In electric vehicle (EV) and hybrid electric vehicle (HEV) application, accurate information of state-ofcharge (SOC) and capacity of each cell are required for elaborate SOC/capacity estimation algorithm of the battery pack. However, the measurement of the states of all cells using sophisticated algorithms increases the computation time beyond practicality, because the computation time required for the SOC/capacity estimation of the battery pack is directly affected by the number of unit cells. In this work, a simple SOC and capacity estimation algorithm for Li-ion battery pack is newly proposed by using filtered terminal voltage. The SOC estimation algorithm using filtered terminal voltage extracts an estimated current information from the terminal voltage of the battery pack through equivalent-circuit model (ECM)-based filters without sensing the current. Consequently, it drastically reduces computational steps for the SOC estimation algorithm. With the fact that all the current flowing through the series-connected cells in pack are identical, the estimated current value of each cell should be identical. As a result, it can be known that this algorithm enables us to obtain the relative proportion of SOC/capacity information of each cells and battery pack with minimal complexity increase. To validate the performance of the proposed approach, a scaled-down HEV profile is used for a pack consists of twelve 18650 series-connected Li-ion batteries (12S1P). The experimental results verify the performance of the proposed battery pack SOC estimation algorithm.
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