 Internal resistance

An effective source resistance in a specified circuit model: a source with electromotive force $\mathcal{E}$ drives $I=\mathcal{E}/(R_s+R_L)$ through a resistive load $R_L$. A biological source or electrochemical interface can require a frequency-dependent or nonlinear model instead.

Internal resistance refers to the opposition to the flow of electric current within a power source, such as a battery or a fuel cell. It occurs due to various factors, including the chemical reactions occurring inside the battery, the physical properties of the materials used in the battery, and any ionic conductivity limitations within the electrolyte. When a current flows from a power source, internal resistance causes some of the voltage to be lost as heat instead of contributing to the output voltage available to an external load.