Just saw you wrote powerwall, my bad... no clue for the powerwall, im amazed there isnt a teardown of one of those yet...
Completely custom development.
Every battery module has its own 6s BMS for balancing and monitoring. This is so every module can be monitored even in case of catastrophic damage where communication cables could be damaged.
Every one of those BMS boards act as a slave to the master board, this one in return controls it all and communicates the the main car computer.
With the Model 3 the packs have been changed to 4 modules, 2x 24s and 2 x 28s (i think, from what i can remember) and i again presume each module has their own slave board.