Rapid Open-Circuit Voltage Measurement Method for Lithium-Ion Batteries Using One-Cycle Bipolar-Current Pulse

Abstract

Accurate estimation of state-of-charge (SOC) of battery is important for battery storage systems. A lookup table between the battery open-circuit voltage (OCV) and SOC has been widely adopted for online SOC estimation. However, it is time-consuming to obtain an accurate SOC-OCV correlation since the battery requires several hours to reach an inner-equilibrium state. Some acceleration methods utilized the measured terminal voltage to predict the final OCV, but they have accumulated errors. Therefore, instead of predicting the OCV, a directly fast OCV measurement method for lithium-ion batteries based on one-cycle bipolar-current pulse, namely, a positive current pulse followed by a negative current pulse with the same duration and amplitude is proposed in this article. Explicitly, a second-order relaxation model, which shows a good balance between accuracy and simplicity, is established for analyzing the relationship between current and polarization voltage. Furthermore, considering the safe operation of the battery and hysteresis voltage issue, a 1 C bipolar-current pulse with 6-min duration is chosen for implementation. Experimental results on LiNiMnCoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and LiNiCoAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> batteries have proven the effectiveness of the proposed method, which can reduce up to 90% (depending on SOC level) of the time as compared with the conventional method.