John, I believe that "Battery Day" is tied in closely with Musk's future announcements about:  "Roadrunner Project".  Rumor?  Fact or fiction?  Guess we will find out in September.. maybe.  Battery Day was originally supposed to come off in April and then it was moved to May and now it has been moved to 15 September.  It has been the trail of the movable Goal Post.  Ok, lots of rumor, but some of it seems to be well founded.  We won't know for sure until "Battery Day" actually becomes a reality and that is only "IF" Battery Day includes a reveal about the Roadrunner Project... but why wouldn't it?  Because with the Roadrunner Project what is the sense of even having a "Battery Day".

Another quote from one of the links I posted ( from Electrek ) about the Roadrunner Project:

"Now sources have told Electrek that the secret project is called “Roadrunner,” and it will aim to achieve mass production of a new battery cell that will be more energy-dense and cheaper.

The cells are using technologies developed by Tesla’s internal teams, including work from its research lab in Canada led by Jeff Dahn, and new technologies recently acquired through the acquisition of Maxwell.

Maxwell’s dry electrode technology would enable Tesla to produce cheaper cells with higher-energy density that would enable them to use less batteries per vehicle and still achieve longer range."

So, the whole thing is on the back burner until the elusive "Battery Day" becomes reality.  Anticipation is a game well played by Elon Musk over the years.  He is a master at it.