I'm guessing you mean 13 to 15 volts, so that would be in the normal range but 15 is little high. Most likely your alternator is good. I'm assuming you are asking because you have a cranking problem. If so, I would suspect if it is not holding a charge, the battery is bad. Take it to a parts store and have it "load tested" to be sure. Bad connections on either of the battery terminals, starter or elsewhere could give similar symptoms.
Yes doc after running the car for a few hours leave it alone for a day and go out to start it it cranks real slow then picks up and starts.If i use acessories like the power windows and heater or radio it seems like its too much of a load and its wants to stall.