A mathematical model is derived for a Li-ion battery wherein there is a number of additives in the electrolyte that are in low concentration enough so that only interactions between an additive and the solvent are accounted for in the multicomponent diffusion equations. The additive is either neutral or charged, and either wanted (e.g., redox shuttle) or unwanted (i.e., contaminant). Just like in Newman's battery model (Dualfoil), the electrolyte is treated as being composed of a single solvent although most Li-ion battery electrolytes are made up of a mixture of solvents. However, unlike in Dualfoil, cation and anion concentrations of the salt are not necessarily equal. The model is applied to a cell sandwich that contains a cationic redox shuttle as an example. It is then compared with an approximated model where charged minor species are neglected in both the electroneutrality and ionic current equations. There is a fair agreement between the simulations of both models in the conditions explored, except for the concentration of lithium ions. (C) 2013 The Electrochemical Society. All rights reserved.