In most cases, a single cell does not provide a high enough voltage and a serial connection of several cells is needed. The metallic skin of the cell is insulated to prevent the 'hot' metal cylinders from creating an electrical short circuit against the neighboring cell.
Nickel-based cells provide a nominal cell voltage of 1.25V. A lead acid cell delivers 2V and most Li-ion cells are rated at 3.6V. The spinel (manganese) and Li-ion polymer systems sometimes use 3.7V as the designated cell voltage. This is the reason for the often unfamiliar voltages, such as 11.1V for a three cell pack of spinel chemistry.
Nickel-based cells are often marked 1.2V. There is no difference between a 1.2 and 1.25V cell; it is simply the preference of the manufacturer in marking. Whereas commercial batteries tend to be identified with 1.2V/cell, industrial, aviation and military batteries are still marked with the original designation of 1.25V/cell.
A five-cell nickel-based battery delivers 6V (6.25V with 1.25V/cell marking) and a six-cell pack has 7.2V (7.5V with 1.25V/cell marking). The portable lead acid comes in 3 cell (6V) and 6 cell (12V) formats. The Li-ion family has either 3.6V for a single cell pack, 7.2V for a two-cell pack or 10.8V for a three-cell pack. The 3.6V and 7.2V batteries are commonly used for mobile phones; laptops use the larger 10.8V packs.
