State of Charge (Coulomb Counting)

Formula

Description

Coulomb counting estimates the state of charge by integrating current over time and comparing it to the battery nominal capacity. Charging current is positive (increases SoC) and discharging current should be entered as negative (decreases SoC). This is the simplest SoC estimation method, used in battery management systems (BMS) for electric vehicles, power tools, and energy storage systems. The main limitation is accumulated error from current measurement inaccuracy and capacity variation with temperature and aging.

Variables

• SoC_init — Initial state of charge (ratio, e.g., 0.5 for 50%)
• I — Current (A, positive for charging, negative for discharging)
• t — Time period (s)
• Q — Nominal battery capacity (Ah, enter in base unit mAh or Ah)

Practical Notes

Coulomb counting drifts over time due to measurement errors and must be periodically recalibrated against voltage-based SoC or other reference points (e.g., fully charged at 4.2 V for Li-ion). The Coulombic efficiency (charge out vs. charge in) is typically 99.5% for Li-ion but only 80-85% for lead-acid, introducing additional error if not compensated.