CMOS Battery or similar for iPhone Location
All power on the iPhone comes from the main battery which is a big lithium ion battery taking up most of the case.
Settings etc are stored in internal nand flash normally and not the CMOS chips that PCs used to use with the real time clock being updated via internet connection (Apples own time servers) or via the mobile phone network.
A full breakdown can be found on the ifixit site here and step six shows you the battery.