The advantages of lithium-based batteries over lead acid batteries have created great interest in developing safe and cost effective drop-in replacements. To achieve the required cost effectiveness and safety of the battery, Battery Management Systems (BMS) are critical to avoid over-charging, over-discharging, and continuously and accurately determining the State of Charge (SOC), State of Health (SOH), and State of Life (SOL) of the battery. In a program funded through a U.S. Army–TARDEC SBIR, the authors developed and tested a military-grade BMS that includes: (1) a Kalman Filter-based SOC estimation algorithm with better than 5% accuracy; (2) continuous cell monitoring to avoid over-charging or over-discharging; (3) active and passive cell balancing; (4) an innovative, low cost, and high-accuracy current sensing method; and (5) vehicle-level communication capability. Our BMS uses a modular, universal architecture that supports any lithium-based chemistry, pack size, or configuration. This is particularly important when multiple packs are series connected to achieve high voltage. This paper presents our design approach, test data which validate performance expectations, and our plans for integrating this BMS with an emerging class of 6T format batteries for U.S. Army tactical vehicle starting and Silent Watch energy storage needs.