OK I know this is years late but I just got my 89f and the battery will run down overnight with a cable in the 1/4" jack, but will keep its charge without the cable plugged in. I talked to Line 6 support and they said if the modeling knob is not turned on it doesn't use any battery, but what i think is the Piezo bridge is in the circuit with the cable plugged in. I don't have a schematic, but my guess is the electronics and the bridge are different circuits, the bridge connected directly to the cable switch and the modeling electronics connected through the modeling knob switch. Seems silly that we can't get a definitive answer after all this time. Love the guitar though.