| Summary: | 碩士 === 國立臺灣科技大學 === 電機工程系 === 106 === In this thesis, a state-of-charge (SOC)-based equalizing circuit for series-connected lithium-ion batteries is proposed. The power stage utilized in this thesis is a bidirectional buck-boost converter. Using this topology, bidirectional energy transfers between two individual cells can be achieved. Comparing with voltage-based balancing strategies which are widely used in equalizing circuits, SOC-based balancing strategies are more effective due to the flatness of open circuit voltage (OCV) versus SOC characteristics. For SOC-based balancing algorithms, an accurate SOC estimation is essential. In this thesis, a compensated OCV-based SOC estimation method is proposed for improving the accuracy of SOC estimation.
However, if the duty cycle of the bidirectional buck-boost converter is kept constant, the balancing current will decrease as the SOC difference decreases; this will correspondingly increase the balancing time. To deal with this problem, two balancing algorithm named varied-duty-cycle method and curve-fitting method are proposed. These presented techniques can adjust the duty cycle value according to the SOC difference and keeps the balancing current nearly constant. Comparing with fixed-duty-cycle approach and varied-duty-cycle method, the curve-fitting method can improve the balancing time by 27.1 % and 18.6 %, respectively.
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