State of Health in Battery Management Systems

Introduction

• Presenter: Eric Topol, president of Stoffels Systems
• Topic: State of Health (SoH) of batteries as calculated by a Battery Management System (BMS)

Definition of State of Health (SoH)

• SoH: Total capacity of the battery pack today / Total beginning of life capacity (Bol capacity)
• Units: Amp hours or units of charge
• Example with cellphone battery:
  • New battery: SoH = 100%
  • After 1-2 years (300-600 cycles): SoH may drop to 60-70%
  • Importance: Indicates how much energy can be discharged over time

Capacity and Cycle Life

• Cycle Life: Number of charge/discharge cycles a battery can undergo before capacity decreases
• Graph description:
  • Y-axis: Capacity (Amp hours)
  • X-axis: Number of cycles
  • Capacity decreases over time:
    • Example:
      • 0 cycles: 50 Ah
      • 200 cycles: 47 Ah
      • 600 cycles: 44 Ah

Types of State of Health

1. State of Health Capacity

• Describes capacity fade over time

2. State of Health Impedance

• Impedance: Resistance in the battery
• Initially low ESR (Equivalent Series Resistance)
• Over time, as the battery is charged/discharged, ESR increases
• Graph description:
  • Y-axis: Impedance (Ohms or milli Ohms)
  • Impedance growth increases over time

Importance of Monitoring SoH

• Monitoring both SoH capacity and impedance helps estimate:
  • Remaining capacity to discharge
  • Voltage drop and thermal output during discharge
• Increased resistance leads to:
  • More thermal output
  • Less efficiency

Estimating Remaining Range for Electric Vehicles

• Example calculation:
  • State of Charge (SoC) = 70%
  • SoH capacity = 80%
  • Initial Bol range = 200 miles
  • Current max range = 160 miles (due to capacity fade)
  • Remaining range = SoC * Current max range = 70% * 160 miles
• Importance of accurate modeling:
  • Incorrect estimation of range can result in up to 40 miles difference

Effects of Impedance Growth on Performance

• Thermal performance as a limiting factor in high-performance electric vehicles
  • Example: Initial thermal limiting may occur after 10 minutes of racing
  • After 50-100 cycles with significant degradation, thermal limiting may change to 5 minutes
• Accurate estimations of battery performance over time are essential

Conclusion

• Importance of understanding SoH and impedance for accurate performance estimates
• Thank you for watching!