Car Electrical Issue 96 Ford Mustang?

As I was driving home on the highway Sunday my electrical systems slowly died. The gauge indicated a very low battery charge. Once I eventually lost everything I pulled to the side, yanked my battery to charge it at Autozone and put it back in to get the car home before I put a new alternator in. With my now half recharged battery I made it back home. Tuesday I install a new alternator and the car drives around just fine and with a healthy reading on voltage. Unfortunately, this morning, as I turned my ignition the car made little to no effort to turn.

The battery is fairly new (about a year old), with no indication of bad terminals and Autozone acknowledged there were no issues with the battery when charging it. I'm clueless as to what it is at this point. Any ideas?

Have your voltage regulator checked.

A healthy reading on voltage, what is your opinion of healthy?

Have whole charging system electrics checked with a Volt- Ohm multimeter. Check continuity/voltage drop operation of battery/alternator cables first with battery disconnected on charge out of car. Any engine management system will revert to default setting with the warning light on for a period unless settings could be off loaded to a temporey storage PC then reloaded.

What is your definition of healthy voltage? To be sure measure the voltage with a meter at the battery with it running you should see at least 13.7 volts to mid 14's. If it is hanging out near 12 then you charging system is NOT working. If this is true EVERY car has a fuse in the charging circuit yours might be blown.
If you have the correct voltage you have a bad battery or something is draining the battery.