Or you could just have the batteries in a separate enclosure away from your house. I think I would be inclined to do this anyway, certainly for Lithium batteries given the possibility of fire.
hydrogen sulfide is not anywhere in the same category. When you consider failure you have to consider what is the most catastrophic possibility and if that is “this battery silently kills people” then you dont make it.
Batteries with Prussian blue cannot kill people silently.
Cyanide could be released only at high temperatures, e.g. if the battery is opened and burned, not during normal operation, even if overcharging is not prevented, as it should.
The sulfuric acid from the traditional lead-acid car batteries is more dangerous than this.
We also have to adulterate that methane with bitter smelling agents too warn people of the danger when there's a leak. The line into the house is also limited by a regulator to ensure the pressure is very low. If gas builds up in a battery, it's either going to leak out slowly or build up and leak out all at once.
The methane is almost always piped in to be burned, and that can easily create odorless carbon monoxide. And the smell is not foolproof either. This does routinely kill people and we keep doing it. The jurisdictions that are banning it are doing so because of environmental reasons, not safety.
> hydrogen sulfide is not anywhere in the same category.
It has the same LD50 dose as HCN. It literally _is_ just as bad. It routinely kills people on oil rigs because in lethal concentrations it immediately shuts off your nose.
How often do you hear about people getting poisoned by it from lead-acid batteries?
The only people with any significant amount of lead acid batteries on their property are off grid types who typically store them away from their primary domicile as a fire safety precaution.
Fast charging a car/chemical weapon in your garage isn't terribly appealing.
Also, I think HCN can be scrubbed by adding a special absorptive cap onto the battery.