I use my 510 PCC for charging regular batts only, not storage of PV materials. TW is going to sell, or has now, PCCs for the 510 mega batts. Not sure of the additional storage space in them.
If I remember the TW video correctly, they have added a hole that will hold another battery (not charging)... but have eliminated one of the cartridge holes. At least that's the way I remember it.