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 on real-time cell status, such as undervoltage, overvoltage, or overcurrent.

Typically, the source of the FET is connected to the battery’s B−, and the drain is connected to P−. The controller IC monitors the B1~B5 lines and outputs a gate signal (usually between 8V~12V) when safe conditions are met.

A schematic diagram illustrating the BMS circuit's gate voltage delivery to MOSFETs, showing charge/discharge FETs, controller IC, and voltage sensing lines labeled in English.

How to Measure the FET Gate Voltage

Using a digital multimeter, follow this procedure:

  1. Identify the three legs of each FET: Gate (G), Drain (D), Source (S).
  2. Put the multimeter’s black probe on the Source (generally B−).
  3. Touch the red probe to the Gate pin.
  4. A normal gate voltage should read above 8V when activated.

If the gate reads 0V when the FET should be conducting, the controller IC may be blocking the gate signal due to an error condition.

 

 

Common Causes of Abnormal Gate Voltage

  • Cell voltage imbalance: One or more cells under 2.5V.
  • Damaged voltage divider resistors: On the B1~B5 sensing lines.
  • Faulty controller IC: Unable to provide proper gate control.

If these issues are identified, replacing the BMS board is often easier than attempting component-level repair.

Final Tips

Always measure and compare gate voltages on both working and faulty battery packs under the same conditions. If the gate voltage is consistently absent on the defective board while present on a good one, you’ve likely found the problem.