Smart and Hybrid Balancing System: Design, Modeling, and Experimental Demonstration

Abstract

Performance of series connected batteries is limited by the 'weakest link' effect, i.e. the cell or group of cells with the poorest performance in terms of temperature, power, or energy characteristics. To mitigate the 'weakest link' effect, this study deals with the design, modeling and experimental demonstration of a smart and hybrid balancing system (SHBS). A cell-to-cell shared energy transfer configuration is proposed, including a supercapacitor bank in the balancing bus, thus enabling hybridization. Energy is transferred from each battery module connected in series to the balancing bus, and vice versa, by means of low-cost bi-directional dc-dc converters. The current setpoints of the converters are obtained by means of a smart balancing control strategy, implemented using convex optimization. The strategy is called 'smart' because it pursues goals beyond conventional state-of-charge equalization, including temperature and power capability equalization, and minimization of energy losses. Simulations show that the proposed SHBS is able to achieve all these goals effectively in an e-mobility application, and are also used to assess the impact of different hybridization ratios and cooling conditions. Finally, an experimental setup is developed to demonstrate the feasibility of the SHBS.

A Novel Modular, Reconfigurable Battery Energy Storage System: Design, Control, and Experimentation

“…Converters have also found their way into hardwired BESS. The studies in [18][19][20] integrate DC/DC converters with the cells to enable cell balancing and power loss minimization. However, the fixed hardwired connections among the cells make them susceptible to single-cell faults.…”

“…However, due to a lithiumion battery's nonlinear characteristics, optimization problems posed for the power management are often non-convex, and thus defy the computation of global optima. To mitigate the issue, the studies in [18,19] choose to leverage a battery model convexification approach proposed in [22] to formulate convex optimal power control problems. The convexification therein involves a linear approximation of the relationship between the state-of-charge (SoC) and the open-circuit-voltage (OCV), thus restricting the proposed methods to only limited operating ranges.…”

“…We then proceed to present a model-based optimal control problem for the RBESS and convexify it for computational tractability. Here, the modeling and optimal control formulation are a refinement of the methodology in [18,19], and improvements are introduced to expand the operating SoC range of the RBESS and ensure the feasibility of the optimization problem. Finally, we present a switching circuit reconfiguration mechanism that dovetails with the power management method.…”

“…For the sake of convexification, we begin with linearizing the SoC/OCV curve. The existing studies, e.g., [18,19] perform the linearization for only the medium SoC range, where the OCV is closely linear with SoC for lithium-ion batteries. However, this treatment excludes the use of the low and high SoC ranges.…”

Hybridizing Lead–Acid Batteries with Supercapacitors: A Methodology

“…Especially for DC standalone microgrids, this is a viable option for getting the energy storage mix right and extending the HESS lifespan as much as possible. It is worth noting that there are some recent alternatives that involve distributed hybridization combined with active balancing, achieved by incorporating supercapacitors into the balancing bus in order to enable cell-level hybridization [23]. However, this is still a new and more complex solution, so far targeted only at Li-ion batteries in e-mobility applications.…”