WOBBL18650 cell
Can a cell at the hottest point of the sealed battery pack run away on its own?
OK
Thermal runaway starts above 155 C, 80.4 K above the 74.5 C the 18650 cell sits in, so self heating cannot run away.
runaway above
155 C
surroundings
74.5 C
margin
80.4 K
Semenov estimate
107 C
Confidence high
- passed lumped cell, energy balance, time step convergence, Semenov bound
Simulation
Checks
| status | check | result |
|---|---|---|
| pass | lumped cell | Biot number 0.00581 for 5 W/(m2 K) over the cell, so its inside stays within 0.291% of one temperature |
| pass | energy balance | heat made equals heat stored plus heat shed to 9.4e-14% over each transient |
| pass | time step convergence | at 155 C the cell runs away after 68.6 min, and 68.6 min with a tolerance 100 times tighter (7.7e-05% apart) |
| info | closed form | at the Semenov point the cell sits 11.4 K above its surroundings, against R Ta^2 / E = 8.92 K for one reaction carrying 67% of the heat, 27% apart |
| pass | Semenov bound | with fresh reactants that never run out the cell would run away above 107 C, below the 155 C the transient needs, since the small store of SEI burns out before it can carry the cell |
Assumptions
- Wobbl guessed: temperature not given: assumed 25 C (298.15 K).
- Wobbl guessed: pressure not given: assumed 1 atm.
- LiCoO2 cell at 100% charge (Moli ICR18650), 18 by 65 mm, with the kinetics of Hu, He, Rein and Restuccia, Numerical Study of Self-Heating Ignition of a Box of Lithium-Ion Batteries During Storage, Fire Technology 2020, Table 2, from Hatchard et al. 2001 and Kim et al. 2007.
- Cooled by 5 W/(m2 K) of convection and radiation at an emissivity of 0.8 over its whole surface, in surroundings taken from the hottest point of the heat study in ../battery-pack/output.
- The cell is lumped, open circuit and at rest, with no internal short, overcharge or outside fire. Runaway is the moment the cell heats itself faster than 10 C/min, the trigger Feng et al. (Frontiers in Energy Research 2018) use.
- The kinetics describe a fully charged LiCoO2 cell. LFP cells run away at higher temperatures and NMC cells at similar ones, so for other chemistries treat the answer as indicative.
- The kinetics carry the cell up to the trigger. What follows it, the separator melting, venting and fire, is beyond them, so the temperatures after the trigger are not a forecast.