The onset of experimental galvanostatic charge/discharge data of Li y FePO4 at low current density and at room temperature is analyzed using a single-particle mathematical model. The model contains only two adjustable parameters, namely one related to solid-state diffusion in the active particle and another one related to the surface resistance of the particle. The analysis reveals that these two parameters depend on the current density in a similar manner, meaning that there exists a correlation between them. An immediate consequence is that the onset of the experimental charge/discharge curves is properly modeled with the particle radius as a unique parameter depending on the current density. Hypotheses are made to shed light on this unusual dependence.