This paper presents a fully embedded state of health (SoH) estimator for widely used C/LiFePO₄ batteries. The SoH estimation study was intended for applications in electric vehicles (EV). C/LiFePO₄ cells were aged using pure electric vehicle cycles and were monitored with an automotive battery management system (BMS). An online capacity estimator based on incremental capacity analysis (ICA) is developed. The proposed estimator is robust to depth of discharge (DoD), charging current and temperature variations to satisfy real vehicle requirements. Finally, the SoH estimator tuned on C/LiFePO₄ cells from one manufacturer was tested on C/LiFePO₄ cells from another LFP (lithium iron phosphate) manufacturer.

The driving range of electric vehicles is a complex issue. In simulation, this range is determined by coupling a battery model and a traction model of the vehicle. But most of the battery models used for such studies do not take into account the interaction between the temperature of the battery (impacting its electrical parameters) and the traction model of a studied EV. In this paper, a battery electro-thermal model (with all electrical parameters dependent upon the temperature) is dynamically coupled with the traction model of an electric vehicle. Simulation results are provided to show the impact of the temperature on the driving range. The initial battery model (without temperature dependence) leads to an overestimation of the driving range of 3.3% at -5°C and an underestimation of 2.5% at 40°C.

Modular multilevel converters (MMCs) have been recognized as the most attractive technology for high voltage direct current (HVDC) applications during the last years. Their advantages include the ability to freely control the internally stored energy in the distributed submodules (SMs) and to independently control AC and DC powers. Recent studies showed that an MMC can take advantage of this degree of freedom to supply ancillary services with low energy requirements such as frequency response or power oscillation damping by connecting energy storage systems (ESSs) to its SM capacitors. This paper proposes a modular DC-DC converter as an interface between the energy storage element and the SM capacitor. As it can be built with switches allowing a high switching frequency, it is shown that the volume of the passive components can be reduced in comparison with interface converters known in the literature. The proposed control strategy avoids to transfer the harmonics of a SM to the ESS that prevents to drastically oversize the system. The proposed control method also provides an appropriate energy exchange without affecting the control philosophy of an MMC and it addresses balancing issues. Simulations and a downscaled prototype confirm the relevance of the proposed control. As the proposed interface converter is able to track the power reference with fast dynamics, it provides the same system-level advantages as other interface converters known in the literature but with a reduced volume.

Lifetime of energy storage systems is a key factor that is extremely influenced by the operating conditions. For this reason, lithium-ion batteries (LiBs) and supercapacitors (SCs) were subjected to accelerated aging tests in several previous research in order analyze their lifespan. Lithium-ion capacitors (LiCs), which fall in between LiBs and SCs, are still considered as a novel technology. Their behavior during accelerated aging tests is not yet well studied in the literature. This paper focuses on studying the degradation of their properties during floating aging. Eighteen samples were tested under three different voltage values and two different temperatures over 12 months. Cells that were discharged to the minimum voltage showed a severe capacitance decrease that was much higher than the one of fully charged cells. The best storage voltage of LiCs was found to be 3 V, which is equivalent to a half charging state. The hybrid structure of LiCs was found to be the major cause of this change in behavior during aging at different voltages. Mechanisms that affect the electrodes and the internal composition of the cells in the diverse conditions were analyzed and predicted.

The process of determination of batteries lifespan for a specific application generally contains some aging tests. Most of the time, these tests used simplified power or current profiles designed specially to be as representative as possible of the reality. The use of a real profile could improve the reliability of the approach but needs to know how to select the typical profile from an available database. This article shows the methodology used to analyze a database of 10 electric vehicles monitored during 2 years. Different mathematical methods (Pearson and Spearman coefficient of correlation, Shannon entropy, principal component analysis, K-means, ...) were applied to sort the essential variables and classify the uses of the vehicle. Representative classes of uses were then gathered to constitute a profile used in aging tests. The aging results with this real-life type profile and with an adjusted WLTC (Worldwide Harmonized Light Vehicles Test Cycles) profile are presented and compared. The effectiveness of the use of the WLTC profile for aging tests in real life automotive application is shown.

Lithium-ion Capacitors (LiCs) that have intermediate properties between lithium-ion batteries and supercapacitors are still considered as a new technology whose aging is not well studied in the literature. This paper presents the results of accelerated aging tests applied on 12 samples of LiCs. Two high temperatures (60 °C and 70 °C) and two voltage values were used for aging acceleration for 20 months. The maximum and the minimum voltages (3.8 V and 2.2 V respectively) had different effects on capacitance fade. Cells aging at 2.2 V encountered extreme decrease of the capacitance. After storing them for only one month at 60 °C, they lost around 22% of their initial capacitance. For this reason, an aging model was developed for cells aging at the lowest voltage value to emphasize the huge decrease of the lifetime at this voltage condition. Moreover, two measurement tools of the capacitance were compared to find the optimal method for following the evolution of the aging process. It was proved that electrochemical impedance spectroscopy is the most accurate measurement technique that can reveal the actual level of degradation inside a LiC cell.