voltage question

ive notice that when i drive my car the voltage meter on the dash reads all the way to the right almost to 18 volts but when i come to a stop it drops down to the half way mark at 13 volts and the lights sorta dim down. How much voltage am i suppose to be reading and should it be fluctuating like that?

By the way the battery is new and the alternator is fairly new and in excellent working condition according to the alternator test that was done at the autoparts store

It should never exceed 15 volts under normal conditions. It sounds to me like the voltage regulator went bye-bye. This is internal in the alternator, have it checked again.

heres couple of pix of what the gauge reads. The first one is what it reads when im driving and the second one is what it reads when i come to a stop and the lights dim down a bit. Is it suppose to go down that low and fluctuate like that?
http://i75.photobucket.com/albums/i291/ls954/whip/DSC00174.jpg
http://i75.photobucket.com/albums/i291/ls954/whip/DSC00175.jpg

Well, that isn't almost to 18 volts. Yes, that's normal behavior if you have your lights on etc.

your head lights should not dim, usually they will dim if the idle is low, bad ground or something. Its probaly not a bid deal but it could be a worning sign.