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.