Methods to Measure Initial State of Charge
- Open Circuit Voltage (OCV) Measurement
This method estimates SoC by measuring the battery’s voltage when it is at rest and not connected to any load or charger. The OCV has a known relationship with SoC, which can be used to approximate the initial charge level. However, this method requires the battery to rest for a sufficiently long period to reach equilibrium, making it less practical for quick measurements. - Coulomb Counting Method
Also known as current integration or ampere-hour counting, this technique calculates SoC by integrating the current flowing in and out of the battery over time, starting from a known initial SoC. The battery current is measured precisely during charge and discharge cycles, and these values are integrated to track the change in SoC. The initial SoC must be accurately known for this method to remain reliable. Losses such as self-discharge and side reactions necessitate periodic recalibration to maintain accuracy. - Model-Based Methods (e.g., Kalman Filters)
Advanced estimation methods use model-based algorithms like Kalman filters, which combine measurements of voltage, current, and sometimes temperature, with a mathematical model of the battery to estimate the SoC more accurately. These methods can filter out noise and correct for inaccuracies in measurements, providing a dynamic estimation even when the battery is under load or charging. - Voltage and Current Curve Methods
During charging or discharging, the voltage and current behavior follows specific curves. By comparing measured voltage/current characteristics during the constant current (CC) or constant voltage (CV) stages of charging (or during discharge at known currents) to manufacturer-provided reference curves, the initial SoC can be inferred. - Impedance Spectroscopy
A less common but sophisticated method is impedance spectroscopy, where the battery’s impedance is measured across frequencies to estimate SoC indirectly. This method can be combined with complex modeling algorithms to improve SoC accuracy.
Summary
| Method | Description | Notes |
|---|---|---|
| Open Circuit Voltage (OCV) | Measures voltage at rest; relates OCV to SoC through known curves | Requires rest period; good for initial SoC |
| Coulomb Counting | Tracks charge in/out by integrating current; requires known initial SoC | Most common; needs recalibration due to losses |
| Model-Based (Kalman Filter) | Uses models and measurements to estimate SoC dynamically | More accurate; computationally intensive |
| Voltage/Current Curve Fitting | Uses charging/discharging voltage-current profiles to estimate SoC | Depends on accurate reference data |
| Impedance Spectroscopy | Measures frequency-dependent impedance to estimate SoC | Advanced; less common in practice |
The accuracy of initial SoC measurement is critical because errors propagate through methods like coulomb counting. Typically, a combination of methods is used in practical battery management systems to improve reliability.
In conclusion, the initial state of charge is primarily determined from open circuit voltage measurements when possible or inferred from manufacturer charge/discharge curves, then tracked using coulomb counting integrated with model-based filtering for ongoing accuracy.
Original article by NenPower, If reposted, please credit the source: https://nenpower.com/blog/what-methods-are-used-to-measure-the-initial-state-of-charge/
