Your battery is fine between 13 and 15 , thats average charging rate.Once it gets higher than 15 , then something is not regulating it properly.Either the alt is going or the voltage regulator.Also , if the alt says its between 14 and 15 and its frying the battery , then your guage isn't working , or something else is contributing to the distruction.Possibly a short somewhere in the charging system?

1996 Jeep Grand Cherokee 5.2 with mods.Hughes Air gap intake , 52MM TB , ford injectors, 2 3/4" lift, Doug Thorley headers , HID headlights with LED , HID fog lights , glow gauges , a few others