There is a problem with these batteries breaking down, venting electrolyte, and overheating. The solution puts the problem battery in a sealed 1/8” thick steel box. What happens to that box when vented electrolyte has nowhere to go? Isn’t the box being pressurized? This sounds like a potential bomb being created if the pressure were to build up sufficiently.
It’s vented to the outside world.
Boeing engineers thought about that:
“Boeing provided details Friday on how it will encase the redesigned power pack in a steel box, pack it with different insulation, heat-resistant material and spacers, drainage holes to remove moisture and to vent any gases from overheating directly to the atmosphere outside the aircraft."
From the article:
“Should a cell rupture within the redesigned steel box, the gases would open a pressure release disc, a valve of sorts, which would vent the electrolyte mixture through a one-inch titanium tube to the exterior of the aircraft. Each of the two batteries has its own vent tube, and the system is designed to eliminate the chance of gasses or smoke emanating from a failed battery entering the aircraft. Boeing says the vents will require placing two small holes in the fuselage, but they will not affect the structure of the aircraft. The FAA requires that the holes do not result in the vented gases reentering the aircraft downstream.”