At least mass-market phones and their batteries by reputable manufacturers go through rigorous testing. The sole exceptions I'm aware of are Samsung's infamous last-ever Note phablets and a few batches of iPhones. Fires of mechanically intact phones are rare.
Shoddy knockoffs and cheap Chinesium garbage however, that's a different thing.
When you only have one battery cell it's also hard to screw up battery management, and phone batteries are rarely driven past their prime (no one wants a phone with 50% remaining capacity). The odds are not zero though, as the battery can develop internal shorts from imperfections like dendrite growth over time.
The odds get much worse when you have tens or hundreds of kWh comprised of hundreds or thousands of cells, especially in OPs jank setup with improper battery management and cells that are all already at the end of their life and likely have significant internal damage already.
At least mass-market phones and their batteries by reputable manufacturers go through rigorous testing. The sole exceptions I'm aware of are Samsung's infamous last-ever Note phablets and a few batches of iPhones. Fires of mechanically intact phones are rare.
Shoddy knockoffs and cheap Chinesium garbage however, that's a different thing.
When you only have one battery cell it's also hard to screw up battery management, and phone batteries are rarely driven past their prime (no one wants a phone with 50% remaining capacity). The odds are not zero though, as the battery can develop internal shorts from imperfections like dendrite growth over time.
The odds get much worse when you have tens or hundreds of kWh comprised of hundreds or thousands of cells, especially in OPs jank setup with improper battery management and cells that are all already at the end of their life and likely have significant internal damage already.