Title: Modeling and Design of A Cost-Effective Redistributive Dual-Cell Link Battery Balancer for Electrical Vehicle Applications
Description: The electric vehicles, as the most promising solution for achieving high fuel economy, have significantly better emission profile than conventional vehicles powered by fossil fuels. However, range anxiety and the limited accessible fast-charging infrastructures mainly restrain the drivers from adopting the electric vehicles that have much higher energy efficiency. Due to the internal and external factors, the cells in the battery pack degrade differently, leading to a usable capacity that is less than the available capacity if they are left unbalanced, which ultimately shortens the driving range. Therefore, an external circuitry, i.e. battery balancing circuit, that manages the unbalanced cells is installed to maximize the usable capacity, and thus, to prolong the driving range. However, the most commonly adopted balancing circuit is the dissipative balancing strategy in the large-scale electric vehicle productions, where the available capacity is underutilized. One of the most efficient redistributive balancing strategies that overcome the drawbacks of the dissipative one is converter-based strategy that monitors and regulates each paralleled-connected cell module.

Nevertheless, installing the individual DC-DC converters on each module is not cost-friendly, and thus, reducing the cost of the converter-based balancing system becomes the priority for large adoptions of the redistributive balancing systems in electric vehicles. This thesis proposes a dual-cell link that integrates the functionalities of the auxiliary power module, battery gauging and battery balancing, leading to a low-cost solution comparable with the dissipative balancing. The topological improvements are made achieving 50% less number of the needed converters compared with the existing topologies. In addition, the integration and minimization are the design targets in terms of the main circuit components. The costly components, such as MOSFETs and magnetic components, are curtailed by 62.5%-75% and 50%-100%, respectively, with no sacrifices on the balancing speed. In order to achieve the magnetic integration, the detailed circuit model is developed using average- and small-signal modeling techniques.

The design procedure for the half-full bridge converter with the cored transformer is firstly discussed, followed by a further minimized dual-half active bridge converter with a coreless transformer. Following the design procedure, two systems are characterized, built, tested and validated with the real batteries. Not only is the cost reduced, but also the balancing process is facilitated, which is realized by an additional balancing path. A DC current offset between the adjoining cells in one link can be introduced to the circuit by utilizing a normally undesired volt-amp imbalance in the transformer, which provides the extra cell-to-cell balancing path. An asymmetric duty cycle control is proposed to regulate the DC current offset so that the different balancing modes can be achieved. With the enabled cell-to-cell path, the balancing speed can be reduced by 50% compared with the conventional cell-to-stack only balancing methods with a state-of-charge difference of 20% between two adjoining cells.

The auxiliary power module requires the proposed converters to work as efficiently as possible within its wide operating range. However, the efficiency of the half-bridge systems drops at light-load conditions due to the loss of the soft-switching capability and high conduction loss. In order to overcome this drawback, the variable frequency modulation is normally preferred. A conduction-loss based control criteria is proposed, inheriting the benefits of the conventional variable frequency modulation while maintaining the optimized conduction loss. It is validated on the converter prototype that the proposed control criteria can achieve 1-2% better efficiency with an extremely simple but robust control logic compared with the critical soft switching.