It is overcomming the G of the reaction with the electical voltage from the charger. That is, we are considering ideal situations and they are rarely achieved. The other day I put out a formula relating G change and electrochemical work. Well, you remembered the right formula with the long number and here is another one. Note that RT has energy units and LnK is dimensionless.
G has energy units.
Both these are OK and in fact we could equate them. This connects the equilibrium constant with the electrochemical potential for a reaction. We are really down to fundamentals here. Yes, the max useable eneryg. Yes, about the fuels.