I'm still not clear what the critical safety issue is that caused the grounding of this fleet.
The FAA/NTSB have not released any technical details of their specific concerns which led to the 787 fleet being grounded. It seems very unlikely that they would ground these aircraft due to the non-availability of electrical power from a battery. My guess it is fire damage (either actual or potential) to critical electrical systems within the EE bay that is the real worry, and the reason for the grounding.
The fire on the 787 in Boston was tackled by airport fire services very quickly and the fire on the ANA aircraft happened shortly after it departed Haneda airport. In both cases, it was possible to extinguish the fires reasonably quickly after their detection. However, the FAA will need to consider what fire damage might result to critical systems from a similar battery fire in mid-Atlantic, at 40,000'.