Solid Phase Diffusion Dynamics Distorting Battery Voltage Lookups
Unrelaxed solid-phase diffusion gradients create particle surface overpotentials that distort battery open-circuit voltage lookups by up to twenty percent.
Open Circuit Voltage Lookup
Meaning
State estimation method utilizes a pre-established database to determine the state of charge of an electrochemical cell from its stable terminal voltage. An open circuit voltage lookup provides a reference point for battery management systems during periods of extended rest. This method relies on the stable correlation between the cell’s equilibrium voltage and its remaining capacity.
Algorithmic Application
Coulomb counting alone accumulates drift errors over time due to measurement noise and changing cell conditions. By executing an open circuit voltage lookup, the battery management system resets the integration error when the vehicle is parked. This correction prevents the estimated state of charge from drifting away from the true value.
Calibration Requirement
Generating the reference tables requires extensive laboratory testing across multiple temperatures and discharge rates. Engineers measure the voltage at increments of five percent state of charge after allowing the cell to relax for several hours to eliminate polarization effects. This database must be stored in the non-volatile memory of the control unit to ensure that the algorithm has access to the correct values throughout the operating lifetime of the battery.
Operational Limitation
Voltage plateaus in certain chemistries like lithium iron phosphate make this method less effective. The flat discharge curve of these cells means that a tiny measurement error leads to a massive error in the estimated state of charge. This limitation requires combining the lookup method with other state estimators.