What happens to the voltage as you increase the number of cells or batteries?

Batteries have different volts because of several reasons. First they differ in size, the bigger the size the more electricity they can store and supply. Second they are made of different types of materials, each type has its own capacity of generating voltage.

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The output voltage depends on the materials which have been used to make the cell. The cells in a car battery produce an output voltage of around 2 volts. Lithium cells produce around 3 volts.

For more information see the answer to the Related Question shown below.