In order for a cell or battery to be able to deliver electrical current to an external circuit, a potential difference must exist between the positive and negative electrodes. The potential difference (usually measured in volts) is commonly referred to as the voltage of the cell or battery. A single lead-acid cell can develop a maximum potential difference of about 2 V under load. A completely discharged lead-acid cell has a potential difference of about 1.75 V, depending on the rate of discharge.