Disconnect Switches in Lithium-Ion Battery Packs

Introduction

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

Overview of Battery Pack Components

1. Cells
2. Battery Management System (BMS)
3. Disconnect Switch

Purpose of Disconnect Switch

• Ensures safe and reliable operation of Lithium-Ion battery packs
• Monitors:
  • Voltage of cells
  • Temperature of cells
  • Current flow in/out of the pack
• Prevents dangerous conditions (e.g., overheating, overcharging) by:
  • Interrupting current flow via the disconnect switch
  • Communicating issues to the application system controller

Types of Disconnect Switches

1. Electromechanical Switches

• Components:
  • High current contacts
  • Isolated coil
• Operation:
  • Closed for current flow, open to interrupt flow
• Characteristics:
  • High current carrying capacity (e.g., 500A to 1000A)
  • Provides electrical isolation

2. Solid State Switches

• Components:
  • N-channel MOSFETs (transistors)
• Operation:
  • No moving parts
  • Blocks or allows current in both directions
• Characteristics:
  • Smaller, lightweight, lower cost
  • Fast response time (milliseconds vs. microseconds)
  • More flexible form factor (can be placed on BMS board)

Disconnect Switch Architecture

• Top and Bottom Side Disconnect:
  • Ensures redundancy and safety in high-voltage applications (e.g., electric vehicles)
• Low Side Switch:
  • Less common; can complicate communication with external systems

Pros and Cons of Each Approach

Electromechanical

• Pros:
  • High current capacity
  • Electrical isolation from BMS
• Cons:
  • Slower response time
  • Larger size

Solid State

• Pros:
  • Fast response (e-fuse functionality)
  • Smaller and lighter
• Cons:
  • Limited voltage handling (difficult for >96V systems)

Manual Service Disconnect (MSD)

• Purpose:
  • Enables safe assembly/service of battery packs
  • Disconnects modules to reduce maximum voltage present
• Operation:
  • Controlled separately from the BMS
  • Last connection made before battery pack completion

Conclusion

• Importance of considering different disconnect switch types in Lithium-Ion battery pack designs.