From WSJ
Dreamliner Batteries Needed Replacement Before Incident - WSJ.com
In the U.S., investigators have collected world-wide data for the 787 fleet indicating that as many as 160 batteries may have been replaced, including before and after delivery, according to people familiar with the details. The primary reason 787 batteries have been swapped out, these people said, are that they were improperly disconnected. They were left to discharge without any other power sources on the aircraft, causing a deep discharge; or they expired, people familiar with the matter said.
Sounds like a fair amount of mishandling rather than unexpected battery failures.