Clarifying the Function of MOSFETs in Battery Management Systems (BMS)

Many people misunderstand the operation of MOSFETs in BMS circuits. A common misconception is that the charging FET controls the ground line of the charger, and the discharging FET controls the ground line of the load. But in fact, their behavior is more precise and nuanced than that.     1. Basic Configuration of BMS with N-channel MOSFETs Most lithium-ion BMS boards use two N-channel MOSFETs for controlling charging and discharging. These MOSFETs are connected as follows: Source (S): Connected to B− (battery's negative terminal) Drain (D): Connected to P− (output/input common ground) Gate (G): Controlled by the protection IC When a gate voltage (Vgs) above the threshold is applied, the MOSFET conducts and allows current flow from the Drain to the Source.     2. Difference Between Charge and Discharge MOSFETs The discharge MOSFET is responsible for connecting P− to B− when load discharge is allowed. The charge MOS...