In this article the Butler-Volmer equation used in describing Lithium-ion (Li-ion) batteries is discussed. First, a complete mathematical model based on a macro-homogeneous approach developed by Neuman is presented. Two common mistakes found in the literature regarding a sign in a boundary conditions and the use of the transfer coefficient are mentioned. The paper focuses on the form of the Butler-Volmer equation in the model. It is shown how practical problems can be avoided by taking care in the form used, particularly to avoid difficulties when the solid particle in the electrodes approaches a fully charged or discharged state or the electrolyte gets depleted. This shows that the open circuit voltage and the exchange current density must depend on the lithium concentration in both the solid and the electrolyte in a particular way at the extremes of the concentration ranges.