Installed a new alternator and now get a message that says "battery not charging"

The first part of the test involves measuring battery voltage with the engine running, which you already observed is falling. The third cause I eluded to is a high-resistance break in the wire going from the generator's output terminal back to the battery's positive post. One test is to measure the voltage at that output terminal when the engine is off. If must have full battery voltage all the time, but that can be misleading. If you find 0 volts, there's a burned-open fuse link wire or a blown very large fuse that is typically bolted into the fuse box.

https://www.2carpros.com/articles/how-to-check-a-car-alternator

It's when you DO find voltage that you have to look further to identify if there's a high-resistance break. For example, when fuse link wires are used and it burns open, it leaves a carbon track behind. That carbon will conduct enough current for a voltmeter to pick up battery voltage, but the generator's output current will never get back through there to charge the battery. The clue to that is with the engine running, you will find considerably higher voltage on the generator's output terminal than across the battery terminals.

Caradiodoc, thank you! Just for shucks I replace the new alternator with another new one (old new one turned out to be shot under bench test). The voltage came back to 14 volts on both the battery side as well as the alternator side of the fusible link. Battery is 28 months old. Now if I hold the accelerator at 2000 rpms everything seems to be half way decent. When I let go, car barely idles and even sometimes dies. Everything was fine prior to the first alternator going out. Now when I read the computer, Code P0102 comes up (Mass Air Flow (MAF) Circuit Low Input). I've researched what it means, symptoms, causes and solutions but just find it weird that it is now a problem. When the first alternator went out the serpentine belt was shredded. Could that have hit the MAF wiring or sensor? To tell you the truth, I don't even know where that is. Any thoughts? I don't blame you for not answering. One person can't monopolize all your time. So thanks for the first response back :-) Dennis