I was just studying physics and came across this in the textbook that “A cell produces its highest p.d. when not in a circuit and not supplying current.” Could you please explain this? If the cell is not in a circuit, it is not supplying energy to the charges to flow through anything as there is no circuit/conductor and since p.d. is defined as amount of potential energy (in joules) given to each coulomb of charge, potential energy should be 0. Right? 

[![The textbook](https://i.stack.imgur.com/K7Yw5.png)](https://i.stack.imgur.com/K7Yw5.png)