Battery and Battery Cell

What is the Battery Cell?

The electric battery cell is a device. It has two electrodes, immersed in the electrolyte. A battery cell produces electricity. The electricity in the battery is due to chemical reactions between electrodes and electrolyte.

What is an Electrolyte?

The electrolyte is an aqua solution. For example, the aqua solution of sulfuric acid is an electrolyte. The dilute sulfuric acid with the specific gravity from 1.25 to 1.27 is suitable as the electrolyte for the standard battery cell. The electric current can pass through an electrolyte. Hence, it is an electrical conductor. For examples, diluted sulfuric acid, diluted nitric acid, and other diluted acids are popular electrolytes.

What is Electrode?

Basically, an electrode is a terminal through which the electric current either enters or leaves the electrolyte. Always there are two types of electrodes. One is the positive electrode. Another is the negative electrode. In a battery, the electrode through which the electric current comes out form the electrolyte is the positive electrode. On the other hand, the electrode through which the current enters in the electrolyte is the negative electrode.

battery-cell

Here, in the above picture, we have shown the symbol of a battery cell. We can observe that the current is leaving from the positive terminal of the cell. Again, the same current is entering through the negative terminal of the cell. It is needless to say that, the positive terminal links with the positive electrode of the cell. At the same time, the negative terminal links with the negative electrode.

What is a Battery?

Electric cells are the main source of electrical energy. A battery is the combination of a number of such electric cells. We combine a number of battery cells to obtain the desired amount of electric energy.

Types of Battery Cell

There are two types of battery cell.

Such as

  • Primary cell
  • Secondary cell

Again, the primary cells have few categories. First, Voltaic cell come. After several developments, other types of primary cells came into the market. They were Daniel cell, Laclanche cell, Weston Cadmium Standard cell, and Mercury cell.

On the other hand, there are two types of secondary cells. These are Lead Acid cell, Nickel Alkaline cell.

What is Primary Cell?

The cells which are not reusable, are the primary cells. After using these battery cells once, we can not use these again. That means, we can not recharge these cells.

Duracell Battery Cells

What is Secondary Cell?

Secondary cells are rechargeable cells. That means we can recharge these battery cells after each cycle of use.

Simple Voltaic Cell

A simple voltaic cell consists of a glass jar filled with diluted sulfuric acid. We immerse one copper rod and one zinc rod partly in this diluted sulfuric acid electrolyte. When we connect the top of these two rods with a conductor, the current starts flowing through the conductor. This is because, as soon as we connect two electrodes (copper rod and zinc rod), electrochemical reactions start in the electrolyte.

simple voltaic cell

In voltaic cell, the copper rod acts as the positive electrode. So, during electrochemical reactions, the current comes out through this copper electrode. Here, the zinc rod acts as the negative electrode. So, the current enters in the electrolyte through this zinc electrode.

Electrochemical Reaction in Voltaic cell

In diluted sulfuric acid, H2SO4 molecules exist in (2H+ + SO42-) form. These SO42- ions react with zinc and form zinc sulfate (ZnSO4). As a result, the sulfate ions give up electrons to the zinc rod. H+ ions absorb electrons from the copper rod and become pure hydrogen gas. So, the concentration of electrons becomes unbalanced between copper and zinc rod. As a result, there is a voltage difference between the electrode rods. This voltage difference is 1.1 volt for the standard voltaic cell.

Emf of a Battery Cell

When we measure the terminal voltage of a battery cell without connecting any load across it, we get the emf of the cell. That means emf is the no-load voltage of a battery cell. The cell creates this electromotive force in it.

battery cell emf

Potential Difference of a Battery Cell

This is the terminal voltage of a loaded battery cell. This means, this is the voltage we measure at load condition of the cell. Actually, there is always some internal resistance in a practical battery cell. When we connect the cell with any closed circuit, the current starts flowing through the cell itself.

potential difference of battery cell

As a result, there must be some resistive drop in the battery cell itself. So, the closed circuit terminal voltage of a cell is less than the open circuit terminal voltage. Again, the open circuit terminal voltage of a battery cell is its emf. So, we can write,

Potential Difference = EMF – Internal Resistive Drop

Internal Resistance of a Battery Cell

Each of the battery cells has its internal resistance. In a cell, the current flows from one electrode to the other. During this flow of the current, the electrolyte provides resistance, to the current. We call this resistance as the internal resistance of the cell. This resistance depends on the size and shape of the cell. Actually, if the distance of the electrode plates becomes less, the internal resistance also becomes less. Besides, if the active area of the electrode plates is more, also the internal resistance becomes less. In other words, the internal resistance of a battery cell depends on the portion of the electrode plates immersed in the electrolyte.

A cell with low internal resistance is more efficient than that with higher internal resistance. We generally measure the emf of a battery cell with the help of a voltmeter. Although, the measurement with a potentiometer gives a more precise result. We take this measurement when no load is connected with the battery cell. But when we connect the cell with a load, the terminal voltage of the cell reduces. This is because, at this condition, the cell supplies the current to the load circuit.

Now if we measure the terminal voltage at this loaded condition, we get the potential difference of the cell. So the voltage drop in the cell is the difference between emf and potential difference. Now, if the current supplied by the battery is I. We can measure this current by connecting an ammeter in the output circuit of the battery cell. Again, we consider the internal resistance of the cell is R. Hence, the voltage drop occurs in the battery cell is IR.

Therefore,

battery-cell

Where V is the potential difference of the cell. E is the emf of the cell.

internal resistance of battery cell

What is the Local Action of a Battery Cell?

It is next to impossible to obtain a 100% pure metal. So, there may always some other metals present in the zinc or any other metallic electrode. These metallic impurities, form bimetallic small cells in the electrode body. So there always some electrochemical reactions occur in a cell even at no load condition. This phenomenon of a normal battery cell is known as local action.

What is the Polarization of a Battery Cell?

When the battery is in service, the copper electrode produces the hydrogen gas. Actually, positive hydrogen ions come to in the contact of the copper electrode. Here, the ions receive electrons. Hence the ions become pure hydrogen gas. The entire hydrogen produced here may not be able to form bubbles to escape from the liquid electrolyte. Because of that, there may be a thin layer of hydrogen accumulated on the copper electrode. As a result, the flow of current is reduced in the cell. Ultimately, the current through the cell may become zero. We call this phenomenon as the polarization of the cell. To reduce this polarization effect, we use manganese dioxide, nitric acid, potassium dichromate, etc.

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