3S 20A BMS board resistance

[grmsyd]

I notice a 100k ohm difference between the negative input and 0V output,
However when connecting an output to a 16850B battery it shows 0 ohm difference,
Is this normal?

[Phil1977]

It´s the purpose of that board to behave that way.

It´s more or less a MOSFET-switch that switches the cells from the load or charger if anything is out of the SOA-windows.

If no cell is detected, you are outside of the SOA-voltage and so the switch is open --> high resistance
If healthy cells are connected, the switch is closed --> low resistance

The FET in the standard BMS-boards usually is on the low-side, so it´s to be expected that you measure these resistances between battery-ground and output-ground.

The only strange seems to me that you say the connection of *one* 18650-cell switches on the output. Usually for a 3S-system all three voltage levels must be present.