The charging voltage will vary depending on the state of charge of the battery. If the battery is low, the charging voltage will be low. If the battery is well charged, the voltage will be high. This is part of OHMS law. When the amps are high (charging) the volts will be low, and when the amps are low (not charging much) voltage will be high.
The rule of thumb for the increase in voltage over the base battery reading (after the surface charge has been removed by turning the lights on for about 30 seconds) is between 1/2 volt and 2 volts.
12.5 volts (your base battery voltage reading) plus 2 volts equals 14.5 volts.
My experience has been that if you are much above 14.6 or 14.7, there may be something wrong. 15 volts is suspecious.
I just checked the shop manual. It says usually charging voltage should be between 14.1 and 14.7. It also says if charging voltage is more than 2.5 above battery base voltage you may have a voltage regulator problem. It sort of leaves a grey area there between 14.7 and 15.2.
I think the style of battery may make a difference. Are you running an Optima?
