There is a popular myth that - after the early 787 Li battery fires - all Boeing did was put the battery in a steel box and left the rest of the system alone.
That's a myth - a popular, often repeated myth - but still very much a myth.
In the aftermath of the original Li fires (and the associated grounding), Boeing and YUASA did a complete redesign - upgrading the charging system and the battery itself. However, due to the extent of the damage to the fire involved batteries, it was impossible to definitively determine the root cause - so in a bit of a 'belt and suspenders' approach, they put the redesigned battery in a stainless-steel box that would vent any fire or resultant toxic gases overboard. The intentionally caused a battery short-circuit to cause the battery to thermally runway in the steel box and verified that the box prevented the resultant fire from propagating out of the box.
Since the redesign, there have been a handful of single-cell battery failures, but none have propagated and spread to the rest of the battery.
It should be noted that there were two battery fires with only 50 aircraft in service for about a year. Since the redesign, there are over 1,000 787s flying around daily - racking up something like 30 million flight hours - without a reoccurrence of a battery meltdown.
And as I've posted repeatedly in the (now closed) threads, even with a 100% aircraft power loss, the engines are sufficiently isolated and independent that they will still operate just fine, so long is there is fuel in the tanks.