Something funky going on.
Voltage at battery before start 12.7
Voltage at battery while idling 17.7
Voltage at alternator while idling 7.8-8.2
Voltage at voltage regulator idling 6.8-7.5
Seems bad ground when checking voltages at alternator and regulator?? Makes me think clean your grounds and any contact points between alt and vr. Also loosen and tighten alternator bracket and VR to fire wall to ensure a good ground. ohm the wires between alt and vr check resistance from alt to AMP guage to battery clean bulk head connector. MY THOUGHTS ARE voltage at alt and vr should be the same as or close to battery. If not why? faulty meter? Just sounds like ground problem to me.
Im off to fallon for tires for daughter cant stay and help more.