NTSB and Boeing have fit a 787 for flight test and peritioned the FAA for waiver to fly.....
The claim is: "We have identified 'Thermal Runaway' and wish to test the system, (BATTERY)"
I'd like to see that flight test plan. IMO, a flight test isn't the right way to verify proper operation of the charger/battery/load system. One flight, or a couple of flights won't prove anything. The 787 flew how many hundreds (thousands?) of cycles without a failure. The same will happen here.
True tests, to the 'coffin corners' of the performance and environment envelopes aren't likely to be run on flight tests. There are too many test cases, so the economics doesn't make sense. At best, Boeing might hope to spot a common load or charging anomaly using actual aircraft loads and systems that prior cert. tests overlooked. If so, an actual re-certification will still be a ways off.
On the other hand, Boeing does have some planes to be delivered in need of checkout flights. And the flight line in Everett is getting crowded. So perhaps they just want to move the inventory around.