A lead acid battery cell consists of three main components. These are two electrodes and the electrolyte. The electrodes are of rectangular plate shape. For this reason, we call positive electrodes as positive plates and negative electrodes as negative plates.
A container contains dilute sulfuric acid of specific gravity 1.20 to 1.27. The negative and positive plates are alternatively immersed in this acid. There are numbers of such negative and positive plates in the construction of lead acid battery. All positive plates are electrically connected together. All negative plates are also electrically connected together. The number of negativity plates is always greater than the number of positive plates by one. This is because of the maximum utilization of active materials during electrolysis. Since these plates are the vital parts in the construction of lead acid battery cells, we should discuss the construction of plates in details further.
Formation of Plates
There are generally two methods by which we can produce electrode plates for a lead acid battery. These methods are
- Plante a Plate Formation
- Faure a Plate Formation
In this process, we immerse two sheets of lead in dilute sulfuric acid. Then we connect these two plates with an external electric source. Due to electrolysis water splits up in hydrogen and oxygen ions. At anode (that plate is connected to the positive terminal of electric source) oxygen ions attack the lead and convert it into lead peroxide (PbO2). But cathode remains unaffected because hydrogen cannot react with pure lead (Pb).
Now we alter the terminal of electric source and charge the system in the opposite direction. In this condition, hydrogen ions react with PbO2 of the positive plate and produce water and pure lead (Pb). At the same time, oxygen goes to negative plate and form lead peroxide (PbO2). Hence, lead peroxide (PbO2) gradually covers the entire negative plate with this thin layer.
Again we charge the system by connecting with external electric source. Then we discharge it again. Consequently the above process of electrolysis repeats. Hence, there is another thin layer of lead peroxide (PbO2) on the same plate. If we continue to do the layer lead peroxide becomes thicker. Therefore positive plate of PbO2 forms. We can make negative plates in the same process but we do not normally use the plante process for making negative plates.
Structure of Plante Plate
The active materials of plante plates consist of a thin layer of PbO2. But it is always desirable to have a larger surface area of the plate to get a significant volume of active material. We can increase the superficial area of the plante plate by grooving and laminating. A plante positive plate generally consists of the grid-like structure.
The construction of this place consists of thin vertical lamination. Such design increases the superficial area by 10 or 12 times than that of a plante sheet of same overall dimension.
Here we mechanically paste active material on the electrode plate. We take grid or lattice type structure of plates. Then we mechanically press the active materials such as red lead (Pb3O4) and litharge (PbO) on the group of plates. After that, we dip both groups of plates in dilute sulfuric acid and supply electricity to them externally. We connect the plates of Pb3O4 with the positive terminal of the external source. At the same time, we connect the plates of PbO with the negative terminal of the source. Due to electrolysis water splits up into hydrogen and oxygen ions. The oxygen then attacks the positive plates of Pb3O4 and produces PbO2. The hydrogen ions come to negative plates of PbO and form sponge lead and water. Therefore positive PbO2 plate and negative sponge lead plate are formed.
Internal Resistance of Lead Acid Battery Cell
When a battery cell supplies current, there is always a potential drop across it. This is due to the internal resistance of the cell. Hence, to improve the voltage output of the cell we always keep the internal resistance as low as possible.
Obliviously, by increasing the size of the plates, we can decrease the internal resistance of the cell. But the increased size of the plates causes the cell bigger. Also, it increases weight. Therefore there must be a limit up to which we can practically increase the size of the plates.
We join all the negative plates together. Similarly, we join all the positive plates together in the cell. Joining of all negative plates together and joining of all positive plates together separately effect as the joining of many cells in parallel. So this parallel arrangement reduces the internal resistance of the cell. Also, this arrangement brings alternative negative and positive plates close to each other. Therefore the gap between one positive plate and its adjacent negative plates becomes quite short. Also, this reduces the internal resistance further.
Capacity of Lead Acid Battery Cell
A standard lead acid battery cell is supposed to supply a minimum of 1.8 volts. That means we take its output until the voltage falls to 1.8 volts. The capacity of the cell is nothing but the energy, that the battery can give before it reaches the minimum voltage level of 1.8 volts. We express this output capacity of the battery in Ampere Hour. In other words, it means that the capability of the battery to give a certain amount of current for a certain time. For example, 10 Ampere-hour (Ah) for 10 hours means the battery cell can supply 1 Ampere current for 10 hours before it reaches its minimum e.m.f of 1.8 volts.
The alternate placing of plates in a lead acid cell also increases this capacity along with decreasing internal resistance.
If there is n number of positive plates, there is (n+1) number of negative plates. This is because the negative plate occupies both ends of the assembly. That means there is a negative plate at both ends of the assembly. This arrangement increases the maximum utilization of active material of positive plates. So far we have seen that the position of negative and positive plate in the construction of lead acid battery cell is quiet close. So, we must arrange something so that the negative and positive plates do not physically touch each other. Otherwise, an internal short circuit may take place. We can solve this problem by inserting a porous insulated separator sheet in between the plates.
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