alternator

Has anyone ran a dual battery system, using the oem or upgraded alternator ? I know there are other options like self contained batteries etc but I'm very set on building this aux bat system with the/an alternator being the only charging source for it. Thanks in advance!

Buddy went off roading with me in his Badlands. Got himself into way too deep of a puddle.. turns out it was like a 30” deep mud hole. Got stuck, and I had to winch him out. he was in there for maybe 5 minutes. totally fried the alternator. Truck had to be towed to a dealer. I know, I know...

2.7 OBX has been showing battery voltage anywhere from 14.5 - 15.1V on my gauges the majority of the time. Seems excessive compared to normal ranges when searching for the answer (couldn’t find anything specific to the Bronco) Is this a typical range for this battery/alternator?