Battery cells, modules and packs

Battery cells, modules and packs are the three layers from which a large battery is built. A cell is the smallest unit that stores energy, like a single cylinder or pouch. A module is a group of cells joined together and held in a frame. A pack is a group of modules, with cooling and safety systems and a computer, fitted into one housing.

Here is how they fit together in general. One cell gives only about 3 to 4 volts, so many cells are joined in series to raise the voltage and in parallel to raise the capacity. The cells are grouped into modules so that they can be handled, cooled and replaced in smaller parts. The modules are joined into a pack. A battery-management system (BMS), a small computer in the pack, measures the voltage and temperature of the cells, stops charging or discharging if something is wrong, and keeps the cells balanced.

The joins are counted in numbers. Say a pack has 96 cells in series, each at 3.7 volts. The voltage is about 355 volts. If four cells are joined in parallel at each position and each cell holds 5 ampere-hours, the capacity is 20 ampere-hours, and the pack stores about 7 kilowatt-hours. This is written 96s4p. Cells come in three shapes: cylindrical, pouch and prismatic. Modules make it possible to replace one failed part, and to stop a fire in one cell from reaching the others. Some newer designs leave out modules and fit cells directly into the pack, to save space.

Take, for example, an electric two-wheeler with a pack of about 3 kilowatt-hours. It contains a few hundred small cylindrical cells, grouped into modules inside one case. Consider an electric bus with a pack many times larger, built from the same layers. Imagine a grid battery for a solar plant, with thousands of cells arranged into module and rack layers; see site batteries.

Details are on the pages for battery cells, battery packs, battery intelligence and sodium-ion storage. Packs are charged through fast-charging packs, using DC power as explained in AC and DC.

How is it different from a power bank or a phone battery? Those are small packs with only a few cells. The layers matter when there are hundreds or thousands of cells that must be kept safe and balanced.