Notes on State of Charge (SoC) of Battery Packs

Introduction

• Speaker: Eric Staal, President of Stoffels Systems
• Topic: State of Charge (SoC) as estimated by a Battery Management System (BMS)

Definition of State of Charge (SoC)

• SoC: Defined as the remaining capacity of a battery pack.
  • Measured in amp hours (Ah) or coulombs.
  • Formula:
    • SoC (%) = (Remaining capacity / Total capacity) x 100
• Example:
  • 100 Ah battery pack, 70 Ah remaining = SoC of 70%.

Importance of Amp Hours vs. Energy

• SoC is expressed in amp hours, not energy.
• Discharge Curve of Lithium-Ion Batteries:
  • X-axis: Amp hours discharged.
  • Y-axis: Voltage (V).
  • Typical voltage range: 4.2V (full) to 2.5V (empty).
  • Curve shows a varying downward slope as amp hours are discharged.
• Energy vs. Capacity:
  • Confusion occurs when using a fuel gauge algorithm based on capacity instead of energy.
  • SOC II (based on energy) is more relevant for applications like electric vehicles (EVs).

Distinction Between SOC and SOC II

• SOC C: Based on capacity.
• SOC II: Based on energy (more accurate for runtime and distance).
  • Example: 50% SOC II may correspond to 42% SOC C due to the discharge curve's shape.

Calculation of State of Charge

• Coulomb Counting:
  • Primary method for calculating SoC.
  • Requires current sensor (shunt or Hall effect) to measure current flow.
  • Integration of current over time to determine amp hours.
  • Area under the current vs. time curve represents charge capacity removed.
• Challenges with Coulomb Counting:
  • Current sensors may have drift and integration errors.
  • Need for Open Circuit Voltage (OCV) lookup for accuracy.

Depth of Discharge (DoD)

• Definition: Inverse of SoC.
  • E.g., 70% SoC = 30% DoD.
• OCV Lookup:
  • BMS uses OCV lookup table to determine SoC based on voltage after the pack has rested.
  • Ensures accuracy of the SoC estimation by recalibrating the Coulomb counting.

Conclusion

• Importance of combining Coulomb counting with OCV lookup for accurate SoC algorithms.
• Prevents issues such as sudden drops in SoC that could lead to user frustration or operational problems.
• Encouragement to watch future videos for more detailed calculations and methods.