[Saris]

It's such a strange thing to do unless they wanted to intentionally make them have short lifespans.

The M12s have the pins too for voltage sensing in the charger (and tool maybe?), but they have 1 mega ohm resistors in line so cannot be used for balancing.

[notjulianjaynes]

DeWalt also makes a series of batteries that will either provide ~20v (5s3p) or ~60v (15s1p) depending on the tool you plug it into. I've taken one of these apart before and how they implement this is just a mechanical switch between the cell groups, which only gets depressed when inserted into 60v tools. Not sure if thats why they put all the circuitry on the tool side, but I can't think of another good way to implement the same. (You'd need a pretty beefy boost/buck controller for the amps these tools pull )