15 volts is on the ragged edge of too much voltage. With a fully-charged battery the max voltage should be around 14.7. After cranking, and especially if the battery has been loaded heavily, charging voltage at the battery posts can be anywhere from about 13V to 14.5 or slightly more. The state of charge of the battery, NOT THE VOLTAGE REGULATOR determines what voltage the charging system develops. The regulator ONLY functions to set the maximum charging voltage against a fully charged battery.

If your voltage is going to 15V immediately after startup, you probably have a shorted regulator and it's capable of boiling the battery dry.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!