The authors developed a physics-based equivalent circuit model of a lithium-ion battery (LIB) whose parameters are continually updated, reflecting the theoretical calculation results of the Butler-Volmer equation, diffusion equations of the lithium-ion and lithium, and Nernst equations of the liquid and solid phases. The developed model was applied to the charge/discharge simulations of a LIB, and the experimental and simulated results of constant current discharges and pulsed-charge/discharge were found to be in excellent agreement.