Equations that describe the voltage variations with time of rechargeable batteries during charging and discharging measurements are used to determine the component values of the equivalent circuit of lithium ion batteries as the battery state of charge (SOC) changes from 100% to 40% SOC. The battery is described as an ideal voltage source in series with a resistor and the parallel combination of a resistor and a capacitor. The model uses different values of resistance and capacitance, in the parallel combination, during the different phases of the discharge-rest–charge-rest sequence. For the 3.7 V 900 mA-hour camera battery resistances vary from 0.01 Ω to 0.39 Ω and capacitances from 1.03 F to 16.23 F. The resistances are higher at 40% SOC than at 100% SOC. For both the charge and discharge phases the capacitor value decreases as the SOC decreases. The component values are useful in the design of circuits driven by the lithium ion battery.