The advantage of using 18650 cells instead of bigger prismatic ones is that:
1. Surface/volume ratio is superb. It is easier to cool.
2. Compartmentalisation is better. It protects against thermal runaway cascading to other cells.
On other side...
1. 18650 themselves have bad thermals. They are simply not designed for any high current use, like EVs.
2. The above dictates the need in an overkill cooling system, and smart monitoring to account for thermal inertia of cell's insides.
3. 18650 have poorer resistance against swelling, and are more likely to fail catastrophically due to it.
4. The above mandates serious compartmentalisation, and engineering in fail safety, and fail tolerance.
5. 18650 are way more quick to become unbalanced over time from uneven thermal regimes in a pack, and inside the cell itself.
6. The above, and especially uneven internal degradation make the part of 18650 closer to the positive electrode to wear out faster, rise the internal resistance.
7. Per-cell balancing of such small cells are uneconomical.
1. Surface/volume ratio is superb. It is easier to cool.
2. Compartmentalisation is better. It protects against thermal runaway cascading to other cells.
On other side...
1. 18650 themselves have bad thermals. They are simply not designed for any high current use, like EVs.
2. The above dictates the need in an overkill cooling system, and smart monitoring to account for thermal inertia of cell's insides.
3. 18650 have poorer resistance against swelling, and are more likely to fail catastrophically due to it.
4. The above mandates serious compartmentalisation, and engineering in fail safety, and fail tolerance.
5. 18650 are way more quick to become unbalanced over time from uneven thermal regimes in a pack, and inside the cell itself.
6. The above, and especially uneven internal degradation make the part of 18650 closer to the positive electrode to wear out faster, rise the internal resistance.
7. Per-cell balancing of such small cells are uneconomical.