RT4 Nav question

hi I have an rt4 nav system I have just fitted in my c4 coupe 2008 and I seem to have come across an issue where the clock isn't keeping time.
do any of you techno wizards have any ideas. everything else seems to be functioning holding last station on radio. I notice they have a battery but not fully sure what that's for , im presuming mainly for the built in telephone system

any ideas welcomed

cheers
John