The charge and discharge characteristics of lead battery:

During discharge the plates are converted into lead sulphate, the water content of the electrolyte increases, the internal resistance of the cell increases and the terminal voltage decreases.

By passing a current through the battery in the opposite direction these effects are reversed. The plates are converted back to their original form, the water content of the electrolyte decreases, the internal resistance decreases and the terminal voltage increases. The process of recharging takes approximately 8 to 10 hours.

During most of the charge and discharge cycle the battery terminal voltage remains constant at 1.95V, it therefore gives no indication as to the battery's state of charge.

The specific gravity of the electrolyte however changes at a regular rate as the battery is charged, or discharged and can therefore be used to determine the battery's state of charge.