I've had my Mini since 2005, with 2 keyless entry keys, I have never had a problem with them, and then a few days ago, both keys just stopped working. My question is do I have to go to the dealer to replace them? Is it just a matter of putting in a new battery? Any help would be most appreciative, thank you!

 

I can see one dying but both at once I doubt both would die at the same time.
If ones been sitting unused for a long time it might have died then the daily use one died.
You can replace the battery and see if that cures it. if that doesn;t do it you might need the dealer to check the reciever