Define the interfaces, so the modules can evolve independently.
Reading the available material on this project so far, I'm struck by a few distinct needs:
There's the "do it with an arduino and through-hole components" logic board, which will meet certain needs but not others.
There's the "run a whole OS on the thing and let it talk over ethernet" logic board, which will meet more needs but cannot be sensibly achieved with exclusively through-hole components.
There's the "stick on any 12v battery" power system, which will serve smaller installations with the batteries most of us likely have sitting around.
There's the "let's go 48 volts" power system, which will find application in higher-wattage installations, and telecom settings where all the gear is built for 48v.
And there's the "c'mon guys it's not 1987 anymore, use lithium already" power system, which someone will make, probably out of a rewired EV pack.
There's also the "actually no batteries, I just want a smart power strip/PDU" power system, which has enough in common that it's likely to exist at some point.
And surely there are more. Rather than try to anticipate and design a one-size-fits-all solution, I think it's most sensible to define the interfaces so any logic board can mate with any power system, and designers can dream up future versions of either.
So, let's use this issue to decide which functions must be on the logic board vs the power parts, and how the interfaces should work. I'll add my own thoughts the comments.
(This issue is NOT about the PC-to-UPS interface, that's #14 (closed).)