NST Pulley Set Alternator Issue

[MGargano]

I picked up and installed a full set of NST pulleys a little over a year ago. To my knowledge, they were not underdrive or overdrive pulleys, just the lightened version, but apparently (thanks to CTScott's diagnosis) my alternator is under-charging the battery.

I'm not sure if this is a new issue or if it has been doing this for the past year- either way, I'm curious why/how the non-underdrive pulley set could be causing this.

When running, the ScanGauge puts my voltage at 14.1 maximum, without any kind of load (amplifiers, head unit, etc.). Even 14.1 is rare- it's usually around low- to mid-13's.

I did a search and saw some people having the same problem with the 10% underdrive pulley, but I'm curious if anyone has run into this with the standard lightened set and what a potential fix might be. Something as simple as tightening the belt? Or something more drastic?