The backup battery is rechargeable and only used while you change the main battery - and this is rarely necessarily if you charge your x3i regularly in the cradle.
It takes quite a while to recharge the bu-battery - so you may need some patience - maybe a week or more.
Go regularly to settings>system>power to keep an eye on the batteries.
Just want to put your attention to the welch of knowledge accumulated on this site by experienced members.
Just ask ...
For further information you may also click on this link to the many excellent
AximSite articles.
BTW:
Congratulations on your first post
Welcome to the AximSite - I'm sure you'll enjoy -
®