Should the usb/power cable be left in all the time, even after the battery is full, or does it not matter?
Complicated. Never fully charge or discharge, though full discharge is the worst
Apparently the deluge batteries are easily replaced by users and not really expensive so i wouldn't worry too much about fully charging. Full discharging is something i always try to prevent.
And if you can keep it connected to power the battery will have less discharge cycles overall so it should last longer.
Korg M1 for deluge 2.1 Preview
Hi @minigoat! I'm just jumping in to confirm that you can't overcharge the Deluge. You should be able to keep it plugged in for as long as you need