The electro-neutrality assumption for Li-ion battery electrolytes has been mathematically investigated. A general system of equations describing the development of ionic concentration gradients in simple binary electrolytes is derived. Considering that the overpotentials and the concentration gradients across the electrolyte are most dominant under steady-state (dis)charging, the analyzes have been performed under these limiting conditions. It was found that deviations from electro-neutrality in terms of ionic concentrations are extremely small and do not exceed 10-7% under normal (dis)charging conditions. Nevertheless this small deviation is responsible for development of an electric field with considerable magnitude. Therefore, a rearrangement of standard Nernst-Planck equations is essential to accurately model these processes at different scales.