How to Check the Gate Voltage of FETs in a BMS Circuit

When repairing or analyzing a 21V lithium-ion drill battery pack, one of the most critical aspects is checking the MOSFETs on the BMS board. Among them, understanding how to measure the gate voltage and confirm switching status is a key skill for troubleshooting battery issues.     Structure and Role of FETs in a BMS Most BMS (Battery Management System) boards consist of two N-channel MOSFETs: one for charging and one for discharging. In a 5S configuration, the board monitors the voltage of five cells via B1~B5 lines and controls the two MOSFETs accordingly. Charging FET: Controls current entering from the charger to the battery. Discharging FET: Controls current exiting from the battery to the drill or other load.     Where is the Gate Voltage Supplied From? Unlike simple switches, the gate of a MOSFET in a BMS doesn’t receive constant voltage. Instead, the BMS controller IC provides a pulse or a fixed voltage depending o...