A Comparative Study on the Influence of DC/DC-Converter Induced High Frequency Current Ripple on Lithium-Ion Batteries

Modern battery energy systems are key enablers of the conversion of our energy and mobility sector towards renewability. Most of the time, their batteries are connected to power electronics that induce high frequency current ripple on the batteries that could lead to reinforced battery ageing. This study investigates the influence of high frequency current ripple on the ageing of commercially available, cylindrical 18,650 lithium-ion batteries in comparison to identical batteries that are aged with a conventional battery test system. The respective ageing tests that have been carried out to obtain numerous parameters such as the capacity loss, the gradient of voltage curves and impedance spectra are explained and evaluated to pinpoint how current ripple possibly affects battery ageing. Finally, the results suggest that there is little to no further influence of current ripple that is severe enough to stand out against ageing effects due to the underlying accelerated cyclic ageing

Design Space Evaluation for Resonant and Hard-charged Switched Capacitor Converters

USB Power Delivery enables a fixed ratio converter to operate over a wider range of output voltages by varying the input voltage. Of the DC/DC step-down converters powered from this type of USB, the hard-charged Switched Capacitor circuit is of interest to industry for its potential high power density. However implementation can be limited by circuit efficiency. In fully resonant mode, the efficiency can be improved while also enabling current regulation. This expands the possible applications into battery chargers and eliminates the need for a two-stage converter.In this work, the trade-off in power loss and area between the hard-charged and fully resonant switched capacitor circuit is explored using a technique that remains agnostic to inductor technology. The loss model for each converter is presented as well as discussion on the restrained design space due to parasitics in the passive components. The results are validated experimentally using GaN-based prototype converters and the respective design spaces are analyzed

Analysis, Design and Control of a Modular Full-Si Converter Concept for Electric Vehicle Ultra-Fast Charging

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The effects of high frequency current ripple on electric vehicle battery performance

The power electronic subsystems within electric vehicle (EV) powertrains are required to manage both the energy flows within the vehicle and the delivery of torque by the electrical machine. Such systems are known to generate undesired electrical noise on the high voltage bus. High frequency current oscillations, or ripple, if unhindered will enter the vehicle’s battery system. Real-world measurements of the current on the high voltage bus of a series hybrid electric vehicle (HEV) show that significant current perturbations ranging from 10 Hz to in excess of 10 kHz are present. Little is reported within the academic literature about the potential impact on battery system performance and the rate of degradation associated with exposing the battery to coupled direct current (DC) and alternating currents (AC). This paper documents an experimental investigation that studies the long-term impact of current ripple on battery performance degradation. Initial results highlight that both capacity fade and impedance rise progressively increase as the frequency of the superimposed AC current increases. A further conclusion is that the spread of degradation for cells cycled with a coupled AC–DC signal is considerably more than for cells exercised with a traditional DC waveform. The underlying causality for this degradation is not yet understood. However, this has important implications for the battery management system (BMS). Increased variations in cell capacity and impedance will cause differential current flows and heat generation within the battery pack that if not properly managed will further reduce battery life and degrade the operation of the vehicle

On the Sizing of the DC-Link Capacitor to Increase the Power Transfer in a Series-Series Inductive Resonant Wireless Charging Station

Wireless inductive-coupled power transfer is a very appealing technique for the battery recharge of autonomous devices like surveillance drones. The charger design mainly focuses on lightness and fast-charging to improve the drone mission times and reduce the no-flight gaps. The charger secondary circuit mounted on the drone generally consists of a full-bridge rectifier and a second-order filter. The filter cut-off frequency is usually chosen to make the rectifier output voltage constant and so that the battery is charged with continuous quantities. Previous works showed that an increase in power transfer is achieved, if compared to the traditional case, when the second-order filter resonant frequency is close to the double of the wireless charger excitation and the filter works in resonance. This work demonstrates that the condition of resonance is necessary but not sufficient to achieve the power increment. The bridge rectifier diodes must work in discontinuous-mode to improve the power transfer. The paper also investigates the dependence of the power transfer increase on the wireless excitation frequency. It is found the minimum frequency value below which the power transfer gain is not possible. This frequency transition point is calculated, and it is shown that the gain in power transfer is obtained for any battery when its equivalent circuit parameters are known. LTSpice simulations demonstrate that the transferred power can be incremented of around 30%, if compared to the case in which the rectifier works in continuous mode. This achievement is obtained by following the design recommendations proposed at the end of the paper, which trade off the gain in power transfer and the amplitude of the oscillating components of the wireless charger output

Improvements in Testing and Performance of Batteries for Automotive Applications

Batteries are increasingly important in modern technologies. This is particularly true in the automotive sector, with hybrid vehicles using batteries to augment the traction power traditionally provided by the internal combustion engine. In such applications, one of the most important factors is the Dynamic Charge Acceptance (DCA) performance of the battery. This study investigates the standard method for establishing DCA performance and determines how the individual parameters of the test procedure and external factors influence the performance of lead-acid cells. This work identifies shortcomings of the standard test, which result in the true DCA performance being better than the standard test suggests. A series of modifications are proposed, which are shown to produce a more representative result. An investigation is performed to determine the effect of cell degradation on charge acceptance. This shows that the DCA test itself is not well suited to assessing the effects of degradation on DCA, and causes the results to appear worse than reality. The work also demonstrates that the usual methods of characterising degradation do not correlate well with DCA performance, and there is very little reduction in charge acceptance over the operational life of the cell. Investigations are undertaken into methods by which DCA performance may be improved. This shows that the application of ac ripple currents to batteries causes a significant increase in charge acceptance, and demonstrates how the frequency of the ripple is important in achieving the best results. This study also shows that the ripple currents have no detrimental effects on the health of the battery. Finally, the work is extended to cover lithium cells. This shows that whilst the DCA performance of lithium is more consistent, maximum charge acceptance is less than lead. It is shown that, by reducing maximum charge voltage, cycle life of cells can be extended without significant loss of stored energy

Custom Maximum Power Point Tracker

The purpose of this project was to design a custom Maximum Power Point Tracker (MPPT) for an off-grid charging application. The custom MPPT used a DC/DC buck conversion topology controlled by a Pulse-Width-Modulated (PWM) signal. The duty cycle of this PWM signal was determined according to a Maximum Power Point Tracking algorithm set by a microcontroller. The implemented algorithm performed power calculations and allowed the solar panel to operate at its maximum efficiency. The design was mounted on a Printed Circuit Board and further testing showed that the algorithm was successful in determining the Maximum Power Point of the solar panel with an efficiency greater than 90 percent

Online impedance spectroscopy estimation of a dc–dc converter connected battery using a switched capacitor-based balancing circuit

This study investigates a novel method of undertaking online electrochemical impedance spectroscopy measurements to estimate battery impedance across the frequency range using a battery balancing circuit. A switched capacitor balancing system is used to generate an excitation signal of low-frequency of variable values from which battery voltage and current can be measured to estimate the impedance