How to Manually Bypass the Discharge MOSFET in a BMS Circuit

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When a battery pack shows 0V across its P+ and P− terminals despite having good cell voltage, it's possible the discharge MOSFET is disabled due to protection lockout. In such cases, temporarily bypassing the MOSFET can help "wake up" the BMS.

 

 

1. Why Bypassing the Discharge FET Works

The discharge MOSFET controls current flow to external loads. If the BMS controller detects a fault (e.g., undervoltage), it shuts off the gate signal, keeping the FET off and output voltage at 0V. A brief manual connection between P− and B− can restore normal state.

A schematic-style diagram showing how to safely bypass the discharge MOSFET by briefly connecting P− and B− to restore output voltage in a BMS.

  • P−: BMS output ground controlled by discharge FET
  • B−: Main battery ground connected to cell negative terminal
  • P+: Positive terminal (usually directly connected to B+)

 

 

2. Step-by-Step Bypass Test

  1. Set your multimeter to DC voltage mode.
  2. Measure voltage across P+ and P−. If 0V, proceed to the next step.
  3. Using an insulated wire or test lead, briefly connect P− to B− for 1–2 seconds.
  4. Measure P+ to P− again. If voltage appears, the BMS has re-enabled the output.

This method is non-destructive when done briefly and carefully. Never leave P− and B− permanently bridged, as it bypasses all protection.

 

 

3. When and When Not to Use This Test

  • Use it when: The pack reads 0V, but cell voltages are individually healthy (e.g., above 3.0V).
  • Avoid it if: Any cell is deeply discharged (below 2.0V), swollen, or physically damaged.
  • Always recharge the battery using a current-limited charger after reactivation.

 

 

Conclusion

Bypassing the discharge MOSFET is a practical and fast way to restore output voltage from a locked BMS. This simple trick can prevent unnecessary replacements and help you diagnose issues effectively—especially for DIY repairers handling lithium-ion packs.