For those interested: I'm going to add a second battery bank to my parents' installation.
This is the existing schematic:
I would modify the system using:
Basically I would connect the now 15s (not 16s as mentioned initially) battery in parallel with the other 15s6p pylontech batteries.
Given that each battery in the future system will be connected in parallel, and each with it's own BMS, I don't see a problem;
The schematic would become (adding my homemade battery):
In practice I would simply add the homemade battery to the 2 copper bars, since there is room (free holes):
In theory I could add the system to VRM using CAN cable between Daly BMS & Victron Venus GX, but I don't think that's really needed. But it would be nice to see the battery in the system overview so that my dad can monitor it.
Please let me know if I'm being realistic or about to make a huge mistake.