Introduction:

A battery is an electrical element where electrical potential is generated due to chemical reactions. Each electrochemical reaction has its own threshold for creating an electrical potential difference between two electrodes.
Battery cells are where this electrochemical reaction takes place to create a finite electrical potential difference. Several cells are to be connected in series to achieve the required electrical potential difference across the terminals of the battery. So it can be concluded like this, a battery is a collection of several cells where a cell is a unit of a battery. For example, nickel-cadmium battery cells typically produce about 1.2 V per cell while lead-acid batteries produce about 2 V per cell. So a 12 volt battery will have a total of 6 cells connected in series.

EMF of Battery:

If one simply measures the potential difference between the two terminals of the battery when no load is connected to the battery, one will produce a voltage across the battery when no current is flowing through it. This voltage is commonly called the electromotive force or emf of the battery. It is also called the no-load voltage of the battery.

Terminal Voltage of Battery:

The terminal voltage of a battery is the potential difference across its terminals when current is being drawn from it. Actually when the load is connected to the battery, the load current will flow through it. Since a battery is an electrical device, it must have some electrical resistance inside it. Because of this internal resistance of the battery, it will have some voltage drops. So, if one measures the terminal voltage of the load i.e. the terminal voltage of the battery when the load is connected, one will get a voltage which is less than the emf of the battery through the internal voltage drop of the battery.

If E is the emf or no-load voltage of the battery and V is the terminal voltage of the load voltage of the battery, then E – V = internal voltage drop of the battery.
According to Ohm’s law, this internal voltage drop is nothing but the product of the electrical resistance offered by the battery and the current flowing through it.

Internal Resistance of Battery:

The total resistance encountered by the current is known as the internal resistance of the battery as it flows through the battery from the negative terminal to the positive terminal.

Series Parallel Batteries:

Battery cells can be connected in series and parallel as well as a combination of both series and parallel.

Series Batteries:

When in a battery, the positive terminal of one cell is connected to the negative terminal of the next cell, the cells are called series connected or simply series battery. Here, the total emf of the battery is the algebraic sum of all the individual cells connected in series. But the overall discharge current of the battery is not more than the discharge current of the individual cells.

Parallel Batteries:

When in a battery, the positive terminal of one cell is connected to the negative terminal of the next cell, the cells are called series connected or simply series battery. Here, the total emf of the battery is the algebraic sum of all the individual cells connected in series. But the overall discharge current of the battery is not more than the discharge current of the individual cells.

Mixed Grouping of Batteries or Series Parallel Batteries:

As we said earlier, battery cells can also be connected in a combination of both series and parallel. This combination is sometimes referred to as a series-parallel battery. A load may require both higher voltage and current than an individual battery cell. To achieve the required load voltage, the required number of battery cells can be connected in series and to achieve the required load current, the required number of these series combinations are connected in parallel. Let m be the number of series, each containing n numbers of identical cells, connected in parallel.

Again assume that the emf of each cell is E and the internal resistance of each cell is r. Since n number of cells are connected in each series, the emf of the battery along each series will also be nE. The equivalent series resistance is nr. As, m number of series connected in parallel equivalent internal resistance of this series and parallel battery is nr/m.