This report has been linked in at least two threads. Read it again, carefully.
Most readers focus on the battery meltdown. Although thermal runaway does appear to be the initial issue, it was not
the proximate emergency. The emergency
was fire in the cockpit, resulting in burn injury to the pilot, his exit of the still moving aircraft, and the ultimate total loss of the airframe due to fire.
Ok, so there was an ignition source (a hot battery) on the engine side of a firewall. The question investigators fail to ask is how did a hot battery result in open flame on the cockpit side of the firewall?
The J170 has a firewall assembly typical of composite aircraft, a bonded-in plywood core faced with a ceramic fiber sheet (fiberfrax) or similar, then covered with a stainless steel sheet. Here's a link to the construction manual; check pages 45 to 53:
I've never burn tested a SS/fiberfrax/plywood assembly, but I suspect the bare stack-up would do moderately well. The problem is practical detail. As noted in this thread and elsewhere, steel fittings and bolts are both capable of point heating. They can glow red and transmit (by conduction) more than enough heat to provide an ignition hot spot.
Look at the photo at the bottom of page 53, a view of the cabin side after installation of all the bolts and fittings:
Now jump to page 234:
Yes, that's foam insulation glued over all those ignition point sources.
Folks, I cannot absolutely state that in this case the fire transfer mechanism
was hot battery-->hot hardware-->foam ignition. We just don't have enough information, all too typical for an EAB crash report. I have watched bolts glow red in an otherwise relatively cool firewall, and if it walks like a duck and quacks like a duck...
So, two lessons.
(1) For the umpteenth time, don't put stupid stuff on the cabin side of firewalls, even when properly insulated on the forward side.
(2) Lithium battery runaways can get very hot. The mere fact that the battery is on the opposite side of the firewall, or within a containment box,
does not guarantee adjacent objects will remain below ignition temperature.