An observer is designed for the Doyle-Fuller-Newman electrochemical model of a Li-ion battery under the assumption of constant ion exchange current density and MacInnes’ equation being concentration independent. For this nonlinear system, the observer is shown to experience both exponential convergence and a degree of robustness. This result extends the class of electrochemical model for battery observer design, generating improved estimates for the state-of-charge and for the spatial variation of the overpotential across an electrode.