Clarifying the Function of MOSFETs in Battery Management Systems (BMS)

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Many people misunderstand the operation of MOSFETs in BMS circuits. A common misconception is that the charging FET controls the ground line of the charger, and the discharging FET controls the ground line of the load. But in fact, their behavior is more precise and nuanced than that.

 

 

1. Basic Configuration of BMS with N-channel MOSFETs

Most lithium-ion BMS boards use two N-channel MOSFETs for controlling charging and discharging. These MOSFETs are connected as follows:

  • Source (S): Connected to B− (battery's negative terminal)
  • Drain (D): Connected to P− (output/input common ground)
  • Gate (G): Controlled by the protection IC
    A circuit diagram illustrating the correct connection and function of MOSFETs in a 5-cell lithium-ion battery BMS system, highlighting Source, Drain, Gate directions and charge/discharge control paths.

When a gate voltage (Vgs) above the threshold is applied, the MOSFET conducts and allows current flow from the Drain to the Source.

 

 

2. Difference Between Charge and Discharge MOSFETs

The discharge MOSFET is responsible for connecting P− to B− when load discharge is allowed. The charge MOSFET controls current entering the battery pack from the P+ terminal. Despite both using N-channel types, they are arranged in different directions.

3. Real Switching Control

- Discharge: The load side (P−) is disconnected or connected from the battery ground (B−) via the discharge FET.
- Charge: The charger’s input (P+) is connected to the battery's + terminal only if the charge FET is conducting.

 

 

4. What Happens When FETs Are Not Active

  • If the gate voltage is not properly applied, the FETs remain open and the circuit is blocked.
  • This results in 0V across P+ and P−, even if the battery is charged.

5. Key Indicators and How to Diagnose

  • Measure voltage between Gate and Source.
  • If Vgs is low, the MOSFET is not conducting.
  • This could be caused by the protection IC disabling the FET due to undervoltage, overcurrent, etc.

 

 

Conclusion

Understanding how MOSFETs truly operate in BMS circuits allows for better repair, modification, and diagnostic decisions. Knowing that they are switching the ground path, not just passively connecting lines, changes your approach to analyzing battery pack problems.