Quoteop-amp that has a very high common mode voltage rejection ratingI thought about something like that, but it requires much more circuitry [45 opAmps,plus etc] than the other approaches. But would be very good for a few sample points only.
op-amp that has a very high common mode voltage rejection rating
45 DPST relays
Check out the linked pdf doc.
separate Arduino processors, powered at different points in the battery chain, and each reading 6 A/D channels,
Or just one with a 2 pole 45 throw switch ahead of the opamp.