A study on the impact of open circuit voltage tests on state of charge estimation for lithium-ion batteries

The open circuit voltage (OCV) is of essential importance for accurate estimation of the state of charge (SoC) in lithium-ion battery (LiB). The OCV-SoC relationship is typically predetermined by fitting offline OCV data. Commonly used two OCV tests are compared in few literatures. Moreover, they only focus on the middle SoC region (i.e., 20% and 90%) of LiNiMnCoO2 (NMC) LiBs, the performances of these OCV tests for other battery types and entire SoC region are failed to be addressed. In this paper, the impact of two OCV tests on SoC estimation for NMC and LiFePO4 (LFP) LiBs is investigated at different temperatures and aging stages. A parameter and SoC joint estimation method is introduced, based on an integrated H-infinity-UKF method. The accuracy and reliability of the proposed method are verified by using two different OCV testing data at various ambient temperatures and aging stages for some commercial NMC and LFP LiBs. The results indicate that the incremental OCV test method results in more accurate SoC estimation than the low current OCV test method, on both NMC and LFP LiBs. Furthermore, to reach equilibrium states and achieve desired SoC estimation accuracy, the relaxation period in the incremental OCV test method needs to be extended at low temperatures.