Novel MMC control for active balancing and minimum ripple current in series-connected battery strings

The performance of a string of series-connected batteries is typically restricted by the worst battery cell in the string and a single failure point will render the entire string unusable. To address these issues, we present a decentralised battery management system based on a modular multilevel converter (MMC) with a distributed inductor. This novel MMC design enables each MMC cell to operate autonomously, without central control, by sensing the local inductor voltage. Batteries are loaded with a current proportional to their capacity, and the MMC cells also work together to minimize the output ripple voltage of the full series stack. We show experimentally that the novel decentralized controller implemented in each MMC cell can reduce the output voltage ripple by 69%. We also give an extensive theoretical analysis of the waveforms produced by the MMC using Fourier decomposition. This analysis has applications in all modular multilevel converters.