run an 8 ga jumper line from batt positive terminal to the alt "batt" threaded stud & see what you get at the battery at a fast idle. EDIT (much) easier yet, hook your large "regular" jumper cables from the batt pos/neg posts over to the alt "batt" stud/housing case (have a helper hold the ends at the alt) & fire it up & see what you get at the batt & the alt. 15+ is way too high for a charged battery. You might even get a handfull of HF mini jumpers with alligator clips & feed the reg/alt field circuits including ground paths & see what develops. this takes everything out of the picture exc the hard components: alt/reg/batt.