Novel Fitting Algorithm for Parametrization of Equivalent Circuit Model of Li-Ion Battery from Broadband Impedance Measurements

The impedance of Li-ion batteries contains information about the dynamics and state parameters of the battery. This information can be utilized to improve the performance and safety of the battery application. The battery impedance is typically modeled by an equivalent-circuit-model (ECM) which provides the dynamic information of the battery. In addition, the variations in the model parameters can be used for the battery state-estimation. A fitting algorithm is required to parametrize the ECM due to the non-linearity of both the battery impedance and ECM. However, conventional fitting algorithms, such as the complex-nonlinear-least-squares (CNLS) algorithm, often have a high computational burden and require selection of initial conditions which can be difficult to obtain adaptively. This paper proposes a novel fitting algorithm for the parametrization of battery ECM based on the geometric shape of the battery impedance in the complex-plane. The algorithm is applied to practical and fast broadband pseudo random sequence impedance measurements carried out at various state-of-charges (SOC) and temperatures for lithium-iron-phosphate cell. The performance of the method is compared to conventional CNLS algorithm with different initial conditions. The results show that the proposed method provides fast and accurate fit with low computational effort. Moreover, specific ECM parameters are found to be dependent on the battery SOC at various temperature.publishedVersionPeer reviewe

Online Impedance Measurement of Lithium-Ion Battery : Applying Broadband Injection with Specified Fourier Amplitude Spectrum

State parameters such as the state-of-charge (SOC) and state-of-health (SOH) play important role in the operation of Li-ion batteries, and accurate estimate of the parameters is of prime importance in evaluating the battery condition and guaranteeing the safe use. While it is difficult to measure the SOC and SOH directly, the battery internal impedance can be used to obtain the parameters. Recent studies have shown methods based on pseudo-random binary sequence (PRBS) with which the battery impedance can be rapidly measured in real time. Using this method, the battery is charged and recharged according to the excitation waveform and Fourier methods are applied to obtain the impedance. The PRBS often produces accurate impedance measurements, but the method may require a high perturbation amplitude, which may easily interfere with the nominal battery operation and create nonlinear distortions. This paper proposes the use of discrete-interval-binary sequence (DIBS) for measuring the battery impedance. The DIBS is a computer-optimized binary sequence in which the power spectral densities are maximized at specified frequencies without increasing the signal time-domain amplitude. Experimental measurements of a commercial Li-ion battery are presented to demonstrate the effectiveness of the proposed method.Peer reviewe

Adaptive Bus-Impedance-Damping Control of Multi-Converter System Applying Bidirectional Converters

Modern dc-power-distribution systems utilizing energy storages are often dependent on the operation of bidirectional power-electronics converters. Such distribution systems typically consist of several feedback-controlled converters prone to experience stability issues due to cross-effects among the different converters. Studies have presented adaptive control-based techniques to prevent such stability issues, but most studies have not fully considered their implementation on a bidirectional converter. The system dynamics may vary significantly depending on the operating point and particularly the direction of the bidirectional power flow. Therefore, specific care should be taken in the design of the adaptive stabilizing control to guarantee that the system’s regular operation is not impeded when the stabilization is implemented on a bidirectional converter. This paper proposes a procedure to implement an adaptive stabilizing control method on a bidirectional converter with minimal changes to the regular controller. We add an adaptive resonance term to the bidirectional converter’s voltage controller that enhances stability and damping around the identified resonance frequency without impeding the converter’s regular operation. The resonance term is adjusted periodically based on online impedance measurements and the chosen design criteria. As a result, the controller can dampen resonances and prevent adverse impedance-based interaction. Experimental measurements based on a multi-converter setup demonstrate the effectiveness of the proposed methods.acceptedVersionPeer reviewe

Broadband Impedance-Measurement Methods in Dynamic Analysis of Dual Active Bridge Converters

The output impedance of a switched-mode converter is an important parameter in the converter small-signal stability analysis and stability-enhancing control. One of the popular methods for obtaining the converter output impedance is to introduce a broadband signal such as the pseudo-random binary sequence (PRBS) to the converter duty cycle; the resulting output voltage and current are measured, and the output impedance is obtained with Fourier methods. However, such techniques have not been fully considered for dual active bridge (DAB) converters. This paper discusses the challenges of broadband impedance measurements on DAB converters, compares different implementation methods, and outlines guidelines for achieving accurate results. Unlike conventional dc-dc converters, the power of a DAB converter can be controlled by both the duty ratio and phase shift, thus allowing several methods to perform the impedance measurement. Yet, nonlinearity between the power transfer and phase shift and duty ratio introduces potential challenges to the measurement implementations. Experimental measurements based on a DAB converter are shown to demonstrate these challenges as well as the effectiveness of the proposed measurement guidelines.acceptedVersionPeer reviewe

Adaptive configuration of generalized nonlinear ECM of Li-ion batteries based on impedance measurements and DRT analysis

An adaptive approach for configuration of generalized battery nonlinear equivalent-circuit-model (ECM) is proposed. In the approach, the distribution-relaxation-times (DRT) analysis is used to configure and initialize the ECM to be fitted to the impedance data. The performance of the approach is validated and analyzed by using experimental battery impedance measurements

Internal Impedance in Determining Usability of Used Lithium-Ion Batteries in Second-Life Applications

Li-ion batteries have become key energy storage devices in powering various industrial processes and consumer products. Increased demand for Li-ion batteries has raised environmental issues because most of the batteries are landfilled upon reaching end of life. However, studies have suggested that most of the used Li-ion batteries can be reused in second-life applications. Battery state parameters such as the state-of-health (SOH) and state-of-charge (SOC) are typically used for assessing the performance. The SOH is particularly useful in predicting the remaining life cycle of the battery and selecting suitable second-life applications for reuse. However, measuring the SOH can be difficult using existing technology. Recent studies have shown that a battery's internal impedance can be used to determine the SOH of the battery. This paper demonstrates the applicability of the internal impedance in evaluating the SOH and suitability of used Li-ion batteries in second-life applications. Experimental measurements from several commercial Li-ion batteries are shown and analyzed.Peer reviewe

Effects of Ramp Rate Limit on Sizing of Energy Storage Systems for PV, Wind and PV–Wind Power Plants

As the share of highly variable photovoltaic (PV) and wind power production increases, there is a growing need to smooth their fast power fluctuations. Some countries have set power ramp rate (RR) limits that the output powers of power plants may not exceed. In this study, the effects of RR limit on the sizing of energy storage systems (ESS) for PV, wind, and PV–wind power plants are examined. These effects have been studied prior for PV power plants. However, for the wind and PV–wind power plants, the effects of the RR limit are studied comprehensively for the first time. In addition, the effects of the size of the power plant are considered. The study is based on climatic measurements carried out with a sampling frequency of 10 Hz for a period of 153 days. The modeling of the PV and wind powers and the simulation of the RR-based control algorithm of the ESS were completed using MATLAB. The results show that as the applied RR limit increased from 1%/min to 20%/min, the required relative energy capacities of the ESSs of the PV, wind, and PV–wind power plants decreased roughly 88%, 89%, and 89%, respectively. The required relative power capacities of the ESSs of the PV, wind, and PV–wind power plants decreased roughly 15%, 12%, and 20%, respectively. The utilization of the ESSs was found to decrease as the applied RR limit increased and as the size of the power plant grew.publishedVersionPeer reviewe

Effects of Ramp Rate Limit on Sizing of Energy Storage System for PV-Wind Power System

The power produced by variable renewable energy power plants (VREPP) can fluctuate heavily and cause issues in the power grid. To prevent the power quality issues in the grid, some countries have set a ramp-rate limit (RR) that the generated output power of power plants may not exceed. The power fluctuations of VREPPs are often mitigated by an energy storage system (ESS) and a power smoothing method. This paper presents how the RR limit value affects the size of an ESS needed for a photovoltaic (PV)-wind power system. Also, the size of the power plant is considered, and how it affects the size of the ESS. The generated power of the PV-wind power system was simulated using measured irradiance, temperature and ind speed. An RR-based control algorithm was used to operate the virtual ESS. It was found that the increase in the RR limit greatly decreases the size of the ESS. The size of the power plant also significantly affects the size of the ESS.Peer reviewe

Realtime Internal-Impedance Measurement of Lithium-ion Battery Using Discrete-Interval-Binary-Sequence Injection

Internal impedance of a Lithium-ion (Li-ion) battery is an important parameter in evaluating the battery characteristics such as the state-of-charge (SOC) and state-of-health (SOH). Recent studies have shown broadband methods based on pseudo-random binary sequence (PRBS) and Fourier techniques with which the battery impedance can be accurately and rapidly measured in real time. Applying the conventional PRBS requires, however, a relatively high injection amplitude that may easily interfere with the normal battery operation and produce nonlinear distortions. This paper proposes the use of a discrete-interval-binary sequence (DIBS) for a battery impedance measurement. The DIBS is a computer-optimized binary sequence in which the energy is maximized at specified harmonic frequencies to minimize the required level of signal injection. Otherwise the DIBS has the same attractive characteristics as the conventional PRBS. Experimental results based on a commercial Li-ion battery are presented to demonstrate the effectiveness of the proposed method.acceptedVersionNon peer reviewe