[MertsA]

>There is no appropriate extinguisher when you have a thermal runaway on 20 kWh of chemical energy.

Yes there is, it's called a hose. But you're right, that 20 kWh isn't just going to up and disappear. I think it's realistically possible to do this with a modicum of safety even for a hobbyist but it needs to be passively safe. It can't rely on cells being inherently balanced with respect to one another, it needs to have some serious thought about what to do with that 20 kWh if the worst occurs but that's not an insurmountable task. Sprinkler heads are cheap and even just a garden hose will put out 25 gallons a minute. A very slight risk of water damage can be acceptable IMHO, I wouldn't have trouble sleeping with something cobbled together sitting in my garage so long as I planned for the worst along the way when piecing it together.