Theory 054/Engineering & Materials /Battery safety
Runaway Stops at a Spacing
Safety is pursued through chemistry. A gap may be enough.
Thermal runaway propagates between cells only when their spacing falls below a critical value set by the balance of heat transfer and cell heat capacity, so packs can be made propagation-proof by geometry rather than by chemistry.
01The evidence
Propagation tests show that packs with adequate spacing can contain a single-cell failure, while tightly packed packs propagate.
02The tension
Battery safety focuses on cell chemistry and on additives. Geometry is treated as a packaging constraint rather than as a safety parameter.
03The mechanism
A failing cell releases heat. Whether its neighbour reaches its own trigger temperature depends on how much heat reaches it, which falls with spacing and with the conductivity of the intervening material.
That makes spacing the controlling variable, and predicts a critical gap above which propagation stops regardless of cell chemistry.
04What this is not
Chemistry models focus on the trigger. This focuses on the transfer, which predicts a geometric threshold.
05Predictions
- P1Propagation should stop above a critical spacing.
- P2The critical gap should scale with cell capacity.
- P3Low-conductivity fillers should raise the gap requirement.
06Refutation
Propagation occurring at any spacing would remove the geometric threshold.
07What it changes
It would make pack geometry a safety specification.