I'm not sure I follow . . . ?

I thought the alternator always provided the extra voltage when the car is running.

It increases the voltage of the 12V system such that if a load or a battery is at a lower voltage, electrons will flow towards that load or battery.

Ah, I see what you were saying before: Yes that'* a problem that your voltage went down to 12.9V after the click. That tells me that your alternator was not charging the 12V system that is being measured by that volt meter.

Yeah that is what I was trying to say, I may have had a good bit of medical marijuana before explaining, it eases my totally obliterated subarachnoid L5/S1, my bad as back.
I am thinking since I never seen a light saying the alternator was bad, maybe there is just some issue with the battery or alternator, my money is the batt as it is a bit old and came from amazon...
But, I am not ruling out the alternator, it is so pretty though, lol

And it eases the mind. Relieves stress and depression. So long as we don't overdue it although at times it seems a necessity.

Yeah it helps my anxiety and stress from work no doubt, if not for it a 60hr week at my plant would kill me, lol
Making transmission reaction discs, friction plates, and torq convertor discs sucks!

I took the isolator out, old batt said Jan 20, so I took it out and put my newer AGM in front, I'll test the other batt later, I really did not need both, or the isolator.
I also cleaned all my cables, and switched to a new nice terminal for my positive, all while getting soaked, damn t-storms, lol
So far it starts up so much better sounding, if that makes sense, maybe I am just hopeful, lol
IF I ever put another batt I will use no isolator, and I will run them parallel of course.

Now I am stumped for sure.
I removed all of that, connect my amps power back up, stars fine, I drive to the store, as I turn the bass up loud the charging system failure shows on the DIC, but the showed volts stay at 14.9 always.. The bass cuts out, so thaty amp alone cuts out. Not sure why yet, it has plenty of power, and never cut out before, but I feel like I am on the tail of a gremlin, lol

I'm starting to think that a wiring diagram is in order.

What I don't understand is how before the amp never ever cut out, so I may need to double check my grounds.
If that does not get me anywhere then I'll look for my book 2 of my FSM. :o

That Charging System Failure on the DIC disappears when I turn the bass down, I can turn the 6x9'* up loud with no issues, they are on a different amp than the sub.
So I'll inspect the ground for that AMP..
The rear batt showed 100% full as I checked it, and the one I took out of the front was at 97%

I do have an extra ground in the trunk, the second battery tied them all together, I think I'll get small ground block and tie all the grounds back together, deff could not hurt.
This all depends on how many hours they try to get out of me this week!