The battery voltage described by the Nernst Equation and battery capacity assumes that the battery is in equilibrium. Since a battery under load is not in equilibrium, the measured voltage and battery capacity may differ significantly from the equilibrium values, and the further from equilibrium (ie the high the charge or discharge currents), the larger the deviation between the battery voltage and capacity equilibrium and the realistic battery voltage may be. The difference between the voltage under equilibrium and that with a current flow is termed polarization. Polarization effects have a significant impact on battery operation, both beneficial and detrimental. For example, polarization effects mean that under normal operation of lead acid batteries the electrolysis of water proceeds slowly and to first order can be neglected during discharge (but not charging since the voltage is higher). However, polarization effects also have detrimental effects on performance, by, for example, reducing efficiency and making the battery capacity sensitive to charging and discharging conditions.

The polarization is comprised of three basic mechanisms, relating to resistive drops in the battery, and to two effects relating to the rates at which a reaction can proceed. These two effects are due to kinetic effects caused by the inherent rates of the chemical reaction (called kinetic overvoltage or activation overvoltage), and by the effects related to the movement of reactants to the electrode (called mass transport overvoltage).  The overvoltage causes a deviation of the voltage and capacity from the equilibrium values calculated earlier. As shown below, during discharging, the battery voltage is lower than that in equilibrium, while during charging, a higher voltage than the Nernst voltage is required. Polarization effects have significant impact on the battery efficiency and how the battery can be charged and discharged.

Cell potential from equilibrium and including polarization effects.