It seems the solution of a Thunderstruck is just to have a master BMS (as the batteries themselves have the slave BMSs, like the master is optional?).. It seems like on the 2s they shouldn't drift off balance if you top balance the pair in parallel prior to hooking them up in series.
This guy seems to give some info about the Thunderstruck, indicating that even with it, you still need a balancing solution to go between your 2s modules:
FYI, new product on market, a BMS for the Valence U27-12XP batteries Much less cost that Valence (hard to find) units, can program yourself easily. Tradeoffs, less overall features, but will probably handle 80% of users out there. https://www.thunderstruck-ev.com/valence-battery-bms.html Many...
diysolarforum.com
Or you could monitor them with your laptop to see if they ever did drift, and add in a master BMS later if you needed to.
The question I'm not clear about, it seems on that thread they might be indicating that the internal cell balancing feature (within a battery module) is not activated without either a master BMS connected or a laptop connected to the Valence com port and app running? Maybe it needs the master for that feature to work (to 'wake' up the slave BMS's and allow its features to work).
Yeah, and it might also need the master BMS for the safety cutoff relay, and do the slave BMS's operate the low/high temp/voltage cutoffs if no master is present, if they are not woken up?...
Did you read both of these threads? The one above, and this one:
#1: I'll try and keep this organized. We're limited to 10,000 characters so this will be a multi part post. If you have an XP battery or a bunch of them this will be a good thread for information on how to treat them properly. We'll discuss what the Valence Brand U-BMS actually does and how to...
diysolarforum.com
Still trying to learn about it, but if I had a battery system where it was designed to have a master BMS to control slave BMS's, I would want to try and make that operational... And then it seems you might still need a balancer solution to go between the 2s on top of that (if you started to notice drift)...