Because I sometimes go a while without a convenient charge, I like to buy the "extended" version of replacement batteries. Sometimes they are rated twice or more. One that comes to mind is 1300 mAH for the regular, 3500 for the extended similar to the above. I have seen where the battery cover is also changed.
ex. "Extended Battery w/Door for Motorola Droid A855 (3500 mAh) -"