MAXX battery in Ultra?

[ckkbleev]

I bought my sister a used Ultra a few months ago and it has been great. The only thing that can be improved is of course, the battery life. I was wondering if it would be possible to fit a 3500mAh MAXX battery into her Ultra with the back cover from a MAXX? Are our batteries the same size/interchangeable? I've been eyeing this for a little while: Replacement Part for Motorola Droid Ultra XT1080 Battery

Also, what's the difference between these two batteries?

http://www.etradesupply.com/motorola-droid-ultra-xt1080-battery-eu40.html
http://www.etradesupply.com/replacement-part-for-motorola-droid-ultra-battery.html

Thanks for your help!