As every aero-modeller will confirm, when you discharge a Lithium Ion cell in minutes as opposed to hours, ( i.e. at a rate higher than 1xC ) it will get warm. The same applies for fast charging.
As the outside vent from the sealed box is sealed with a rupture plug, Is it true that the whole box has no natural or forced cooling air-flow? I have not been able to find any reference to a cooling system.