System Theoretic Analysis of Battery Equalization Systems

Battery equalizers are widely used in multi-battery systems to maintain balanced charge among individual battery cells. While the research on the hardware realization of battery equalizers has received significant attention, rigorous analysis of battery equalization from the system\u27s point of view remains largely unexplored. In this research, we study three types of battery equalization system structures: series-based, layer-based, and module-based. Specifically, we develop mathematical models that describe the system-level behavior of the battery equalization processes under these equalization structures. Then, based on the mathematical models, analytical methods are derived to evaluate the performance of the equalization processes. We also carry out statistical analysis to compare the performance of the three equalization structures considered. In addition, these systems will be studied with energy loss. Note to Practitioners--This research work develops computationally efficient tools to evaluate the performance of battery equalization systems under series-based, layer-based, and module-based structures, respectively. Using these tools, engineers and designers can predict the equalization time of a battery system in real-time with high accuracy. In addition, based on these tools, one can compare the equalization performance under different structures without using time-consuming simulations. Numerical experiments suggest that the layer-based and module-based structures have better average performance than the traditional series-based equalization structure

Thermal modeling of industrial-scale vanadium redox flow batteries in high-current operations

A cell-resolved model that simulates the dynamic thermal behavior of a Vanadium Redox Flow Battery during charge and discharge is presented. It takes into account, at a cell level, the reversible entropic heat of the electrochemical reactions, irreversible heat due to overpotentials, self-discharge reactions due to ion crossover, and shunt current losses. The model accounts for the heat transfer between cells and toward the environment, the pump hydraulic losses and the heat transfer of piping and tanks. It provides the electrolyte temperature in each cell, at the stack inlet and outlet, along the piping and in the tanks. Validation has been carried out against the charge/discharge measurements from a 9kW/27kWh VRFB test facility. The model has been applied to study a VRFB with the same stack but a much larger capacity, operating at \uf0b1400 A for 8 h, in order to identify critical thermal conditions which may occur in next-generation industrial VRFB stacks capable to operating at high current density. The most critical condition has been found at the end a long discharge, when temperatures above 50\ub0C appeared, possibly resulting in \u3016VO\u3017_2^+ precipitation and battery faults. These results call for heat exchangers tailored to assist high-power VRFB systems

Active Charge Equalizer of Li-Ion Battery Cells Using Double Energy Carriers

In this work, a new active balancing circuit is proposed. This circuit consists of a cell-access network and an energy-transfer network. The cell-access network requires 2n + 6 switches, where n is the number of cells, and creates an energy-transfer path between unbalanced cells and the energy-transfer network. The energy-transfer network has double energy carriers and simultaneously implements cell-to-pack and pack-to-cell balancing operations without overlapping. As a result, a high power rate and fast balancing operation can be achieved by using two energy carriers in a single balancing circuit. The prototype of a proposed balancing circuit was built for six cells and then tested under various conditions; all cells in the state of charge (SOC) region of 70% to 80% were equalized after 93 min, and one charging/discharging period in the SOC region of 10% to 90% was increased by 8.58% compared to the non-balancing operation. These results show that the proposed circuit is a good way to balance charges among batteries in a battery pack.11Ysciescopu