A wet cell battery, often referred to as a “flooded cell” battery, is a type of secondary, or rechargeable, battery that utilizes a liquid electrolyte to facilitate the electrochemical reaction. The most common example is the lead-acid battery, which provides the high surge current necessary for starting internal combustion engines in automobiles. These batteries also see heavy use in backup power supplies and large-scale energy storage systems due to their reliability and relatively low cost.
Essential Physical Structure
The functional core of a wet cell battery is contained within a durable, acid-resistant plastic casing, typically polypropylene, that prevents leakage of the corrosive internal components. Inside this casing, the energy-storing elements are submerged in a liquid electrolyte, which is a solution of sulfuric acid (\(\text{H}_2\text{SO}_4\)) diluted with water. The concentration of this acid mixture is a factor that determines the battery’s voltage and capacity.
The battery’s internal structure consists of alternating positive and negative plates, which serve as the electrodes. The positive plate, or cathode, is composed of a lead grid coated with a paste of lead dioxide (\(\text{PbO}_2\)). Conversely, the negative plate, or anode, is made from a porous form of pure lead (\(\text{Pb}\)). These plates are arranged in groups and connected in series, with six individual cells typically creating a standard 12-volt automotive battery.
To prevent a short circuit, an insulating material known as a separator is placed between each positive and negative plate. This separator is porous, allowing the sulfuric acid electrolyte to flow freely between the plates, ensuring continuous contact for the chemical reactions. The arrangement of these chemically distinct plates immersed in a conductive liquid creates an electrical potential difference, which is the stored energy ready for release.
The Discharge Cycle: Generating Power
When the battery is connected to an external load, the discharge cycle begins, converting stored chemical energy into usable electrical energy. This process is driven by a spontaneous oxidation-reduction reaction that occurs simultaneously at both the positive and negative plates. The overall chemical reaction is often referred to as a “double sulfation” process.
At the negative plate, the spongy lead (\(\text{Pb}\)) reacts with the sulfate ions (\(\text{SO}_4^{2-}\)) from the electrolyte, which causes the lead to oxidize and release two electrons. This chemical change forms lead sulfate (\(\text{PbSO}_4\)) and is the source of the electrons that flow out of the battery and through the external circuit. The formation of lead sulfate is what reduces the available surface area of the active material over time.
The electrons then travel through the external circuit and enter the positive plate. Here, the lead dioxide (\(\text{PbO}_2\)) reacts with hydrogen ions (\(\text{H}^+\)) and the incoming electrons, also forming lead sulfate (\(\text{PbSO}_4\)) and water (\(\text{H}_2\text{O}\)). This entire process consumes sulfuric acid from the electrolyte as the sulfate ions are chemically bound to the plates. The electrolyte thus becomes less concentrated and more diluted with water.
The flow of electrons from the negative plate, through the load, and to the positive plate constitutes the electrical current that powers the device. As the reaction continues, the voltage gradually drops because the active materials on the plates are progressively converted into lead sulfate. When both plates are covered in lead sulfate and the electrolyte is significantly diluted, the battery can no longer deliver sufficient power and is considered discharged.
The Charging Cycle: Reversing the Flow
The wet cell battery’s rechargeable nature stems from its ability to reverse the chemical process that occurs during discharge. When an external power source, like an alternator or a battery charger, is applied, it acts as an electrolytic cell, forcing a current back into the battery. This external electrical energy is converted back into chemical potential energy.
The applied current reverses the sulfation process at both electrodes. At the negative plate, the electrical energy converts the lead sulfate back into spongy lead (\(\text{Pb}\)) and regenerates the sulfate ions, which return to the electrolyte. Simultaneously, at the positive plate, the charging current converts the lead sulfate into lead dioxide (\(\text{PbO}_2\)), also regenerating sulfate ions and releasing hydrogen ions.
As the chemical reactions reverse, the concentration of sulfuric acid in the electrolyte increases, and the water content decreases, restoring the battery to its charged state. This regeneration of the original active materials and the electrolyte is what allows the battery to be used repeatedly.
Near the end of the charging cycle, when most of the lead sulfate has been converted, a secondary reaction begins. If the charging voltage is too high, the electrical energy will begin to electrolyze the water in the electrolyte, splitting it into hydrogen gas (\(\text{H}_2\)) and oxygen gas (\(\text{O}_2\)). This phenomenon, known as gassing, can lead to water loss from the electrolyte, which is why traditional wet cell batteries require occasional topping off with distilled water.