I'm seriously considering ordering
this 1500 mAh battery. It's kind of expensive but the XV6900 only has a 1100 mAh battery so it's bound to last longer. And it's the same size so you don't need a new back for the phone.
I always make it through a day but sometimes just barely. and I'm always plugging into a USB charger in my car or office otherwise I would never make it through a day. This (and the original battery as a spare) just might do the job.