Voltage of lead acid battery upon charging. The charging reaction converts the lead sulfate at the negative electrode to lead. At the positive terminal the reaction converts the lead to lead oxide. As a by-product of this reaction, hydrogen is evolved.
The battery should not, therefore, be discharged below this voltage. In between the fully discharged and charged states, a lead acid battery will experience a gradual reduction in the voltage. Voltage level is commonly used to indicate a battery's state of charge.
A lead acid battery consists of a negative electrode made of spongy or porous lead. The lead is porous to facilitate the formation and dissolution of lead. The positive electrode consists of lead oxide. Both electrodes are immersed in a electrolytic solution of sulfuric acid and water.
The anode is transformed into lead peroxide (PbO 2) and cathode into the spongy lead (Pb). Water is consumed and sulphuric acid is formed which increases the specific gravity of electrolyte from 1.18 to 1.28. The terminal voltage of each battery cell increases to 2.2 to 2.5V.
The charge time is 12–16 hours and up to 36–48 hours for large stationary batteries. With higher charge currents and multi-stage charge methods, the charge time can be reduced to 8–10 hours; however, without full topping charge. Lead acid is sluggish and cannot be charged as quickly as other battery systems. (See BU-202: New Lead Acid Systems)
The following are the indications which show whether the given lead-acid battery is fully charged or not. Voltage : During charging, the terminal voltage of a lead-acid cell When the terminal voltage of lead-acid battery rises to 2.5 V per cell, the battery is considered to be fully charged.