Disconnect Switches in Lithium-Ion Battery Packs

Introduction

• Presenter: Eric Stoffel, President of Stoffel Systems
• Topic: Disconnect switches in Lithium-Ion battery packs.

Key Components of a Battery Pack

• Cells: Group of cells that store energy.
• Battery Management System (BMS): Ensures safe and reliable operation of the battery pack.
  • Monitors:
    • Voltage of cells
    • Temperature of cells
    • Current flow in/out
  • Functions:
    • Prevents conditions like overheating or overcharging.
    • Uses disconnect switches to interrupt current flow when needed.

Types of Disconnect Switches

1. Electromechanical Switches

• Types: Relay, Contactor
• Operation:
  • Has two high current contacts and an electrically isolated coil.
  • BMS controls the switch via the coil drive line.
• Advantages:
  • High current carrying capacity (e.g., 500-1000 amps).
  • Provides electrical isolation for safety.

2. Solid State Switches

• No moving parts; utilizes semiconductors (e.g., N-MOSFETs).
• Operation:
  • Typically configured with two source-coupled MOSFETs controlled by a gate drive.
• Advantages:
  • Smaller, lighter, and lower cost for lower currents (e.g., 10 amps).
  • Rapid response time (less than 5 microseconds).
  • Flexibility in form factor; can be mounted on BMS board or elsewhere.
  • Can offer e-fuse type functionality for rapid current interruption.
• Limitations:
  • Less suitable for high voltage systems (typically up to 96 volts).

Architecture Considerations

• Top and Bottom Disconnect Switches: Important in high voltage applications (e.g., electric vehicles) for redundancy and safety.
• Low Side Switch: Less common due to ground reference issues affecting communication.
  • Requires isolated communication interface.

Pros and Cons of Electromechanical vs. Solid State Switches

Electromechanical Switches

• Pros:
  • High current capacity.
  • Electrical isolation from BMS.
• Cons:
  • Slower response time (50-200 milliseconds).
  • Larger physical form factor.

Solid State Switches

• Pros:
  • Quick response time for fault protection.
  • Smaller and lighter.
  • Flexible mounting options.
• Cons:
  • Limited high voltage capability.

Manual Service Disconnect (MSD)

• Purpose: For assembly and servicing of the battery pack.
• Placement: Between two halves of the battery pack.
• Functionality:
  • Reduces maximum voltage present in the battery pack.
  • Prevents current flow during servicing.
• Control: Operated separately from the BMS, typically the last connection made before battery assembly.

Conclusion

• Disconnect switches are critical components in Lithium-Ion battery pack design, each type offering various benefits and considerations depending on use cases.