We tend to use thermal runaway to describe a single cell and thermal propagation to describe the cell to cell spread.
Thermal Runaway
If you heat a battery cell to somewhere above 130°C then exothermic chemical reactions inside the cell will increase the temperature and further reactions will take place. The result is an uncontrolled runaway and increase in temperature. The cell should vent in a controlled manner with fire and molten material. In severe cases the cell may explode. The energy released from one cell failing is likely to heat neighbouring cells that again could be triggered into thermal propagation.
Propagation Mechanisms
Most of the propagation mechanisms involve thermal transfer from the cell in thermal runaway to neighbouring cells.
- Conduction cell to cell
- Hot vent gas
- Particle or core projection
- External failure propagation
Design to Inhibit Propagation
There are a number of design approaches to inhibit propagation:
- Cell to cell air gap
- Cell to cell thermal barrier
- Cell clustering
- Vent gas exhaust management
- Electrical fuse