The large-scale deployment of stationary battery storage is critical for enabling renewable energy integration, yet life cycle assessments (LCAs) of these systems often overlook the contributions of balance-of-system (BOS) components. This study establishes a life cycle inventory (LCI) model for two container storage systems (CSSs), a liquid-cooled and an air-cooled system. Consequently, a full LCA is carried out with four different cell types, focusing on sodium-ion batteries (SIBs). The resource and environmental impacts of all BOS subsystems, including containers, thermal management systems (TMSs), power converters, control systems and auxiliary components, are investigated in detail. The results show that BOS components contribute between 32 and 58% of impacts in global warming potential (GWP) and 63–88% in resource use, minerals and metals, emphasizing their significant role in the sustainability of grid-scale storage. Due to its high demand for copper and steel, a transformer is particularly important in this regard. However, the environmental impact attributable to its production can be reduced through correct dimensioning and adequate recycling. It is further shown that SIBs are competitive with lithium-ion batteries in CSSs with regard to environmental impacts, despite their lower energy densities. By shedding light on previously underexplored system elements, this work provides a robust foundation for more comprehensive LCAs of containerized battery storage solutions and enables more informed design, scaling, and policy decisions for future energy systems.