Rashly assuming the voltmeter is accurate, the 1/2 volt difference from the battery to the alternator could just be the nornmal drop caused by a diode isolator. Which is 1950's technology and a waste of power at best today, better to get rid of it and use something like an Echocharger or West/Yandina Battery Combiner.
But the 15.7 on the alternator itself would indicate a defective internal regulator
. Anything over 14.4 is a defective regulator
, normally. And that excess voltage will damage your batteries, blow out your light bulbs, kill your onboard electronics
All very good reasons to pull the alternator without using it at all any more, and get it shoreside for bench testing and an overhaul
or replacement. And in the meantime, you can search on how and why to replace that battery isolator with some new 30-year technology instead.(G)