The lithium-ion batteries eliminate a power cable, which provides the mobility, portability, and convenience of electronic devices and appliances. As a result, battery-powered electronic devices and appliances, such as portable computers, smart phones, camcorders, and cameras, are ubiquitous and essential in the daily lives. Since the charging technique affects the charging time, rising-temperature, and aging of lithium-ion batteries in battery-powered electronic devices and appliances, many charging techniques have been proposed to improve the charging characteristics until now. Of them, the innovative charging techniques based on alternating current (ac) such as the pulse-ripple-current (or pulsed current) and sinusoidal-ripple-current have recently received a lot of attention from academic and industrial fields. The charging characteristics and effects of theses charging techniques are being highly debated. Therefore, in this dissertation, the pulse charging technique and the sinusoidal-ripple-current charging technique are studied for lithium-ion batteries. By analyzing the overpotential voltage (OPV) across the battery impedance from an electrical engineering perspective, the charging characteristics and effects of theses charging techniques are investigated and defined. The study is divided into two parts as follows.
Part I. Overpotential Voltage Analysis of Pulse Charging Technique for Practical Applications
The pulse charging technique uses a pulsed current (PC) periodically falling to zero current. Since the zero current condition of the PC, commonly called relaxation time or rest time, allows ions in battery electrolyte to be evenly distributed, the PC could suppress dendrite formation and improve deposition morphology. For this reason, the pulse charging technique has received considerable attention from academia and industry.
However, since the pulse charging technique has been mainly studied from an electrochemical perspective, electrical engineers who design the battery charging systems have difficulty in understanding the characteristics, benefits and drawbacks of the pulse charging technique from electrochemical analysis. Also, the characteristics and effects of the pulse charging technique are being highly debated. Furthermore, it is not clear whether the pulse charging technique needs the constant voltage (CV) charging for full-charge. Moreover, there is no design guideline to select the well-tuned parameters of the PC. Therefore, this study analyzes an overpotential voltage (OPV) to apply the pulse charging technique.
The OPV analysis based on an equivalent circuit-based battery model helps electrical engineers to understand the characteristics of the pulse charging technique and explains the necessity of constant voltage (CV) charging for full-charge. The alternating current (ac) of PC generates the ripple of OPV, which causes the CV charging to be applied earlier in comparison with a traditional CC-CV charging technique. It means that the open circuit voltage (OCV) and state-of-charge (SOC) are low at the end of the pulse charging technique. Thus, the CV charging should be added for full-charge after the battery is charged by the PC. Due to the OPV ripple on the PC phase, the CV charging current increases dramatically at the beginning of the CV phase to hold closed circuit voltage (CCV) at upper voltage limit. As a result, the pulse charging technique reduces the charging time at the cost of the battery temperature. In this study, the ripple of OPV is optimally reduced by the PC with low frequency and large duty-cycle according to the OPV analysis. In comparison with the traditional CC-CV charging technique, the PC with low frequency and large duty-cycle improves the charging time and maximum-rising-temperature by 4.0% and 10.5%, respectively.
Part II. Battery Impedance Analysis Considering DC Component in Sinusoidal Ripple-Current Charging Technique
A scientific method to optimize the frequency of PC has not yet been found. Until now, empirical and trial-and-error methods have been used to search for the optimal frequency of PC. Electrochemical impedance spectroscopy (EIS) has been used in many studies to define the electrochemical properties of batteries and to understand characteristics of batteries in ac analysis. Recently, new charging techniques are introduced to use the frequency where the battery impedance reaches a minimum in EIS. This frequency is called the minimum-ac-impedance frequency. Instead of the PC, a sinusoidal current is used to set the frequency of the charging current to the minimum-ac-impedance frequency. Since the battery is not charged by the sinusoidal current, the sinusoidal current is superposed with a direct current (DC) as a charging current in practice. This new optimized charging technique is called as the sinusoidal ripple-current (SRC).
However, in analyzing the effect on the SRC charging, the DC component of the SRC has not been considered until now. This study presents a battery impedance analysis when the DC component is considered in the SRC charging. The real overpotential voltage across the battery impedance is analyzed by using an electrical second-order RC battery model and overpotential voltage waveforms. The result shows that the real battery impedance is not minimized at the minimum-ac-impedance frequency during SRC charging. Due to this, in comparison with the CC-CV charging technique, the charging time, charging amount and charging efficiency of the SRC-CV charging technique are not significantly different from those of the CC-CV charging. Rather, due to the ac component (or sinusoidal component) of the SRC, the SRC-CV charging deteriorates the RMS current and maximum rising temperature by 22.5% and 18%, respectively. Also, since the DC component of the charging current charges the battery practically, this study presents that the CC-CV charging using a slightly larger DC is more suitable for practical applications. This study shows the current stress, charging time, and maximum rising temperature of the CC-CV charging based on a slightly larger DC are improved by 2%, 9.7%, and 8.5%, respectively, in comparison with the SRC-CV charging.
This study clarifies the characteristics and effects of the pulse and sinusoidal ripple-current charging techniques for lithium-ion batteries. As a result, this dissertation contributes to the study on the charging characteristics of the lithium-ion batteries.
본 논문은 전원소스로 많이 사용되고 있는 리튬이온 배터리의 충전방법에 관한 연구이다. 특히, 최근에 많은 주목을 받고 있는 펄스나 정현파형태의 충전전류를 이용하는 펄스충전방법 (Pulse charging technique)과 정현파-리플전류 충전방법 (Sinusoidal ripple-current charging technique)에 관해 2개의 부분으로 나누어 집중적으로 다룬다. 전기적인 배터리 임피던스 모델을 이용하여, 이러한 충전방법들의 특성과 효과를 이론적으로 분석하고 이론결과를 실험적으로 뒷받침한다. 또한, 기존의 방법과 비교를 통해 충전방법 결정에 도움을 준다. 이 연구의 결과는 배터리 충전연구와 배터리 충전시스템의 설계에서 충전방법을 결정하는데 많은 기여를 할 것으로 예상된다.
Part 1. 실제 적용을 위한 펄스충전방법의 과전압 (Overpotential voltage) 분석
배터리 이온을 고르게 분포하기 위한 펄스충전방법은 최근 많은 주목을 받고 있으며, 영전압을 주기적으로 오가는 펄스전류를 이용한다. 하지만 펄스충전방법은 화학적인 분석을 통해 연구되어 있었기 때문에, 배터리충전시스템을 설계하는 전기엔지니어가 화학적인 분석결과로부터 펄스충전방법의 장단점을 파악하는 것이 매우 힘들다. 또한, 현재 펄스충전방법의 특성과 효과에 대해 찬반논쟁이 있으며, 완충되지 못하여 CV충전의 필요한지 불필요한지도 불투명하다. 게다가, 정확한 분석이 없기 때문에 너무나도 다양한 주파수와 시비율이 사용되고 있다. 그러므로 본 논문에서는 전기적인 배터리 임피던스 모델을 이용하여 분석하기 때문에 전기엔지니어가 쉽게 이해할 수 있으며, 과전압 분석을 통해 펄스충전방법의 특성과 효과에 대해 알아내며 이로부터 완충을 위해 CV충전이 필요함을 보인다. 또한 펄스충전방법의 특성으로부터 설계디자인가이드를 제시한다.
펄스전류는 직류성분과 교류성분으로 되어 있는데 중첩원리에 의해 펄스전류의 직류성분은 직류-과전압을 유발하고 펄스전류의 교류성분은 교류-과전압을 유발하여 과전압의 리플을 형성한다. 이러한 과전압리플은 이른 시간에 완충을 위한 CV충전에 들어가게 하며, 갑작스러운 전류상승을 일으킨다. 이 갑작스러운 상승전류는 충전속도를 향상시키지만 반면에 배터리 온도를 상승시킨다. 그러므로 본 논문에서는 분석결과를 바탕으로 낮은 주파수와 높은 시비율을 가지는 최적의 펄스전류를 이용하여 기존의 CC-CV충전방법보다 충전속도와 최대상승온도를 각각4.0%와 10.5%씩 향상시킨다.
Part 2. 정현파-리플전류 충전방법에서 직류성분을 고려한 과전압 분석
펄스충전전류의 주파수를 최적화하기 위한 과학적인 방법이 없기 때문에 이전까지 실험적인 방법과 경험적인 방법으로 주파수를 최적화하였다. 이를 과학적으로 접근하기 위해 전기화학적 임피던스 분광법(Electrochemical impedance spectroscopy)을 적용하여 배터리 임피던스가 최소화되는 주파수를 찾아서 최소 배터리임피던스 주파수를 가지는 정현파와 직류전류를 중접시켰다. 이를 정현파-리플전류 충전방법이라고 한다. 하지만 이러한 방법은 정현파-리플전류에서 직류성분을 고려하지 않고 교류성분만을 고려했다. 그러므로 본 논문은 정현파-리플전류 충전방법에서 직류성분을 고려하여 분석하며 배터리 임피던스 양단에 걸리는 과전압을 분석함으로써 직류성분을 고려하면 배터리 임피던스가 최소화되지 않음을 보인다. 또한 기존의 CC-CV충전방법에서 전류크기를 약간 증가시키면 정현파리플전류 충전방법보다 빠르면서 배터리온도 적게 상승시키면서 충전할 수 있음을 보인다.
