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

  1. Set your multimeter to DC voltage mode.
  2. Connect the black probe to B− (usually battery negative terminal).
  3. Touch the red probe to the gate leg of the MOSFET (usually the leftmost or rightmost pin).
  4. Check the voltage. Typically, a Vgs above 2–4V indicates the MOSFET is in conduction (on) state.

If gate voltage is 0V or near-zero, the FET will remain off and block power flow.

 

 

3. Interpretation of Results

  • Gate voltage = 0V: Control IC may have shut down the gate intentionally.
  • Gate voltage = high (e.g. 8–10V): FET is likely on. If no voltage output, the problem lies elsewhere.
  • Unusual fluctuations: Could suggest a broken gate resistor, zener diode, or IC failure.
    A digital schematic showing how to measure gate voltage of discharge MOSFET using a multimeter on a BMS circuit step by step.

4. Measuring Tips

  • Measure during charging or discharging simulation for live conditions.
  • Don’t forget to check both the charge FET and discharge FET separately.
  • If you have a working battery pack, compare its gate voltages as a reference.

 

 

Conclusion

Checking the gate voltage of MOSFETs on a BMS board is one of the most effective ways to pinpoint why a battery pack is not outputting voltage. It’s a key step in DIY battery diagnostics and helps prevent unnecessary board replacements.

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