Automotive electrical question

[bucolic_frolic]

For an occasional weekend amateur, with Youtube and Haynes, I do ok.

But this week ... I successfully repaired the main lights and circuits integrated card on a 90s Honda. Tinned up a few traces. Did an AMP draw test, lots of clicking there, and left it with open circuits overnight under the dash. Next morning ... battery voltage ... 7.59 volts. Lowest I've ever seen. That's not a slow leak. I've had slow leaks ... takes 10 days to draw down to 11.2 volts with a dome light on.

Charging battery while disconnected, prepping up fuse, amp draw, tests. But what could cause such a massive drawdown overnight? I'm thinking alternator/diode?

Thanks for a quick take.