Due to the adjacent cell-to-cell equalization, the classical switched-capacitor equalizer (SCE) has seriously low balancing speed and efficiency for a long battery string. Therefore, a series of star-structured switched-capacitor equalizers are proposed to achieve high balancing speed and efficiency independent of the cell number and the cell voltage distribution without significant influences on the size, cost, control, and reliability compared with the classical SCE. The proposed topologies use the least mosfets and capacitors to achieve the most direct balancing paths between any two cells, being the optimal switched-capacitor structures. An analysis method for the slow switching limit and the fast switching limit impedances of the proposed equalizers is proposed to provide a theoretical guidance for the selection of the switching frequency. The inherent advantages of the proposed systems are the small size, low cost, simple control, high efficiency, fast speed, easy modularization, and high reliability. A prototype for four lithium-ion battery cells is set up. Experimental results show improved balancing performances with strong robustness. Moreover, the measured peak balancing efficiency is about 93.1% despite faster balancing speed.

Owing to the operational characteristic of Vienna rectifier, it is found that the traditional modulation method would result in the current distortion. This paper therefore proposes a novel zero-sequence component injection modulation method for improving the input quality of Vienna rectifier with balanced or unbalanced dc-link voltages. In specific, the degree of unbalanced dc-link voltages increases the size of abnormal regions in slant and asymmetrical space vector diagram, which leads to more serious current distortion. The proposed method identifies the abnormal intervals by average duties accurately without detecting the position of the reference vector. Considering the operational characteristics, three-phase compensation components are calculated respectively depending on unbalanced factor and added to three-phase average duties in abnormal intervals. As a result, the current harmonics of Vienna rectifier are mitigated significantly with balance or unbalanced dc-link voltages. The effectiveness and the performance of the proposed zero-sequence component injection method for Vienna rectifier are verified by simulation and experiments.

The existing switched-capacitor (SC)-based equalizers either have slow equalizing speed and low efficiency or have bulky volume and low reliability. Therefore, an optimized mesh-structured switched-capacitor (SC) equalizer (MSSCE) is introduced to achieve a trade-off between the equalization performances and size as well as cost. The proposed MSSCE achieves all the shortest balancing paths from one cell to any cell in a battery string, significantly improving the equalization speed and efficiency. The switching impedance of the proposed MSSCE is derived and analyzed, which provides a design guidance for the switching frequency and the capacitance. A four-cell prototype was set up. Experiment results confirm that the proposed MSSCE enhances the balancing speed with a high efficiency of 91.7%, moreover which is robust to the imbalanced cell voltages.

Due to the low switching loss, compact size, and low price, the LC series resonant converter (LCSRC) as an energy carrier is promising in battery equalizers. However, the conventional equalizers based on LCSRC only can realize the one-cell-to-one-cell balancing for a high-voltage series-connected battery pack, causing a low equalizing speed and efficiency. Therefore, two direct multicell-to-multicell (MC2MC) balancing topologies based on matrix LC converter and half-bridge LC converter are proposed, respectively, which achieve the direct energy transfer from a consecutive more-charged-cell group to a consecutive less-charged-cell group, significantly improving the equalizing speed and efficiency. An analytical model for the LCSRC is developed to provide a guidance for the optimal design of the proposed equalizers. The prototypes for the proposed two balancing topologies are constructed. The MC2MC balancing performances are verified though the comparative experimental results at different initial voltage distributions. In particular, the proposed equalizers simultaneously achieve the fast balancing speed and high equalization efficiency. The measured peak efficiency is about 87.9% at the four-cell-to-four-cell equalization.

With numerous lithium-ion batteries retired from electric vehicles, the studies on the battery second usage are extremely imminent. However, existing screening approaches on plenty of cells fail to guarantee high efficiency and high accuracy simultaneously. This paper proposes a quick and accurate screening method based on the improved fuzzy c-means algorithm. Firstly, the partial charging curve of every single cell is selected optimally based on the incremental capacity analysis, which is frequently used to detect battery aging mechanism. Secondly, four important features are extracted from the partial charging curves, including key point, curve gradient, voltage energy and volatility. Furthermore, feature optimization is done through observing the relationship between capacity and feature. Lastly, the retired batteries are screened with the optimal features using the improved fuzzy c-means algorithm. The screening result on 176 LiFePO4 batteries proves the high accuracy and high efficiency of the approach. Compared with the Support Vector Machine and neural network approaches, the proposed method has better generality and higher efficiency without pretreatment training. The screening accuracy can reach 90.9%. With permitted error of 1%, it can be as high as 95.5%. The screening efficiency is about 7.6 times higher than the supervised screening method.