These ran NiCd cells. Here's some TL;DR from NASA about a variant of NiCd they use(d), not sure if it applies here.
Fancy NiCd, Higher density and sealed. They rely on precise chemistry to be hermetically sealed units (lean on one element, for limiting and only O2 production).
High pressure at full charge (~60PSI at room temp), higher if things go south, Pressure drops with charge state.
Excess discharge causes hydrogen production.
So, tin can, pressure changing with charge cycles (metal fatigue over many cycles?), H2 production, O2 production... maybe there is some chance for catastrophic failure there.