Chargery's reply is not true!
What's going on here? This is not politics where anything goes. Statements like that are easily proven false.
Powering BMS using sense wires does affect cell voltage measurements.
I just connected a lab power supply to the external power connector. At 15V, supply current is 110 mA with BMS display active. Current drops to 68 mA with display off. Measurements are taken with an active display.
Cell--- Fluke------int powr------ext power
1------ 3.330 ----- 3.312 ------- 3.330
2------ 3.330 ----- 3.332 ------- 3.331
3------ 3.330 ----- 3.330 ------- 3.330
4------ 3.329 ----- 3.318 ------- 3.331
With external power there are no measurement errors associated with cell1 1 & 4.
There clearly is a measurement error when using the sense wires to power the BMS. Cell 4 is low by more then 10 mV. This error is caused by only 110 mA. If two relays combined draw 1A, then measurement error could be 100 mV.