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A simplified electrochemical model is developed as a rapid tool to describe and maximize the specific capacity of Li+ ion cell fabrication through the variation of active (LiFePO4)/inactive (carbon conductor and binder) ratios. The model is able to capture the cooperation arising between liquid and solid phases determining cell performance. Thus, the capacity of first-principle models is extended for the first time to not only account for C-rates, but also electrode composition. The simplified nature of this model is obtained through the correct application of integral current balances, theoretical equations to adjust the conductivity of the electrolyte phase, and material balances in both the electrolyte phase and the solid phase of the electrode agglomerates. The model demands very low computational costs, whereby it can be efficiently solved in a spreadsheet. Simulations of experimental discharge plots reveal that the model adequately describes the cell behaviors at different composite contents and C-rates, using a minimum number of kinetic and transport parameters. The magnitudes of these parameters controlling cell performance depending on electrode composition are discussed.