I am OK with the battery fix so long as they test if for worst case scenario
I really don't care about flight tests, what I care about is worst case period testing. I want all safeties disabled, the battery charged at the highest voltage the plane can deliver, until it totally fails. Then without any intervention, I want to see it run it course until it goes out and cools off by its self with the following conditions met. No damage to surrounding equipment, and all smoke and nasties vented outside of the plane. This is how I want them to prove it is safe for flight.
The major fix to be tested is the modification to the battery to prevent thermal runaway betwwen cells. How they will do this is anyones guess at this point. But it will be more than the original "drive a nail into the cells to cause a short". Once that is done, then they have to test the changes to the cell monitoring system work. Then they need to test the battery enclosure can withstand a battery meltdown and the vapor venting system.
Some of this can be done in a simulated environment prior to testng on an actual plane. Since they do not know what caused the short in the original battery that caused the grounding of the fleet, they will probably have to figure out several scenarios to short the battery and test for thermal runaway (if they have not already figured that out.) If the battrery cell monitoring system upgrade can detect a cell problem, then they will probably have a method to isolate that battery from the planes electrical system. They will have to have all these features on both the forward and rear batteries. Not sure how they will vent the rear battery fumes.
This should all become a lot clearer once they have the tests run and the FAA makes thier final report prior to allowing the planes back in the air.
The testing should also require the plane to fly safely for at least 3 hours after the battery has failed before ETOP's certification is granted,( along with a potential failure of the backup system depending how robust that system is). 5+ hours for the ETOP's certification that the plane was designed for.
I think it should be able to fly as long as it has fuel without the battery working. I think the backup is in the way of both engines, and last resort backup the RAT which can provide power so long as the plane is moving in the air. So I guess they could kill the generator on one of the engines, at that point with battery failed and only one generator working, I suspect they would deploy the RAT in case the other engines generator failed. I highly expect such testing was done long before the battery became an issue, and if not, this not only needs to be done, it should be invetigated as to why it wasn't.