As a battery cell discharges, what happens to its internal resistance?

As a battery cell discharges, its internal resistance increases. This occurs due to several factors such as the depletion of active materials within the battery, the buildup of reaction products that can inhibit the flow of charge, and changes in the electrolyte properties. As the chemical reactions that generate electrical energy progress and the battery's state of charge diminishes, the battery's ability to deliver current efficiently is reduced, resulting in higher internal resistance.

In a typical discharge cycle, as the battery’s capacity decreases, the energy barriers for ion movement increase, contributing to the rise in internal resistance. Therefore, the relationship between internal resistance and the state of charge is significant, as it affects the performance and efficiency of the battery.