How to Measure Gate Voltage of MOSFETs in a Battery BMS Circuit
When diagnosing battery packs, especially BMS boards that are not outputting voltage, checking the gate voltage of MOSFETs is a fundamental diagnostic step. This test can distinguish between a FET issue and a control IC issue. 1. Why Gate Voltage Matters MOSFETs act as electronic switches. Unless a sufficient gate-source voltage (Vgs) is applied, the MOSFET remains off and no current flows between drain and source. Gate: Controlled by the BMS's protection IC Source: Usually tied to B− (battery negative) Drain: Connected to P− (load or charge return path) 2. Step-by-Step Gate Voltage Test Set your multimeter to DC voltage mode. Connect the black probe to B− (usually battery negative terminal). Touch the red probe to the gate leg of the MOSFET (usually the leftmost or rightmost pin). Check the voltage. Typically, a Vgs above 2–4V indicates the MOSFET is in conduction (on) state. If gate voltage ...