I've probably learned more about Powerwall from the comments in various articles than from the articles themselves. A lot is inferred from the specs, comments made by Musk and other company execs, the fact that they're sourcing them from Panasonic (and others) and will eventually build them in the same Gigafactory that will produce their auto cells.
The real trick with large lithium ion packs is battery management and thermal control. This extends cycle life and help avoid "flamethrower mode". Tesla has far more experience than anyone in this arena and they used this knowledge when designing Powerwall..
This article has some info: