The voltage measured between certain cells is incorrect.

Cell 24 is by far the worst, with a digital reading 33 mV less than the actual cell voltage.
Cell 24 is a mid-pack connection between two LTC6804s... but there's no voltage measured across the long trace from one IC to the other (R144 to R188 = 0.001 mV max).

Cell 36 has a digital reading 5 mV less than actual cell voltage.
Cell 36 is the top-most cell on an LTC6804 IC and is also the top-most cell on an 18S pack.

Cell 48 has a digital reading 4 mV less than actual cell voltage.
Cell 48 is the top-most cell on an LTC6804 IC and is also the top-most cell on a 12S pack.

Also, the first cells on 3 of the 4 LTC6804s all read between 1 & 2 mV low:
Cell 01 is -1.4 mV
Cell 13 is -1.0 mV
Cell 37 is -2.0 mV

	Cell 25 - the 4th LTC6804's first cell - is dead on.  Cell 25 is mid-way between an 18S pack.

...

Update: This is due to trace resistance.  Need to route separate traces from each battery connector to the sensed lead, versus the identical net name that's powering the LTC6804.

Update: There is virtually no voltage drop on the external sense leads all the up through into LiBCMs soldered connection joints... the voltage drop is entirely on the LiBCM PCB itself.

Cells affected:
C1- (-1.4 mV error)
C12+@42S (did not test)
C13+ (-1.0 mV error)
C24+ (-32.9 mV error) Fixed
C36+ (5.0 mV error) Fixed
C37+ (-2.0 mV error) Fixed
C48+ (-4.0 mV error) Fixed

Cells not affected:
C12+@48S (-0.5 mV error)
C25+ (-0.1 mV error)
These traces are (accidentally) routed well.  Should still look at optimizing them further, to minimize error... but don't bend over backwards if difficult.

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Update: I removed R144 and connected a lead - with series 50 Ohm resistor - directly from the BMS connector's 24+ to R144 (net C12_B).  This is a direct sense to the LTC6804 for C24... but unfortunately it only reduced the measurement error 10 mV (from 33 mV to 23 mV.  Something else is wrong here.

Going to play around with ADC update rate, etc.

...

Solution: There was a short from highest cell on IC to Vin.
