The current is subjected to resistance when passing through the inside of the battery, and the voltage of the battery is lowered, and this resistance becomes the internal resistance of the battery.
The internal resistance of the battery is not constant and changes over time during the discharge process because of the composition of the active material. The electrolyte concentration and temperature are constantly changing.
The internal resistance of the battery includes ohmic internal resistance and planned internal resistance, and the planned internal resistance includes electrochemical polarization and concentration polarization. The presence of internal resistance causes the terminal voltage at the time of discharge of the battery to be lower than the battery electromotive force and the open circuit voltage, and the terminal voltage at the time of charging is higher than the electromotive force and the open circuit voltage.
The ohmic resistance follows Ohm's law; the polarization resistance increases with increasing current density, but is not linear, and increases linearly with increasing logarithm of current density.