Volt (V), short for voltage, is a measure of electrical force or electrical pressure. The terminal voltage, measured in volts, of a battery is the electrical potential difference across its two terminals. Battery cells of different chemistries have different voltages. A nickel-based cell, like NiCd and NiMH, provides a nominal cell voltage of approximately 1.2 V and a Li-ion cell provides 3.6V/3.7V.
The voltage of two battery cells, of the same chemistry, is always the same. Their capacities may be different, but the voltage always remains the same. Higher terminal voltages are obtained by assembling two or more cells in a series, parallel or a mixture of both. For example, a 7.4V, Li-ion battery would be made up of 2 cells connected in a series.
2 cells x 3.7V Li-ion cell = 7.4V Li-ion battery
It is important that the voltage of a cell or a battery pack always matches the voltage requirement of the device in which it is used. If there is any mismatch of the two voltages, the device will not function. Excessive exposure to a different voltage may also damage the equipment. Equipment users should take special care that the batteries they are using matches the voltage requirement of their device.