“the driver and passengers are exposed to a toxic vapor cloud?”
I’m pretty sure these cells do not out-gas during normal operation the way lead-acid automotive batteries used to do. I grew up in an era of lead-acid auto batteries and I was taught in the 60s to NEVER make ground contact on the battery terminal because of the risk of a hydrogen explosion. I never actually heard of a battery exploding due to hydrogen off-gassing, but was warned about it. You always attached the red jumper cable to the positive battery terminal, then the black jumper cable to the car chassis away from the battery. That kept the sparks away from the battery.
If the battery sends out a Vapor Cloud, it happens fast and big. You’ll know it’s time to abandon ship if you see that happening.
Vent caps on modern (since at least the 1980s) lead-acid batteries are of the flame-arresting type.
Lead-acid batteries CAN explode internally. This happens when the electrolyte is low and one of the plates or bus bars cracks, causing a spark. This will result in the vent caps being blown off. I have only seen this happen once. It happened while cranking the engine. Low electrolyte was the cause, or at least what allowed the spark to ignite the hydrogen vapors. If it’s submerged in electrolyte there is no spark that will ignite anything.
I’m partially into a bourbon so don’t trust timelines, only physics. In ‘70s when I was PhD biophysics grad student i messed around more than I should have if degree were my only objective.
Friend had a Fiat X-1-9(?) into which we dropped a full-race Abarth racing engine. Campus police were used to me snaking oxy-acetylene hoses out the lab windows ...
The redline went up many thousand RPM. I needed to modify his tach. Chucked the distributor up in a lathe in the mechanical shop and began the calibration process - known rpms from lathe head RPMs. Easy peasy.
Had a small auto battery (I think from my VW Rabbit) powering the setup. 5 or so minutes in had a nasty explosion. The arcing from the distributor points ignited the gasses from the battery.
All OK. I was young. Worried about the lathe and the chuck and ways. Now I’d worry only about my eyes. Had safety glasses on but only because I needed Rx all the time.
I owed him. He helped me drop a Chevy small-block into my FJ-40 Landcruiser.