It turns out my favourite head torch has a bit of a design issue that relies on anodisation and solder resist as insulators. The PCB is literally rammed up hard against the metal. Initially I thought this was what had shorted out the lithium cell, but after the video I went further and used a microscope to try and work out where the short circuit had occurred.
The culprit is actually most likely the connection point between the two PCBs. There are a few connections and it looks like a solder ball may have been almost but not quite bridging the battery positive and negative connections. It was black and very slightly charred between the two pads. I think the heat may have melted the solder away and destroyed part of the evidence.
I still don't feel comfortable about the close contact between the PCB and -ve case though.
I peeled more from the end of the 18650 cell and it doesn't look like the end pip has been vented through. It looks more like the heat from the burning springs has burned the plastic and caused a bit of corrosion where that happened. I'm cautious about using that cell now as a high temperature incident could potentially have caused the internal separator films to pull back from the edges of the foils they insulate between.
Shame. I really liked that light. And now I'm wary about using its twin. The PCB against the aluminium is too shady. A shim of plastic could have helped greatly.
I think the cases end cap was applied with a press. There was no really obvious glue, so it may have been a friction fit and the copper clip may have been to mate the PCB to the case for the battery negative connection.
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