Meaning
Process by which a battery cell returns to a state of chemical and thermal equilibrium after a load is removed. During this period, ion concentrations equalize across the thickness of the electrodes. Voltage readings taken during thermodynamic relaxation provide a more accurate measure of the state of charge than those taken under load.
Potential Decay
Measured voltage changes rapidly in the first few minutes after the current stops. The voltage drop happens because the overpotential required to drive the reaction in thermodynamic relaxation dissipates. The curve of this decay reveals the internal resistance and the diffusion rate of the ions.
High resistance cells take much longer to reach a stable resting potential.
Measurement Timing
Accurate state of health diagnostics require a waiting period of several hours. If a technician measures the cell too soon, the reading reflects the surface concentration of ions rather than the bulk state. Thermodynamic relaxation must be complete for the open circuit voltage to be valid.
Most automotive standards specify a rest period before capacity testing begins.
Voltage Recovery
Potential rises after a discharge as the ion gradients within the active material particles smooth out. This recovery allows the battery to deliver more energy during pulsed discharge cycles. Software models use the rate of voltage recovery to estimate the remaining life of the cell.
Efficient thermodynamic relaxation is a sign of a high quality electrode with good porosity. Electrode materials with poor conductivity show a slow voltage rise that limits power output in heavy load applications. Fast recovery times indicate that the lithium transport pathways are clear and functional.
