Constant Current Voltage Research Articles

Data-driven smart charging for heterogeneous electric vehicle fleets

The ongoing electrification of mobility comes with the challenge of charging electric vehicles (EVs) sufficiently while charging infrastructure capacities are limited. Smart charging algorithms produce charge plans for individual EVs and aim to assign charging capacities fairly and efficiently between vehicles in a fleet. In practice, EV charging processes follow nonlinear charge profiles such as constant-current, constant-voltage (CCCV). Smart charging must consider charge profiles in order to avoid gaps between charge plans and actual EV power consumption. Generally valid models of charge profiles and their parameters for a diverse set of EVs are not publicly available. In this work we propose a data-driven approach for integrating a machine learning model to predict arbitrary charge profiles into a smart charging algorithm. We train machine learning models with a dataset consisting of charging processes from the workplace gathered in 2016–2018 from a heterogeneous EV fleet of 1001 EVs with 18 unique models. Each charging process includes the time series of charging power. After preprocessing, the dataset contains 10.595 charging processes leading to 1.2 million data points in total. We then compare different machine learning models for charge profile predictions finding that XGBoost yields the most accurate predictions with a mean absolute error (MAE) of 126W and a relative MAE of 0.06. Simulations show that smart charging with the integrated XGBoost model leads to a more effective infrastructure usage with up to 21% more energy charged compared to smart charging without considering charge profiles. Furthermore, an ablation study on regression model features shows the EV’s model is not a necessary attribute for accurate charge profile predictions. However, charging features are required including the number of phases used for charging.

Pulsed Current Based Fast Charging Methods for Li-Ion Battery

The increasing rate of greenhouse gas emissions from conventional cars and its impact on global warming in the recent years has led to the technological advancements in the evolution of electric vehicles. Developing fast charging methods for Li-ion batteries is one such technological advancement and soon it will be imperative to promote the usage of electric vehicles in increased numbers. However, fast charging of Li-ion battery accelerates aging mechanisms [1] and triggers thermal runaway condition. As a result, this study investigates pulsed current based fast charging methods employed on C/NMC batteries. Pulses of current at low frequency (mHz) [2] and high frequency (KHz) are used with an extended pulse current period, thanks to higher voltage limit based on ohmic drop compensation [3]. The findings of the experimental investigation clearly establishes that a compromise has to be found between relaxation period and temperature in order to achieve fast charging. For instance, on one hand, a 2C current pulse with an on duration of 3s and relaxation duration of 1s allows to save 45% of recharging time when compared with standard constant current-constant voltage charging (CC-CV) at 0.5C as shown in the figure below. On the other hand, the temperature variation does not exceed 20°C beyond ambient which is lower than that of its counterpart CC-CV at 2C. Hence, the experimental investigation successfully opens the doors to implement fast charging methods on C/NMC batteries by limiting the temperature rise, there by arresting the ageing of batteries and thermal runaway condition.Keyword: Li-ion, Fast charging, Current pulse, Ohmic drop compensation, Temperature, Thermal runaway. Figure 1

Realizing Constant Current and Constant Voltage Outputs and Input Zero Phase Angle of Wireless Power Transfer Systems With Minimum Component Counts

In both normal and fast wireless electric vehicle charging systems, constant current/constant voltage (CC/CV) charging profile, regardless of the variation of the battery state of charge, is one of the most essential characteristics to ensure the battery performance and reliability. The input zero phase angle (ZPA) is able to minimize the system volt-ampere rating, enhance the power transfer capability, and make it easy to achieve soft-switching operation over the full range of battery charging profile. Therefore, the load-independent CC and CV output characteristics with ZPA conditions are necessary for wireless charging systems. However, the existing methods that can achieve these functions either add power switches or need a large number of compensation components, which make the system inefficient, uneconomical, and bulky. In this paper, a unified resonant tuning configuration with minimum passive component counts is proposed to achieve CC and CV outputs at two ZPA operating frequencies. According to the proposed configuration, all the possible inductive power transfer (IPT) and capacitive power transfer (CPT) topologies are analogized. With these topologies, both CC and CV outputs with ZPA are achieved using a minimum number of compensation components and no additional power switches. Among all the simplest IPT topologies, a primary Series-secondary Series and Parallel (S-SP) compensation topology is illustrated to demonstrate the analysis.

Cycle Life Study of Li-Ion Batteries With an Aging-Level-Based Charging Method

This study presents a preliminary investigation report on an aging-level-based (ALB) charging strategy that is possible to extend the battery lifespan by varying the charge current according to the state of health of a cell as opposed to the standard constant-current constant-voltage (CC-CV) and a four-stage constant current (4SCC) methods. Experimental tests have been carried out, which employed Sanyo UR14500P lithium-ion batteries in the cylindrical shape with a nominal voltage of 3.7 V and a rated capacity of 840 mAh. The results demonstrate that the ALB charging strategy with a CC charge profile is able to slow down the cell capacity fade by 3.84% compared with the standard CC charging method after 120 cycles, which increases the lifetime of 40 cycles to 50 cycles. Besides, the ALB charging strategy with a 4SCC charge profile is able to slow down the cell capacity fade by 2.55% compared with the standard 4SCC charging method after 100 cycles, which prolongs the lifetime of 30 cycles to 40 cycles. Nevertheless, the ALB charging strategy with both the CC and 4SCC charge profiles has much longer charging time than those with the standard charging method, and the charging time increases by 22.5% and 10.71%, respectively.

A 6.78-MHz Single-Stage Wireless Charger With Constant-Current Constant-Voltage Charging Technique

A 6.78-MHz fully integrated single-stage wireless charger with constant-current constant-voltage (CC-CV) charging technique for resonant wireless power transfer applications is presented. Based on the principle of three-mode reconfigurable resonant regulating ( $R^{3}$ ) rectifier, the proposed charger achieves high efficiency and low cost by realizing power rectification, voltage regulation, and CC-CV charging in a single power stage. The power stage of the proposed charger consists of only four nMOS power transistors. A bootstrapping technique using a single-input-dual-output voltage doubler with adaptive phase control is also proposed to integrate bootstrap capacitors on-chip. The CC loop and the CV loop share the same pulsewidth modulation (PWM) controller with smooth transition from the CC mode to the CV mode. An on-chip current sensing and estimation circuit is specially designed for the CC loop. Fabricated in a standard 0.35- $\mu \text{m}$ CMOS process with a die area of 8 mm2, the charger delivered a maximum charging current of 1.5 A, and the measured peak efficiency reached 92.3% and 91.4% when the charging currents were 1 A and 1.5 A, respectively.

A Reconfigurable On-Board Power Converter for Electric Vehicle With Reduced Switch Count

This paper proposes a compact reconfigurable on-board power converter for low voltage electric vehicle. The proposed converter serves the purpose of a battery charger as well as a 3-phase voltage source inverter (VSI) to drive the traction motor. The 3-phase VSI is the backbone of this converter that can be either reconfigured to a propulsion unit or to a battery charger by adding few more power electronics components. During charging mode, the topology is reconfigured to form a front-end boost power factor correction rectifier which is cascaded by a DC-DC converter through a DC link. The same power stage is reconfigured to operate as a 3-phase VSI to drive the traction motor during propulsion mode. Thus, it reduces the switch count by eliminating the requirement of a separate inverter stage. Simultaneously the weight, volume, and cost of the overall system are reduced. Unified control technique is implemented which is capable to achieve near-unity power factor operation of front-end boost rectifier as well as optimal battery charging without sensing the DC link voltage. This control scheme implements a single control loop for both stages by eliminating the need of two separate control loop for each stage of the charger. A 470 W scaled-down prototype is developed and its performance is validated with Constant Current-Constant Voltage charging of a 24 V, 30Ah battery pack in charging mode and with a 400W BLDC motor in propulsion mode.

Optimal Multistage Charging of NCA/Graphite Lithium-Ion Batteries Based on Electrothermal-Aging Dynamics

Lithium-ion (Li-ion) batteries have been extensively used in electric vehicles, portable electronics, cell phones, and laptops. The charging protocol, as one of the most critical technologies for Li-ion battery systems, has a significant impact on battery performance. Charging current affects battery degradation and charging time, and therefore, it needs to be carefully optimized. To this end, a novel charging protocol using a series of constant charging currents has been developed, which considers the charging time and the battery capacity fade simultaneously. These two conflicting charging objectives are traded off by solving a multiobjective optimization problem based on battery electrothermal-aging behavior. Particle swarm optimization has been applied to obtain the optimal charging current profile. Three optimal charging strategies for minimum charging time, minimum battery aging, and balanced charging performance are obtained by changing the weight factor. The proposed balanced charging is capable of reducing the charging time significantly with a negligible increase in capacity degradation compared with the 0.5 C constant-current constant-voltage (CC-CV) strategy recommended by the manufacturer.

Reconfigurable WPT system for load‐independent CC and CV output with transmitting‐side control

This study proposes a reconfigurable wireless charging topology, which achieves load-independent constant current (CC) and constant voltage (CV) output without any communication link between transmitting and receiving coils. The proposed topology is suitable for CC–CV modes of battery charging. Inductor (L), capacitor (C) and inductor (L) connected as T type (LCL) compensation with an auxiliary capacitor on the transmitting side and series (S) compensation on the receiving side (LCL-S) is used to achieve load-independent CC and CV output. Reconfiguration has been achieved using single AC switch on transmitting side. Control has been implemented with the information on transmitting side. Load-independent CC and CV output operations have been analysed analytically. The merits of the proposed topology are fixed-frequency operation, simplified control scheme for load-independent CC–CV operation without any communication link. Feasibility of the proposed topology is verified by the experimental prototype with a CC output of 12 A and CV output of 56 V.

Improved Performance of Li-ion Polymer Batteries Through Improved Pulse Charging Algorithm

Pulse charging of lithium-ion polymer batteries (LiPo), when properly implemented, offers increased battery charge and energy efficiencies and improved safety for electronic device consumers. Investigations of the combined impact of pulse charge duty cycle and frequency of the pulse charge current on the performance of lithium-ion polymer (LiPo) batteries used the Taguchi orthogonal arrays (OA) to identify optimal and robust pulse charging parameters that maximize battery charge and energy efficiencies while decreasing charge time. These were confirmed by direct comparison with the commonly applied benchmark constant current-constant voltage (CC–CV) charging method. The operation of a pulse charger using identified optimal parameters resulted in charge time reduction by 49% and increased charge and energy efficiencies of 2% and 12% respectively. Furthermore, when pulse charge current factors, such as frequency and duty cycle were considered, it was found that the duty cycle of the pulse charge current had the most impact on the cycle life of the LiPo battery and that the cycle life could be increased by as much as 100 cycles. Finally, the charging temperature was found to have the most statistically significant impact on the temporarily evolving LiPo battery impedance, a measure of its degradation.

Analysis on pulse charging–discharging strategies for improving capacity retention rates of lithium-ion batteries

The capacity fade of lithium-ion batteries (LIBs) are intimately dependent upon charging–discharging strategies. In this work, a pseudo-two-dimensional model coupled with thermal effects was developed to investigate the effects of pulse current charging–discharging strategies on the capacity fade for LIBs, in which the growth of solid electrolyte interphase (SEI) and the lithium ion migration process are highlighted. LiFePO4/graphite (LFP) batteries were taken as samples. The capacity fading processes of the LFP batteries under different pulse current charging–discharging strategies were studied by numerical simulations. The impacts of the growth of the SEI layer at the negative electrode on the capacity degradation were studied by using eleven charging–discharging cases. The results show that, compared with the constant current-constant voltage (CC-CV) strategy, the pulse current-constant voltage (PC-CV) strategies can effectively alleviate the capacity fade of the LFP batteries due to the relaxation process in the pulse charging process. Under the PC-CV strategies, the relaxation duration and the peak current were altered. The relaxation process renders the decrease of the current density of side reactions in the batteries, which is conducive to the recovery of the aged batteries, thus improving the capacity retention rates of the batteries. The advantages of the PC-CV strategies were demonstrated in this work. The results will provide theoretical guidance for the rapid charging–discharging strategies of LIBs.

Cooperative CC–CV Charging of Supercapacitors Using Multicharger Systems

The constant-current constant-voltage (CC–CV) charging method is a common approach to charge batteries with a single charger. Recently, with the emergence of high power-density batteries, e.g., supercapacitors, several chargers are typically connected in parallel to provide the required charging power in many applications. However, the classical decentralized control scheme for the multicharger system leads to the current imbalance, which deteriorates the reliability of the charging system. In this article, we extend the CC–CV charging protocol to multicharger systems using a cooperative control method, which can alleviate the current imbalance among chargers effectively. First, the motivation and charging system modeling of this work are provided. Then, a cooperative CC charging protocol and a cooperative CV charging protocol are presented, respectively, and a switching logic is designed to achieve the switching between them based on the supercapacitor voltage. The closed-loop model of the proposed charging system is developed using the block diagram. A laboratory testbed is built to verify the effectiveness of the proposed method. Experiment results show that the proposed method provides a better current balancing and voltage regulation performance when compared with existing decentralized control methods.

Evaluation of Charging Strategies for Valve Regulated Lead-Acid Battery

The present paper considers the evaluation of temperature regulated and unregulated charging strategies to select the appropriate one to ensure extended battery life with reduced charging time. Temperature regulated pulse charging (TRPC) and temperature regulated reflex charging (TRRC) are compared with the Constant current-constant voltage (CC-CV) charging strategy. In the case of CC-CV charging temperature of the battery rises with the magnitude of the current being injected and cannot be regulated without any external cooling arrangement. Impact on the State of health (SOH) and the expected lifespan of the battery are considered as the parameters of evaluation. Temperature regulated strategies are implemented through a discrete electro-thermal model, which acts as a temperature estimator. The co-efficient of the estimators corresponds to the battery parameters such as internal resistance and thermal time constants, entropy, etc. Temperature regulation is ensured in the three identified sections of charge deposited vs magnitude of the injected current. Three sections are identified as sections where the injected current is not sufficient to raise the battery temperature to set limit or not and the level of charge submitted as compared to normal charging. Experimentation is carried with 12 V, 26 Ah Valve regulated lead-acid battery to justify that increase in temperature reference of regulation allows submission of higher charge for the same charging rate. It is demonstrated that TRPC results in a significant reduction (≈60%) in charging time as compared to CC-CV and TRRC. For the same charging time as achieved with TRPC, TRPC results in almost double the expected life of operation and better SOH as compared to CC-CV and TRRC.

Experimental and Theoretical Analysis of the Fast Charging Polymer Lithium-Ion Battery Based on Cuckoo Optimization Algorithm (COA)

Fast charging of the electric-vehicles is one of the paramount challenges in solar smart cities. This paper investigates intelligent optimization methodology to improvise the existing approaches in order to speed up the charging process whilst reducing the energy consumption without degradation in the light of the outrageous demand for lithium-ion battery in the electric vehicles (EVs). Two fitness functions are combined as the targeted objective function: energy losses (EL) and charging interval time (CIT). An intelligent optimization methodology based on Cuckoo Optimization Algorithm (COA) is implemented to the objective function for improving the charging performance of the lithium-ion battery. COA is applied through two main techniques: The Hierarchical technique (HT) and the Conditional random technique (CRT). The experimental results show that the proposed techniques permit a full charging capacity of the polymer lithium-ion battery (0 to 100% SOC) within 91 mins. Compared with the constant current-constant voltage (CCCV) technique, an improvement in the efficiency of 8% and 14.1% was obtained by the Hierarchical technique (HT) and the Conditional random technique (CRT) respectively, in addition to a reduction in energy losses of 7.783% and 10.408% respectively and a reduction in charging interval time of 18.1% and 22.45% respectively. Experimental and theoretical analyses are performed and are in good agreement on the polymer lithium-ion battery fast charging method.

New Temperature-Compensated Multi-Step Constant-Current Charging Method for Reliable Operation of Battery Energy Storage Systems

Battery lifetime represents a significant concern for the techno-economical operation of several applications based on energy storage. Moreover, the charging method is considered as one of the main critical elements in defining and influencing the operating lifetime of batteries. Several charging techniques have been addressed in the literature, however almost all of them are suffering from lack of temperature feedback in order to maintain battery lifetime. This paper presents a new high-reliable charging method for battery energy storage systems (ESSs). The proposed temperature compensated multi-step constant current (TC-MSCC) method is developed based upon the modified (MSCC) charging method. It enhances the operating lifetime of batteries by employing a feedback from the battery temperature to control the duration and starting time of each charging current step. Compared with the traditional charging methods addressed in the literature, the proposed TC-MSCC method achieves faster charging than the conventional constant current (CC) and the constant-current constant-voltage (CC-CV) methods. Moreover, the proposed TC-MSCC method possesses longer operating battery lifetime with reduced thermal stresses compared to the traditional MSCC methods. The proposed charging method is verified by simulation and experimental results using 9 Ah lead-acid battery. However, the new proposed TC-MSCC method is generalized and can be applied to various types of batteries. The detailed performance comparisons and results show the superiority of the proposed methods over the most widespread charging methods in the literature.

The Charging Control and Efficiency Optimization Strategy for WPT System Based on Secondary Side Controllable Rectifier

The equivalent resistance of the battery will change during charging in practical applications, so it is difficult to design a wireless power transfer (WPT) system with accurate constant current (CC) and constant voltage (CV) output characteristics. In addition, the WPT system efficiency is also affected by the change of equivalent resistance. Therefore, this paper studies the charging control and efficiency optimization strategy for the WPT system with dynamic loads. The WPT system with a structure of double-sided LCC compensation topology and containing controllable rectifier circuit at the secondary side is established, and the working process of full-bridge controllable rectifier circuit is analyzed. Then the output characteristics of the WPT system are analyzed. The expressions of the optimal transmission efficiency and the optimal equivalent impedance of the full-bridge rectifier are derived. The relationships between the equivalent impedance of the capacitor-filtered full-bridge controllable rectifier circuit and the phase shift angle, the charging current and the phase shift angle are also obtained. In this paper, the CC and CV charging strategies based on phase shift control of controllable rectifier circuit and efficiency optimization strategy based on dynamic equivalent impedance matching are proposed, which can improve the WPT system transmission efficiency while realizing CC and CV charging. Finally, the effectiveness of the proposed control strategy is verified by simulation and experiment. The WPT system realizes CC charging mode and CV charging mode in turn when the load resistance changes from $5~\Omega $ to $30~\Omega $ , and the overall efficiency is up to 90.71% at the transmission distance of 15cm.

Nonequilibrium statistical theoretical analysis method of TDDB of gate oxide

As one of the important factors affecting the reliability of Complementary metal oxide semiconductor integrated circuits, the time-dependent dielectric breakdown of gate oxide has always been the focus of domestic and foreign scientists. The previous researches have shown that when the electrons pass through the silicon dioxide lattice and collide with it, part of their kinetic energy will be dissipated, then among of which can create some sort of defects that behave as electron traps. However, when the electron traps density reaches a critical value in the gate oxide, the destructive thermal effects open a low-resistance ohmic path between the electrodes, thus, triggering the breakdown conducting mechanism. In this paper, in order to employing a statistical theoretical analysis method, the behavior related with time-dependent dielectric breakdown of gate oxide was investigated in detail. The following useful results are obtained, (i) According to the microscopic mechanism of the time-dependent dielectric breakdown of gate oxide under electrical stresses associated with the randomness and irreversibility of the electron traps generation in gate oxide, a theoretical analysis method has been proposed, which is based on the equation of the electron traps generation rate combined with the Langevin equation in random theory and the Fokker-Planck equation in the non-equilibrium statistical theory. (ii) In light of the dynamic equilibrium model, the generation rate equations of electron traps under constant current stress and constant voltage stress as well as the probability density distribution functions of electron traps density were determined. By integrating these distribution function, the cumulative failure rate was further obtained. (iii) Taking the specific metal oxide semiconductor capacitors as examples, the relationship between the most probable lifetime of the gate oxide and the constant current stress, constant voltage stress, as well as its thickness have been revealed. And the concept of "breakdown limit" was defined by analogy with the concept of "fatigue limit" in the phenomenon of solid fracture. (iv) The dependence of the cumulative failure rate on the current stress, voltage stress, and time has been presented. A characteristic value of the time was introduced to indicate the time when the cumulative failure rate reaches 0.63. At this time, most devices have breakdown failure. Besides, the test parameters are not affected by previous failures, and they are close to that of the cumulative failure rate of 1. The results show that the probability density distribution function of electron traps density satisfies lognormal distribution, and the obtained distribution of failures fits well with the experimental statistical data.

Optimal Fast Charging Control for Lithium-ion Batteries

Fast charging has gained an increasing interest in the convenient use of Lithium-ion batteries. This paper develops a constrained optimization based fast charging control strategy, which is capable of meeting needs in terms of charging time, energy loss, and safety-related charging constraints. To solve it with less computational effort, a two-layer optimization strategy is proposed, where a charging time region contraction method is utilized to search the minimum expected charging time in the top layer, and the bottom layer uses the barrier method to calculate the corresponding optimal charging current with the charging time given by the top layer. Through utilizing this two-layer optimization method, the optimal charging current can be obtained that leads to the shortest charging period while guaranteeing the charging constraints with relatively low computational complexity. Extensive simulation results are provided to validate the proposed optimal fast charging control strategy, which well outperforms the constant current-constant voltage method.

A Novel Charging Algorithm for Lithium-Ion Batteries Based on Enumeration-Based Model Predictive Control

Lithium-ion (Li-ion) batteries play a substantial role in energy storage solutions for modern-day technologies such as hand-held consumer electronics, aerospace, electric vehicles, and renewable energy systems. For Li-ion batteries, designing a high-quality battery charging algorithm is essential since it has significant influences on the performance and lifetime of Li-ion batteries. The objectives of a high-performance charger include high charging efficiency, short charging time, and long cycle life. In this paper, a model predictive control based charging algorithm is proposed, the presented technique aims to simultaneously reduce the charging time, and the temperature rise during charging. In this study, the coulomb counting method is utilized to calculate the future state-of-charge and an artificial neural network trained by experimental data is also applied to predict the future temperature rise. Comparing with the widely employed constant current-constant voltage charging method, the proposed charging technique can improve the charging time and the average temperature rise by 1.2 % and 4.13 %, respectively.

Investigation of lithium-ion battery degradation mechanisms by combining differential voltage analysis and alternating current impedance

18650-type cells with 2.5 Ah capacity are cycled at both 25 °C and 0 °C separately, and at 25 °C two charging protocols (constant current, and constant current-constant voltage charge) are used. Differential voltage analysis (dV/dQ) and alternating current (AC) impedance are mainly used to investigate battery degradation mechanisms quantitatively. The dV/dQ suggests that active cathode loss and loss of lithium inventory (LLI) are the dominating degradation factors. Significant microcracks are observed in the fatigued cathode particles from the scanning electron microscopy (SEM) images. Crystal structure parameters of selected fatigued batteries at fully charged state are determined by in situ high-resolution neutron powder diffraction. Obvious increases of ohmic resistance and solid electrolyte interphase (SEI) resistance occur when the battery capacity fade falls beneath 20%. Continuous charge transfer resistance and Warburg impedance coefficient (W.eff) increase are observed in the course of cycling. Correlation analysis is performed to bridge the gap between material loss as well as LLI and impedance increase. The increase of the charge transfer resistance is related to both active cathode loss and LLI, and a functional relationship is revealed between LLI and W.eff regardless of the used cycling protocols.

A Modified Luo Converter-Based Electric Vehicle Battery Charger With Power Quality Improvement

Most of the single-stage single switch power factor correction (PFC) solutions have limited applications in high power ac–dc converters as an increased voltage across and current through the semiconductor devices are seen, which, in turn, results into lower efficiency. This article deals with a modified isolated Luo PFC converter-based electric vehicle (EV) charger with two switches and input clamping diodes at the primary of an isolation high frequency transformer (HFT). This configuration facilitates the advantage of comparatively low switch voltage stress, which is clamped at the input voltage. The proposed converter is selected to operate in discontinuous conduction mode (DCM) to obtain built-in PF correction capability at ac mains. The operation of the converter in DCM, considerably reduces the control complexity due to reduced number of sensors. A prototype is implemented to verify the improved power quality (PQ) performance of the proposed modified Luo converter. An improved low ripple-based charging is provided in constant current-constant voltage modes, which improve the long-term battery service life. The performance of the converter is found robust enough to validate the efficacy of charging at steady state and at varying line voltage variation range. The improved PQ indices are observed to meet the international standards of IEC 61000-3-2.