An e53 X5 doesn't require battery registration. Your X5 falls on the "old" side of things...prior to the newer LIN & BSD communication bus systems used on BMWs since the new generations started (e90/e65/e60/e70/etc).
Your e53 X5 DOES NOT have an IBS (intelligent battery sensor) nor the LIN or BSD bus where the battery, alternator, and DME are in constant communication of each other.
The IBS started with the e60 5 series and every model that debuted after. The early e90 & e65 models that debuted before the e60...still need battery registration due to communication over the LIN and/or BSD bus...but again, these 2 models are new generation BMWs.
And don't let folks at the local BMW dealership rip you off either...I've read enough threads where owners of e38s/e39s/e46s/e53s have been told & charged for battery registration on these older models...and it is NOT needed. Just swap in your new battery and go.
EDIT: here's info about the IBS & models that need battery registration