PolyStor has teamed with Hawker Eternacell (US) to develop a BB-2590/U rechargeable lithium-ion battery under contract with the US Army CECOM (Ft. Monmouth, NJ, USA). The concept involves using commercially available ICR-18650 cylindrical lithium-ion cells. The individual cells have a high specific energy of 135 Wh kg(-1) and an energy density of 335 Wh dm(-3). Electronic circuitry was developed to provide pack protection, charge equalization and battery management (fuel gauging). PolyStor＇s rechargeable BB-2590/U battery provides 4.5 Ah at 28 V nominal or 9.0 Ah at 14 V nominal, translating into 108 Wh kg-l and 150 Wh dm(-3). The key developments are discussed in this paper.