Alternator Issues

[Retro-Cow]

Hey all, while driving home during a crazy rain storm I hit a huge puddle of water. The car sat for 2 days after and when I went to start her up I couldn't.
I ended up replacing my battery for a Yellow Top Optima first.
Then I tested the alternator by disconnecting the negative terminal with the car running and it stalled so I ended up replacing the alternator with a used OEM one.
After installing it I did the same test and the car was able to stay on. However, after I did that I decided to tackle a long overdue turbo replacement as well.
I got the engine back in and started her up to make sure everything was still good but unfortunately forgot to unplug the battery afterwards.
The car sat for about a week like this resulting in a dead battery.

After getting the rest of the car put back together and a jump start I went for a drive and she died in the middle of the road.
Then I got the battery charged (it tested fine) and did the whole "disconnect a terminal" test only to have the car stall.
I tested the voltage with the car running at the battery terminals and it's reading about 12.6 volts.
As I understand, it should be reading a couple volts higher with the car running.
I also tested the battery connection at the alternator and it's still giving the same ~12.6v.

So this leads me to believe the replacement alternator is now shot.
I should mention that while I was doing the turbo replacement I also replaced the power steering line which had been leaky for some time, which may have attributed to the first alternator going out.
But this doesn't explain why the new one unexpectedly cut out on me.
I do have a single subwoofer and HIDs but I've driven the car for so long on the first alternator that I don't think I would need a high-output alt.

This leads me to 2 questions.
1. Do I need to get a high output alternator?
2. Would leaving the battery plugged in for about a week with the passenger door open (dome light off) cause the battery to die? Or do I have something that is not turning off when it should?
The first time the battery died after the rain storm I don't recall having left the lights on or anything...

Am I missing anything that I should be considering?

Thanks and sorry for the novel.

[DCaff300ZX]

Hmmmm, I intensely dislike battery/electrical issues because of the nature of electrical stuff, always fuzzy!
I myself went through a similar thing when I bought a brand new battery for the NA and then the engine spun a bearing, and I let it sit for a month+ before realizing I had left the batt connected and went to disconnect it and found it dead. It would take a small charge but die quickly, the problem was that the plates inside will apparently go bad and create enough sediment in the bottom of the battery case to short out the plates, so the battery was shot after only a couple weeks use and the month+ left dead. The alternator charge readings were similarly low, as my brother explained without the battery plates in working order it is basically shorted out and therefore takes a smaller charge to whatever of the plates are functional, and won't hold that charge long if at all.
So my advise would be to try a new/good battery and see if your charge numbers jump into the 14.5-14.9 range, meaning the alt is good AND battery will take and hold a correct charge. If the charge numbers don't get better, I'd then assume a bad alternator which is not abnormal these days unfortunately with parts-house rebuilt stuff.
Hope this helps!