Distributed Control of Active Cell Balancing and Low-Voltage Bus Regulation in Electric Vehicles Using Hierarchical Model-Predictive Control

Abstract

Electric vehicle (EV) battery performance near end of life is limited by mismatched cell degradation, leading to an estimated 5-10% cell capacity variation across the pack. Active cell balancing hardware architectures incorporating a low-voltage (LV) bus supply have been introduced to unlock lost capacity due to cell imbalance at reduced cost, through elimination of the vehicle's 400-to-12 V dc-dc converter. In this article, a hierarchical model-predictive control scheme is applied to a time-shared isolated converter active balancing architecture that incorporates LV bus supply. The proposed controller efficiently divides computation among the battery management system (BMS) hardware components. The energy-buffering capability of the lead-acid battery, which is connected to the LV bus, is used to tradeoff balancing and bus regulation objectives, reducing peak power and improving the system cost-effectiveness. Simultaneous state-of-charge balancing and LV bus regulation is verified in simulation and experiment using real-world drive and LV load data collected from a GM Bolt EV. Similar controller performance compared to a central scheme is achieved in simulation. The experimental setup includes a custom 12S2P, 3.9 kWh, liquid-cooled Lithium Nickel Manganese Cobalt battery module with an embedded BMS. The controller performance is evaluated with an initial maximum state-of-charge imbalance of 6.8%.