A lumped parameter electrical component model of a 26650 size LiFePO4 cell was developed based on charge and discharge plots at various rates. Step responses of the cell from seconds to hours were considered. An impedance element model was developed with components associated with the current collection grid, the active plate materials, and the electrolyte. Model elements may be approximated as resistors and capacitors; however some of the values of capacitor and resistor elements vary as a function of state of charge, load current, and temperature. Voltage dependant voltage sources were included in series with the bulk capacitor to model the nonlinear characteristics of the open circuit voltage at high and low states of charge. Constant current charge and discharge data were used to develop components of the open circuit voltage model and the total value of cell impedance as a function of current. Resistance components and pulse capacitance were determined using data from higher rate pulse test cycles. Charge and discharge plots at various temperatures in addition to cell temperature plots were used to develop the thermal model.
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