15 volts may be OK for your particular vehicle but is considered too high but most experts.  It will lead to over production of battery gases and corrosion.  Your reading should be not more than 14.8 volts at idle rpm.

it's better to measure voltage to at least one decimal place. 15V could mean anything from 14.5 to 15.4 , and those fractions make a difference.

Also, you need to measure the voltage at multiple times. The battery can go through various charging stages, depending on the battery state of charge, temperature, load from running accessories, and engine speed. 15.0V (or more) is ok for short periods of time and is actually good for the battery from time to time. (desulphation and destratification)

If you go to Autozone they will test your charging system for free.