A combined state of charge estimation method for lithium-ion batteries used in a wide ambient temperature range

Abstract

Ambient temperature is a significant factor that influences the characteristics oflithium-ion batteries, which can produce adverse effects on state of charge (SOC)estimation. In this paper, an integrated SOC algorithm that combines an advanced ampere-hourcounting (Adv Ah) method and multistate open-circuit voltage (multi OCV) method,denoted as "Adv Ah + multi OCV", is proposed. Ah counting is a simple and generalmethod for estimating SOC. However, the available capacity and coulombic efficiency inthis method are influenced by the operating states of batteries, such as temperature andcurrent, thereby causing SOC estimation errors. To address this problem, an enhanced Ahcounting method that can alter the available capacity and coulombic efficiency accordingto temperature is proposed during the SOC calculation. Moreover, the battery SOCsbetween different temperatures can be mutually converted in accordance with the capacityloss. To compensate for the accumulating errors in Ah counting caused by the lowprecision of current sensors and lack of accurate initial SOC, the OCV method is used forcalibration and as a complement. Given the variation of available capacities at differenttemperatures, rated/non-rated OCV-SOCs are established to estimate the initial SOCs inaccordance with the Ah counting SOCs. Two dynamic tests, namely, constant- andalternated-temperature tests, are employed to verify the combined method at differenttemperatures. The results indicate that our method can provide effective and accurate SOCestimation at different ambient temperatures. © 2014 by the authors.