A cell produces its maximum PD when not in a circuit and not supplying current.
Voltage is the amount of energy the cell gives the electrons it pushes out. Voltage is measured in volts (V) and is measured by a voltmeter (connected in parallel).
If a cell has 1 Volt, it delivers 1 Joule of energy to each coulomb of charge (J/C).
The formula for voltage is: Voltage=ChargeEnergy or V=CE