Don't panic sports fans, after reminding myself that fools rush in
I took a step back from stripping out the loom and started back at the Battery.
A reminder that just because a Battery shows an apparent healthy voltage doesn't mean it will do its job under any load. (it also calls into question the value of leaving a Battery connected to an Optimate longterm).
My scientific demonstration of what happens to a Battery which was showing 12.75 Volts after connecting a low wattage lamp to it.