A measurement to sum up all internal resistance in a battery.
Each voltage source has an internal resistance. In detail, the internal resistance is composed of the polarisation resistance of the electrochemical reaction (the energy required for the chemical process), the flow resistance of the ions and the ohmic resistance of the electrodes. The electrical equipment therefore does not receive the entire terminal voltage, as a part of it is lost in the internal resistance of the battery. If the current flow increases, the voltage drop at the internal resistance grows and the voltage decreases.