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A simple active battery management system for lithium ion battery stacks is described, that is robust and scalable. The architecture is based on an isolation unit consisting of a small equal turns ratio high isolation transformer with two diodes. One isolation unit is connected to each series connected cell in the battery stack and enables both accurate cell voltage monitoring and active cell balancing. The output of the isolation unit is completely galvanically isolated and thus suitable for high voltage packs with stringent safely requirements such as automotive and industrial electrical vehicles. Detailed measurements results are presented and a complete prototype for a LiFePO4 16s2p 2kWh battery module is constructed and its performance measured.