Back to free energy. While electrons are doing their work only some of the cell energy is being accessed under normal operating conditions. The rest is typically lost as heat which increases the entropy of the surroundings. This continues until the balance is restored between the tendency for low enthalpy and high entropy. At that point the system is at equilibrium and the free energy available is 0. For a galvanic cell that means no more voltage produced: a dead battery.

Because the E^o value represents the potential for the reaction to move toward equilibrium and also is a measure of free energy it should be possible to connect all three concepts: DG^o = -RT ln K_c = -nFE^o

This expression looks at once scary and very powerful. However it suffers from a few limitations. First and foremost is the fact that most of these reactions go very nearly to completion or not at all. So values for K_c tend to be either enormous or miniscule.