Open Circuit Voltage Characterization Methods for Iron Phosphate Cells
Accurate iron phosphate state of charge tracking relies on temperature compensated open circuit voltage hysteresis mapping measured after four hour rest periods.
Quasi Open Circuit Voltage
Meaning
An electrical measurement technique approximates the equilibrium potential of an electrochemical cell by applying an extremely small current continuously during a charge or discharge cycle. The quasi open circuit voltage profile represents a practical alternative to true open circuit voltage curves which require long rest periods at multiple state of charge points. This continuous measurement saves time while still capturing critical thermodynamic transition markers.
Thermodynamic Deviation
A small overpotential is always present due to the continuous flow of current, meaning the measured curve deviates slightly from the true equilibrium voltage. To keep this error minimal, quasi open circuit voltage is acquired using currents around one-twentieth of the nominal capacity of the cell. This low rate ensures that the deviation remains small enough to preserve the visibility of phase transitions.
Application Value
Battery diagnostic software uses this continuous curve to construct differential capacity plots. From these plots, quasi open circuit voltage measurements reveal the peaks associated with specific electrochemical reactions within the electrodes.
Process Limit
This technique cannot be used during high-throughput assembly line inspections because each run requires several hours to complete. Sourcing teams use quasi open circuit voltage data primarily during the initial validation of new cell designs.