This is normal operation of the IPDM module. It allows the battery to fully charge then cuts back the voltage to prevent overcharging. I use a Scangauge II to monitor the system voltage and it shows the same behavior as you described. If you are running the AC or defrosters the IPDM will allow full alternator output voltage for a longer period of time. All this is done to prolong the life of your battery. I have read that some Infiniti
shows disabled this feature as some customers complained that their batteries weren't be charged (the affected customers made many short drives and the IPDM didn't allow their batteries to fully charge because the drives were too short).

 

interesting. i just wired in a new radar that shows voltage and was surprised that, while driving, it never really goes above 13.5v or so (2007 sedan). i thought maybe the radar's reading was off, so took a multimeter and sho nuff, it's showing 13.1xV while idling. i thought my alt took a dump, but no lights ever displayed on the dash. this is definitely reassuring to read. thank you.