Motorcycle Repair: voltage guage, volt ohm meter, electrical system

Question
While riding my 2004 Electra Glide Ultra Classic I observed the voltage gage jumping up and down between 12 and 15 volts.The gage typically stays at about 15.5 volts.I am not much of a mechanic,especially when it comes to the electrical system.Is this a problem I should take directly to a repair shop or can you suggest some trouble shooting tips and possible fixes? Thank you in advance for your help.

Answer
15.5 volts is a little high.  It could boil the electrolyte out of the battery shortening the battery life.  I'd check the voltage at the battery with a volt ohm meter to verify the gage is reading accurately.  It could be just gauge error.

Regards
Rich