Safety Batteries Aboard
the 787

All necessary electric power to operate the 787 in flight is supplied from generators installed on the engines. Power from the batteries is not used during flight. Therefore, in the event of a battery failure in flight, it will not immediately affect flight safety.
However, a battery failure that could send smoke into the cabin, such as the case in January 2013, must be absolutely prevented, as the heat could damage the aircraft or pose a risk of a possible fire.

Correlation between the Generators and Batteries