If the battery of your iBeacon is empty you don't have to purchase a new one. Just replace the battery as follows:
1) Open the iBeacon case by removing the back:
It's best to use a pointed object, a letter opener, for example. The back should come off easily.
2) Replace the battery. Start by removing the double round cell battery from the case.
NOTE: You can't by the double round cell battery used by the producer of the iBeacon in the store. However, it's possible to use a standard single round cell battery (CR2477) which should be available in an electrical supplies store.
Insert the new battery into the case.
3) Now close your beacon case again. Take care to press the back into the small slots on the rim of the beacon case.
That's it. Now you are good to go for approximately another 2 years. Have a good trip!