hi everyone, i'm trying to find a decent soul in this world to help me analyze what the problem is with my alternator overcharging.

a while ago i had both batteries low and i couldn't start the boat, took them both to test to an autoparts store and both were shot. so i bought a new starting battery and a new deep cycle, both flooded lead acid. i fire up the engine and it started first try, awesome! i go for a ride and when i come back i put my voltmeter to the terminals while engine was still running just to check the battery situation and i realized my valeo alternator wasn't charging.

took the original valeo alternator off to take it to a test at an autoparts store and they said it's not charging at all, 0 volts! gave it to my good friend who works at a german alternator factory so he could test if just regulator went or if diodes were shot. diodes were dead, and he told me he can build me a brand new alternator with same specs, so he built me a 135amp one, tested it in factory on cool and hot for 20minutes and was charging 14.2V.

i mounted the brand new alternator, connected all wires same as they were on original valeo alternator, started the engine, tested the volts at the battery and it read 14.2v so i thought the problem was fixed! went for a ride w boat 2 separate days, each time it showed 14.2 at mooring so each time i left dock with my peace of mind. the third time i went to do some work to the boat and might as well tested the voltage again, fired it up and voltage was slowly rising withing a 10 minute period of the engine running and in the end it read 15.62v and i gave up.

my buddy told me there's no way the regulator would be faulty cos it was working in the factory. so out of my frustration he built me another brand new alternator, this time a 70amp, he tested it cool and hot on the bench at his factory lab and even showed me pics of the monitor, it read 14.15v and 70a.

went back to my boat few days later, tested the faulty alternator again while it was still connected and this time it read 14.5v and after 10 minutes is maxed to 14.6v but not 15.6 like it did last time i checked. these tests were done under 2500 up to 3500rpms!
here i was already confused why it read 15.6v and then few days later 14.6v, however anything over 14.2v is too high and don't wanna damage my new starting battery.

i went to install the new new 70amp alternator, fired it up and it read 15.2v right away and i'm just dumbfound now, don't know what to do and where the culprit lays.
today i went down to the boat again to check all wiring and details and all seems fine, the ground to the engine block shows 13.8v, the + from the starter solenoid shows 13.8, the alternator terminals b+ and b- show 13.8v, the b+ and the alt housing shows 13.8v, all is correct.

what's weird is the battery reads 13.8v and i haven't been at the boat for a week and haven't been charging it with the ac nor i have solar panels. i thought the battery should read 12.8v when fully charged?
i fired it up again and it still read 15.2v

my 3 dashboard lights light up when i put key to position I, my glow plug light come on when i move key to position II and all 3 check lights are still on. when i start the engine, all lights shut off as normal.

i took pics and did some videos to show the dashboard lights situation and also the voltmeter various tests, but don't know how to upload a video on here.

does anyone know what could be the cause or has anyone had the same problem before and figured it out? please someone have a soul and help me, i spent so many hours testing and so many hours wasted to my friend to build 2 alternators, i don't even know what else to check.