DHCP allows you to 'reserve' an IP address which basically gives the device a permanent lease. This has the advantage of allowing you to configure the device as if it had a static IP address. I'm not familiar with your particular DHCP server but you should be able to create a reservation entry to a specific address and give it the MAC address of the device you wish to reserve the address for.

Also keep in mind that even though the lease time is only 15 days, the device will try to renew that address as the lease nears expiration. So as long as you don't leave the device off for more than a week or so it will effectively have a static IP address.

Good luck.