An isothermal reduced order physics-based Li ion battery model is developed by incorporating solution phase charge and material balances into single particle (SP) model. Li ion concentration and potential profiles in electrolyte phase are approximated by polynomial functions. A cubic polynomial is used for electrolyte concentration and potential inside electrodes, while separator liquid phase potential and concentration are estimated by parabolas. Diffusion inside solid particles is also simplified using an approximate solution based on analytical solution for the solid concentration. Applying the aforementioned reduction techniques decreases the degree of freedom of full order model by more than 100, while the cell voltage relative error of proposed model is less than 1% at different charge–discharge rates up to 5C.