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Questions -
How many miles on the vehicle?
What is the voltage across the battery, engine off?
What is the voltage across the battery, engine on? At warm idle speed? At 2000 RPM?

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Mileage: ~109,000 miles
Voltage across battery
Engine off: 12.08V
Engine on: 14.27V
Warm idle: 14.22V
2000RPM: 12.5V

I also went in search of some parasitic draws. Removed every fuse and relay that I could find. There seemed to be a decrease in amps drawn when removing Relay 1, Fuse 103, and Fuse 114.

Relay 1: Switched system power relay #4
Fuse 103: SSP4 Relay, SSP4 Relay (Coil)
Fuse 114: SSP3 Relay, SSP3 relay coil

After doing all of this, we noticed something awry related to the hood. The little button that is supposed to detect when the hood is closed, had been rotated out of the way. It was no longer making contact when the hood was closed. It appears that the hood light had also burned out, so if it was drawing power, I would not have been aware of it.

I'm not sure yet what to fully make all of this. I won't really know if the hood issue was the culprit in the parasitic drain until a couple of more days have passed and I can try to fire up the battery.

 

The voltage at 2000 RPM is a problem. It should be in the 14 range.
At 12 1/2 the system is no longer charging the battery unless it is a smart charging system.
In my opinion you have found a problem

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Thanks! I will look into that. I might have to re-measure the voltage at 2000RPM just to be sure.

Any idea what might cause this? Bad alternator? I replaced both the alternator and the battery within the past 6 months. Is it possible I got a bunk alternator? 😓

 

The alternator is working, at least at idle speed.
Issue could be alternator, regulator or connections.
And having the system tested for free would be next. If you are in the US a parts store like AutoZone or OReilly's

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One possibility is if the replacement alternator was not Motorcraft and even if it was a Motorcraft, there is the possibility of having the wrong voltage regulator being installed. (It does have the Smart-Charge system)

Once you find and repair your problem, IF there is a residual draw problem, there are things to consider and things that you can do to avoid killing your battery prematurely.

The Windstar, due to the amount of interior lights, charge rates and placement of the alternator (going through a water puddle on the passenger side can shut charging down for a few minutes while the wipers and exterior lights continue to draw on the battery), passive anti-theft, power sliding doors and a few other things, is hard on a group 65 battery, no matter what brand the battery is. Add-in additional drains if you tow trailers, small campers, run inverters and other things that have electrical requirements (kids wanting to sit and listen to the radio in a campground, parking lot or whatever) and going through periods of opening the doors multiple times without the engine running and you can find yourself hoping that you can find someone with a set of jumper cables in a hurry.

Battery-wise, I discovered in 2009 on a camping trip, where we were in just such a predicament, that a Group 29 Everstart Maxx (from our pop-up camper) will just fit into the battery compartment. Not only will it fit, it handles the electrical loads much better than a group 65 battery.

Installing a group 29 Marine Starting battery is my first recommendation and the Everstart Maxx (Wally-World but made by Johnson Controls) in particular. No, I don't work for Walmart. But the three year replacement and 5 year warranty WILL pay to have. It's a good battery for under $100. That is the only battery I use in the Windstar now.

Second, replace any interior bulb that you can with an LED equivalent. I don't recommend this for tail lights due to the LEDs not being bright enough during the day. (My preference is Warm White light. Yours may differ.)

Next, you can totally turn off the interior lights with the dash dimmer control and leave them off during the day and generally when they aren't needed at night. You can also leave it in the position where only the floor lights come on. This will considerably lower battery demand.

If most of your trips are local, I recommend getting a good battery charger and use it AT LEAST once a month to fully charge the battery. We generally do ours every other week on all of our vehicles.

If you run an inverter, make sure that the supply cable is at least 4 guage wire, 0 guage if you plan on using a coffee pot or hair dryer, with a 10 amp breaker in line. (10 guage wire will melt and . . . well, ask me sometime how I know).

Even if you don't get the group 29 battery, the other tips will help keep your battery from being run too low too fast. IF you get the battery, then you don't need to be quite as strictly observant of the other tips.

The Windstar is a great all around vehicle but it does use a lot of 12 Volt power.
(We have a 2001 with 168K. I took it over when my wife got her Navigator )

I hope you find and fix the problem. Keep us posted.