2.1 Battery modules; cells, modules, racks, and why lithium-ion dominates

The physical energy storage hierarchy

The energy in a BESS is stored in a hierarchy of physical components. At the base are individual battery cells — cylindrical, prismatic, or pouch format depending on the supplier. Cells are grouped into modules, which handle a defined voltage and capacity range and include cell-level monitoring connections. Modules are stacked into racks, which are the structural and electrical units you will see when a container door is opened. Racks connect to the container-level BMS and to the DC bus that feeds the PCS.

Lithium-ion dominates utility-scale BESS for three reasons. Cost has dropped dramatically over the past decade driven by electric vehicle manufacturing scale. Cycle life is sufficient for the daily cycling that utility-scale applications require — a well-managed LFP system can deliver thousands of cycles over a twenty-year project life. And the supply chain and manufacturing infrastructure for lithium-ion is mature enough to support the gigawatt-scale deployments the market now requires.

The specific lithium-ion chemistry you will most commonly encounter at utility scale is Lithium Iron Phosphate, or LFP. The chemistry lesson covers that in detail. What matters here is that the physical hierarchy — cell, module, rack, container — is consistent across suppliers even when the form factors and specifications differ.