The mass transport overvoltage has a significant impact on batteries, particularly at high rates of charge and discharge. As the battery discharges, it depletes the region around the electrode of some of the reactants. The concentration gradient between the region surrounding the electrode and further away in the electrolyte causes reactants to diffuse towards the electrode. However, if the discharge rate of the battery causes the reactants to be used at a greater rate than they can diffuse towards electrode, then the concentration near the electrode will continue to drop as the battery discharges. This drop in concentration is greater than that expected voltage drop if the reactants were uniformly distributed through the electrolyte and therefore, according to the Nernst equation, the battery voltage decreases more rapidly than that calculated by equilibrium. The more rapidly a battery is discharged, the more rapid the fall in voltage compared to that from equilibrium. Rapid discharging affects not only the battery voltage, but also battery capacity. Since the some of the reactants are not used in the reaction before the voltage drops below the minimum voltage, then the available battery capacity is also reduced.

Put in pix from battery book (graph)

During charging, a similar process occurs, except that charging increases the concentration surrounding the electrode. Consequently, a higher voltage is required to charge the battery than expected by equilibrium calculations. The mass transport overvoltage has a significant effect on the battery parameters relevant to a photovoltaic system. The lower voltages during discharge and higher voltages during charging reduce the battery efficiency. Further, mass transport effects alter the available battery capacity, as the battery capacity is reduced under high discharge rates. Because of these effects, mass transport has a significant impact on the optimal use of a battery, limiting both the charge and discharge currents.