What is BMS passive balance?

Generally we name it charging balance, as the balance function only start-up when charging. 

The BMS with passive balance have electric circuit for each cluster, mainly components are IC, Mosfet and Resistance. The IC is for detecting balance start-up voltage, typically it is 4.18V (4.20±0.025V). The Mosfet is the switch to turn on the balancing. The resistance for burning off excess energy, normally the power is 1W and balance current 40-50mA. So a passive system potentially burns off excess energy from the high voltage clusters through a resistive element until the charge matches the lower voltage clusters in the pack.

See in below sketch map, the Series 1 and Series 4 reach 4.18V, but the Series 2 is 4.1V and Series 3 just 4.0V. At this time, both IC on Series 1 and Series 4 detected the voltage 4.18V, then Mosfets turn on and the resistances start heating for burning off energy with current 40-50mA. Theoretically, Series 2 and Series 3 will reach the same voltage as Series 1 and Series 4, the battery pack will be full charged with all clusters 4.2V.

Passive balance charging