Predicting available energy from lithium-ion cells requires a number of measurable parameters. The Impedance Track (IT) model measures, records, and predicts available capacity or energy from these parameters. The model provides the means to accurately determine available energy over the life of a cell by accounting for aging and present environmental conditions. Aging can be accounted for by measuring QMax and cell impedances as the cell is cycled. QMax is the amount of charge available in a fully charged cell. The present conditions include consideration of cell and ambient temperature as well as the present or predicted power requirements. There are three major components of Impedance Track.
- One is measurement of QMax.
- Two is measurement of cell impedance.
- Three is the calculation of capacities.
All three components rely on the relationship of the open circuit voltage (OCV) versus depth of discharge (DOD). DOD is the percent of QMax removed from a cell during a no load discharge and goes to 100% when the cell is fully discharged. The OCV versus DOD relationship is fixed for each particular cell chemistry. QMax is determined by measuring the charge passed between two DOD points; the DOD points being determined from OCV measurements during Relax mode. Cell impedance is measured during constant load times during discharge from the difference of measured voltage to the OCV voltage based on the known DOD. The DOD determined from the most recent OCV reading and adjusted by the passed charge during the discharge. In general, capacities are recalculated based on changes in temperature, load or impedance. Capacity calculations use the known DOD, load, temperature, end-of-discharge voltage,
TermV, and
QMax.