Since you specifically asked about OEM batteries, the
UTStarcom BTE6800 is the largest at 2100ma. This is the battery sold on the Sprint web site for $49.99. I bought one last week. Although in this link they say the back is black, I received silver. From the first complete charge at 6:30pm Friday, the battery didn't reach 10% until 8:23pm on Sunday. I charged it overnight Sunday and topped it off at work at about 3pm on Monday and as of 6am Wednesday as I post this, it's still at 70%. I don't believe it either!