Requirements around thermal runaway propagation have moved from advisory to mandatory in most major markets. The detail varies by jurisdiction and continues to evolve, so treat this as orientation and confirm the current requirement for your market rather than relying on any supplier summary, including this one.
The common intent
Across the major regimes the underlying intent is consistent: a single cell entering thermal runaway must not cascade into a pack-level event without adequate warning time. For vehicles that means occupants must be able to get out. For stationary storage it means an incident should stay inside one enclosure and suppression should have time to work.
Why this is hard for pack designers
Every millimetre of barrier is a millimetre not available for active material, and the trade-off is not linear. Losing cell count changes pack voltage, current density and cooling behaviour, so a barrier that is merely adequate at a large thickness is often not a viable answer at all. This is why barrier thickness gets argued over so hard in design reviews.
What a barrier has to survive
- Direct flame from a venting cell, briefly at very high temperature
- Solid and molten ejecta impacting at speed
- Sustained heat from neighbouring cells after the initial event
- Mechanical compression from cell swelling across the whole pack life
- Vibration and thermal cycling over years of normal service
Why a dual-function barrier helps the space budget
Most packs already include a compression pad to manage cell swelling. If the thermal barrier also performs that function, the net volume cost is much lower than the barrier thickness suggests, because the pad it replaces was already consuming stack height. This is frequently the argument that makes the numbers work.
Practical advice
- Confirm the current requirement for each market you sell into, not a generalised summary
- Test with your own cells and your own geometry. Supplier data uses their test setup
- Get sample material for propagation testing before committing to tooling
- Involve the barrier supplier at design stage, not after the pack architecture is frozen
