3.5 Series and Parallel Configurations
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.
There has been a trend towards lower voltage batteries for light portable devices, such as mobile phones. This was made possible through advancements in microelectronics. To achieve the same energy with lower voltages, higher currents are needed. With higher currents, a low internal battery resistance is critical. This presents a challenge if protection devices are used. Some losses through the solid-state switches of protection devices cannot be avoided.
Packs with fewer cells in series generally perform better than those with 12 cells or more. Similar to a chain, the more links that are used, the greater the odds of one breaking. On higher voltage batteries, precise cell matching becomes important, especially if high load currents are drawn or if the pack is operated in cold temperatures.
Parallel connections are used to obtain higher ampere-hour (Ah) ratings. When possible, pack designers prefer using larger cells. This may not always be practical because new battery chemistries come in limited sizes. Often, a parallel connection is the only option to increase the battery rating. Paralleling is also necessary if pack dimensions restrict the use of larger cells. Among the battery chemistries, Li-ion lends itself best to parallel connection.