Some batteries are better than others. Some due to a manufacturing defect may lose a cell or two earlier than expected...I generally get about three years on my daily driver battery versus five to six on the light duty vehicles.

If your alternator is doing its job, you should see between 13.5-14.4 V at the battery terminals with the engine running (and accessories off). 12V nominal with the engine off. Anything beyond those parameters indicates a problem.

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car