Alternator not charging after engine rebuild

[doowatchyalike]

I just had my engine rebuilt and the car starts and runs just fine. The issue is the car isn't recieving a charge from the alternator. Initially I thought maybe the coding for the battery was lost because the battery was unplugged for so long, so I replaced the battery, coded it with BimmerCode, and registered it with BimmerLink. The problem is still there. I can drive the car fine. Using a monitor, I can see the voltage slowly go down as I run the car. Searching I couldn't find any information on this. Any guidance is appreciated!

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[Vertex]

I'd make sure all hardware and connectors are secured for the alternator. That would be my first go to or grab a multimeter and see if the alternator is doing anything with the car running for whatever amount of time you can