Here is the condition that requires the presence of the upgraded BOX.
(6) Each Lithium ion battery installation must have provisions to prevent any hazardous effect on structure or essential systems caused by the maximum amount of heat the battery can generate during a short circuit of the battery or of its individual cells.
This is why I have made such an issue of the nomenclature, "Cell" v. "Battery".
As written, the language prevents (prohibits) a single short circuit. A single short circuit in the current design will (should) take the entire eight cell "Group" off line, "loss of essential system".
Technically, then, there should be:
Eight separate "boxes" that isolate a short circuit from failing the "essential system" (the other seven "cells") and a means for keeping the power storage at usable level. OR,
""Short Circuit" must be proven to be "One in Ten Million flight hours".
This is regulatory, and hasn't anything to do with "Fire".
Again, the major problem is the 'current' design as to suitable (and "demonstrable") "Reliability", not the prevention of fire.
If the APU battery is an agglomeration of eight cells, comprising a "battery", then going off line is a major failure with or without any fire issue. If it is a group of eight individual batteries, and can be made to perform if one battery shorts out, the system is (becomes) reliable by definition...
The potential to catch fire must still be one in one BILLION flight hours, regardless the design of the enclosure.
EXCEPT. The discussion will be: "What causes the fire". The battery cannot be the source, (cause) of fire, this is fatal to its installation. It is by definition "flammable" in and of itself, and if exposed to extraneous ignition, may ignite, this is allowed.
imo....