Abstract
Entering the 21st century, new & renewable energy, electric vehicle (EV) and energy storage system (ESS) have emerged due to more regulations on CO2 gases and depletion of fossil fuels after climate changes. With a leap towards the 'IT Era,' uninterruptible power supply (UPS) has developed into an essential equipment, an emergency power system designed to prevent a blackout. A key element of these facilities is a secondary battery which can be divided into lead-acid battery, Ni-MH, Ni-Cd and lithium ion battery (LIB). In particular, there have been numerous studies on lithium batteries with the following advantages: i) low price for unit volume and energy density by weight, ii) very stable lead-acid and high energy density, iii) long life expectancy. The valve-regulated lead-acid battery (VRLA) designed for power storage has a long life expectancy that is 3,000 times or more at 70% depth of discharge. Since it is a closed type, it can suppress the decrease of the electrolyte level, making maintenance unnecessary. In this kind of the VRLA designed for power storage (e.g. photovoltaic power generation, wind power generation, load leveling, etc.), battery performance and life are dependent upon lead alloy-based grid casting and electrolyte 'gel' mixing technologies. Regarding energy accumulation for optimum system operation, the technology developed to figure out the progress of battery failure is a key factor. The conventional large stationary lead-acid battery is mostly used as a backup against a blackout. Even though its life expectancy increased to 15 years, it is not appropriate for power storage in which charge and discharge are repeated every day. These lead storage batteries have critical effects on battery life and performances depending on their charging system. Therefore, the design of an optimum charge system is crucial for operating an optimum system. In general, the suitability of an optimum charge system is assessed by measuring the degree of battery aging. Hence, this study attempted to derive the optimum charge system setting after assessing the degree of battery aging under diverse load conditions. According to high-temperature accelerated life testing, battery internal resistance almost doubled compared to the early-stage battery when the rated capacity decreased to 80% or less. At the same time, battery surface temperature increased by almost twice as well. The VRLA/GEL was primarily heated in the middle area. It appeared that a gel electrolyte was dried out because of heating, causing increase in temperature. In LIB, diverse cathode materials have been applied since the lithium ion secondary battery comprised of LiCOO2 was first developed by SONY. However, LiFePO4 and ternary battery are currently used most widely. LiFePO4 is 3.2V in operating voltage and 170mAh/g in theory capacity. Even though it is slightly lower than a ternary battery in terms of energy density, it has a low risk of ignition and explosion. Therefore, it is great in terms of battery safety. With the aforementioned properties, a lithium ion battery is advantageous in industrial battery sectors such as x-EV, ESS and UPS. Even so, LiFePO4 reveals very low electric conductivity as an olivine material (LiMPO4, M=Mn, Ni, Co, Fe). This weakness has been greatly improved by coating the surface of the active material with carbon. However, degradation becomes more severe because of increase in battery internal resistance as a cycle proceeds. The objective of this dissertation was to clarify failure mode of the secondary battery for ESS. Therefore, this study located the source of the heat which occurs at battery charge or discharge, using IR SnapShot Model 525, one of the non-destructive testing (NDT) and analyzed the progress of the degradation. Next, the degradation behavior and durability of lead storage battery were compared through the measurement of battery internal resistance. It was found that battery internal resistance almost doubled compared to the early-stage battery when the rated capacity decreased to 80% or less. At the same time, battery surface temperature increased by almost twice as well. The surface temperature and internal resistance of the lithium battery almost doubled respectively because of problems in the manufacturing process and the materials themselves.
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