When I lived in Tampa, 2 years was all I could get out of a battery.
Generally speaking, those numbers look good.
If the belt was slipping, you would be underdriving the generator/alternator and the output on the voltage regulator would increase to keep the battery charged. If you see a spike right after starting the motor, it may be the regulator making up for the loose belt, but that's speculation.
To rule out the belt, check the deflection on the belt to be sure it's in spec and look for cracks on the ribs.
If everything looks good, the alternator will need to be removed and tested.
There's a small chance it could be a slowly failing relay, fuse, or starter.
2017 Civic Type R
2012 MDX Tech
2004 TSX Base
1997 240SX SE