battery light: anything more I can look into?

[wwjd]

Truth told: I'm NO mechanic at all. I have a couple tools, and zero experience.

That said... my BATTERY light came on, on dash. My battery was 4 years and 1 month old, so I figure it was down. I did NOT have it on any Battery Minder yet*. I replaced the battery myself HOPING that would be all it was. Test drive, and about 5-8 minutes in, battery light came on again. It FELT like it was powering down just a little then - OR it was my paranoid imagination.

I stuck a meter across the new battery while off: around 12.8v while running: about 13.8v

I have a Durametric (that I have no clue how to use) and it BRICKED after I updated the software and required a FIRMWARE update. This fail was unrelated and not attached to car.

Also have that ODB2 thing and TORQUE on phone which tells me nothing useful that I could find.

I've read the alternator could need replaced (I can not do that), and/or some other part, maybe called "regulator"? I forget.

Is there anything else I might be able to look into for this? I'm fine taking it in and getting it fixed, just wondered if I have done all I could.
Thanks!

*I used this work to install the Battery Minder and the new battery is plugged in