Lithium Iron Phosphate Cell Open Circuit Voltage Equilibrium Mapping
Lithium iron phosphate open circuit voltage equilibrium maps two-phase hysteresis using multi-hour relaxation testing for state of charge estimation
Path Hysteresis
Meaning
The difference in the equilibrium potential of a battery cell when it is charged versus when it is discharged arises from distinct internal state trajectories. This history-dependent voltage behavior is known as path hysteresis, and it affects both state estimation and thermal calculations. The terminal voltage does not lie on a single curve but depends on the previous direction of current flow.
Thermodynamic Cause
Active material phase changes during lithium insertion and extraction generate mechanical stress that shifts the chemical potential. In graphite and iron phosphate electrodes, this phase coexistence creates a resistive barrier to the transition, resulting in a persistent voltage separation. This difference remains even after the cell has rested for many hours, meaning that voltage alone cannot uniquely identify the state of charge.
Current Dependence
Partial charging or discharging cycles create intermediate curves that reside within the main hysteresis envelope. These minor loops complicate the task of mapping open-circuit voltage to state of charge. Estimators must track the history of current throughput to determine where the cell sits between the upper charge and lower discharge limits.
Modelling Strategy
Equivalent circuit models include mathematical operators to adjust the predicted voltage. These functions transition between the upper and lower limits as current reverses.