have real design authority over, and it is also the part the model represents most abstractly. This page explains what the tool actually knows about the valve you fit to a battery pack.
The Valve Is a Flow Curve, Not a Geometry
PRV-Sizing never sees an orifice diameter, a spring rate, or a poppet mass. It sees the valve as a relationship between the differential pressure across it and the volumetric flow through it, tabulated as a curve. Everything the valve contributes to the pressure equation arrives through that curve.
This is why the tool cannot compute a required vent area for you: area is one of many things that produce a given flow curve, and the curve is the quantity a manufacturer actually measures and publishes.
Two Curves, Not One
Every valve in the tool carries two flow curves.
| Curve | When it applies | What it represents |
|---|---|---|
| P–Q Curve Before Opening | While the valve is closed | The small flow that passes anyway, through the breathing membrane and any leakage path |
| P–Q Curve After Opening | Once the valve has opened | The relief flow through the fully open path |
The difference between them is typically two or three orders of magnitude. The “before” curve barely relieves anything on the timescale of a venting event, but it is not zero, and including it is what makes the pressure rise before the setpoint physically correct rather than merely adiabatic.
Spring and Membrane
The tool offers two valve types, which differ in one respect: whether the valve can close again.
Spring. A spring holds a plug against a seat. Pressure above the setpoint lifts the plug and opens a flow path. When pressure falls back, the spring reseats the plug and the valve seals again. It can open and close repeatedly during one event.
Membrane. A one-time device. A diaphragm ruptures, or is pierced, once its opening pressure is reached, and the flow path stays open for the rest of the event. There is no reseating.
Real products come in more variants than these two names suggest, but for a
pressure calculation the taxonomy collapses to the question above. A burst disc
and a pin-pierced diaphragm both behave as Membrane. A spring loaded poppet
behaves as Spring.
Hysteresis, and Why a Spring Valve Can Cycle
A spring valve does not reclose at exactly the pressure at which it opened. It recloses lower. This is not a manufacturing imperfection, it follows from the force balance on the plug: while the valve is closed, the seal contributes resistance that the opening pressure has to overcome, and once the plug lifts that contribution is gone. The opening pressure is therefore always higher than the closing pressure, a result derived and measured by Wang et al. (2026).
The practical consequence is visible in your results. If gas arrives fast enough, the valve opens and stays open until the event is over. If gas arrives slowly, pressure can fall below the closing threshold while venting continues, the valve reseats, pressure builds again, and the valve reopens. Venting becomes intermittent, and the valve status trace in your charts shows several steps rather than one.
Neither behaviour is automatically the safer one. A membrane valve guarantees the flow path stays open, at the cost of a pack that is open to the environment afterwards. A spring valve recloses, which keeps outside air, and therefore oxygen, from being drawn back into a pack full of hot flammable gas.
What You Choose, and What You Do Not
This distinction decides how you use the whole tool.
| Property | Who decides it |
|---|---|
| Opening pressure | The valve manufacturer. It is a catalogue property of the part |
| Flow curves | The valve manufacturer. Measured on a flow bench |
| Valve type | The manufacturer, through the product you select |
| Which valve you fit | You |
| How many you fit | You |
When you enter a valve manually, the opening pressure field is there to record the value from your supplier datasheet. It is not a design variable to tune until the case passes. Sizing here means selecting among real parts, which is what Choosing a Valve covers.
Several Valves
Number of Valves multiplies the flow curve. Two identical valves pass twice the flow at the same differential pressure, and they open at the same threshold, since they are the same part seeing the same pack pressure.
The model assumes the valves are identical and see identical conditions. It cannot represent a mixed installation of different valves, nor a valve positioned where the local pressure differs from the pack average.
The Breathing Function Is Not Modelled Here
Most pack relief valves also carry a microporous membrane that lets air pass slowly in and out during normal service, equalising pressure as temperature and altitude change, while blocking liquid water and dust. Sizing that function is a separate calculation, driven by your thermal and altitude duty cycle rather than by a venting event, and PRV-Sizing does not perform it.
Two consequences are worth carrying into a real design review:
- A valve that passes the thermal runaway check may still be wrong for the pack, if its breathing capacity does not match the duty cycle.
- The opening pressure cannot be chosen freely low. A setpoint close to the everyday pressure swing invites spurious actuation, and each spurious opening exposes the pack interior to dust and water. This constraint on real valve design is discussed by Song et al. (2024).
That same study reports a further effect the model cannot capture: vaporised electrolyte can be blocked by the breathing membrane rather than passing through it, so the real relief path may behave worse than a clean flow curve suggests. Treat a marginal pass as marginal.