Stackable ‘shield’ design is a great idea. Module balance module would also allow active/passive balance options, or perhaps stacking several balance boards together for higher balance currents.
Oh man you're a genius, multiple balance boards to increase balance current, how come I didn't think about that?
Now there's some limitations (i.e. you can't stack boards forever...) and the first one will be the current limitation of the inter-board connectors.
Stackable connectors are expensive (quick looking yesterday told me about $5 minimum per connector for a classic 0.1" pitch connector) and there isn't 1 million models available, so high current ones would be very hard to find and very pricey. Then we can do stackable connectors for everything but balancing wires and have one connector per board for the balancing wires. It would solve the problem and also solve the other problem of current limitation of the balancing wires, but you would need a lot of balancing wires so the cabling would be a bit less clean.
Then there's the power supply limit: the main power source is a DC/DC converter who takes B+ and outputs +12 V @ 250 mA max and -12 V @ 250 mA max. There's a second DC/DC converter who takes the +12 V and outputs +5 V @ 400 mA max (mainly to avoid the bigs losses of the linear regulator of the arduino).
I currently use half of the max available on the + 12 V, only a few mA on the -12 V and half of the max on the +5 V. I still need to add some things so I'll probably be around 2/3 of the max on the +12 and +5 V when I'm finished. The balance board shouldn't use much (wild guess 10 to 20 mA) but the DC/DC converter isn't a beast either so sooner or later we will cross the limit.
I'd say stacking more than 3 balance boards will be hard. But each balance board will have a decent balance current (I planned 2 to 5 A but there's some serious heat limitations if I keep the dump resistors on the board, so I may downgrade to 0.5 or 1 A, still better than 90 % of the ready made BMS if I'm not mistaken) so even 3 boards should be plenty enough for a big battery.
NB: I'll not do active balancing because it's costly, complex and not really useful for what I do. But if someone wants to design a board for active balancing I'm all for it
I was pondering the benefits of comparing charge/discharge currents between packs to identify capacity differences.
Ok, so you can definitely do that with a central device, that would be the easier way (it even makes more sense than each BMS comparing itself with each other).