I've seen several comments on not knowing the root cause of the battery failure, yet about ten days ago All Nippon Airways made this statement:
Japan probe finds miswiring of Boeing 787 battery on ANA flight that made emergency landing | StarTribune.com
"TOKYO - A probe into the overheating of a lithium ion battery in an All Nippon Airways Boeing 787 found it was improperly wired, Japan's Transport Ministry said Wednesday.
The Transport Safety Board said in a report that the battery of the aircraft's auxiliary power unit was incorrectly connected to the main battery that overheated, although a protective valve would have prevented power from the APU from doing damage."