poorjohn:
Changing topic quickly: it would be interesting if a third-party STC'ed a replacement using e.g. one of the newer, safer Lithium variants.
It would indeed, but because of the way the battery, the contained BMU and the charger interact, any replacement would probably have to involve all three.
Given the added size and weight of the firebox, it might actually have been possible to design, for example, an LiFePO4 battery in the first place.
But the design was committed back when iron-phosphate batteries were not a well used and characterized as the LiCo, unsatisfactory at that chemistry was.