The behavior of battery voltage with respect to the time in hours of charging or discharging at normal rate is indicated by the curves called charge and discharge curves.
During discharge of the lead acid cell, the voltage decreases from about 2.1 V to 1.8 V, when cell is said to be completely discharged. The discharge rate is always specified as 8 hours, 10 hours etc.
During charging of the lead acid cell, the voltage increases from 1.8 V to about 2.5 V to 2.7 V, when cell is said to be completely charged. If the discharge rate is high, the curve is more drooping as voltage decrease faster. Such typical charge and discharge curves for lead-acid cell are shown in the Fig.1. while discharging the voltage decreases to 2 V very fast, then remains constant for long period and at the end of discharge period falls to 1.8 V. while charging initially it rises quickly to 2.1 to 2.2 V and then remains constant for long time. At the end of charging period it increase to 2.5 to 2.7 V.
