I had thermal runaway on a set of Vetus wet cell batteries that were eighteen months old.
The incident happened at the top of the Sea of Cortez under sail, there was no explosion, I was alerted to it by the smell.
The battery box was hot to touch and I was terrified to open it. The inside was covered in battery acid, all the cables were destroyed, four batteries were cracked, eight batteries had buckled cases.
Four were serviceable which I hooked together to allow me to return to Mazatlan for repairs.
It was a harrowing experience as the temperatures of each battery kept rising for hours as they sat overnight on a dock.
I have met a number of people with variations of the same theme.
Good luck with your new engine.
Former owner of Seafever