Just a note, per the Motorola website, the BP7X is the extended battery for the Droid Pro, so if you use the code listed above, you can purchase the extended battery $39.99 and Extended battery door $9.99 for $20 before shipping and tax.
Most of the portable batteries are simply rechargeable batteries that can be used to recharge the battery in your device. You charge the portable battery, then connect it to your device and it charges your battery, or you can use it so the device will drain the portable battery and save your device battery. I use it when I know I will not be able to plug my device in to charge it, and also when I am using my phone on a plane and want to watch a movie or play a game.
Most of the portable batteries charge via the USB port on your phone, so as long as it has a micro USB adapter you should be ok. The Motorola P790 that I use has a Mini USB adapter, so I had to buy a Mini to Micro USB adapter to make it work on my Droid X.