BATTERY VOLTAGE EQUALIZER IMPROVEMENT USING DC-DC CONVERTER AND ADDRESSABLE SWITCHED BASED ON SWITCHED CAPACITOR WITH TWO COMPLEMENTARY-PHASE NETWORKS

• Main Author: Bagaspratomo, Langgam
• Format: Theses
• Language: Indonesia
• Subjects: Teknik (Rekayasa, enjinering dan kegiatan berkaitan)
• Online Access: https://digilib.itb.ac.id/gdl/view/77261
• Institution: Institut Teknologi Bandung

The needs for batteries as an energy storage system (ESS) is increasing, with the development of electric vehicle technology and renewable energy generation. Lithium-Ion battery is one type of battery that is quite often used because it has a high energy density, high voltage per cell, relatively low self-discharge rate, and long life cycle. Battery Energy Storage System (BESS) is many battery cells arranged in series and parallel to meet the voltage and capacity requirements of an energy storage system. However, every battery cell in the BESS, especially in a series circuit, has a tendency to experience an imbalance of charge distribution. This has the potential to cause overcharging and overdischarging which results in reduced battery life. To prevent this from happening, a Battery Management System (BMS) is needed which one of its functions is as a equalizing system for the State of Charge (SoC) on the battery cells, which is represented by battery voltage. Recent trend in battery equalizer systems is the switched capacitor (SC) based equalizing system due to its small size, easy and flexible implementation, and relatively high efficiency. In general, the SC functions as a temporary energy storage from the power supply, which will be passed on to the battery cells with a lower charge. If the power supply is sourced from the same battery pack, this SC also functions as an isolation between the power supply and the target cell by separating the charging and discharging phases of the capacitor. In this study, a single SC-based balancing system was developed with a modification of the Zero Current Switching (ZCS) mechanism to minimize switch losses, the use of a DC-DC converter on the SC input side as a voltage regulator, and the use of addressable switches based on 1-wire digital communication protocols for increased network scalability and flexibility. Tests were carried out experimentally on four Lithium NMC LG HG2 batteries with a nominal capacity of 3000 mAh per cell and a nominal voltage of 3.6 V. These four batteries were connected in series with an initial voltage of 3.4, 3.5, 3.55, and 3.6 V, respectively. The equalizing algorithm used is a battery terminal voltage based strategy. The research parameters observed were the effect of SC switching frequency, the use of DC-DC converter, and the number of SC lines in the balancing circuit. Switching frequency testing and the use of DC-DC converter were conducted to determine the role of each on the balancing current. Meanwhile, in testing the number of SC lines, a balancing scenario was carried out until the two cells with the lowest voltage reached equilibrium by observing the voltage of each cell and the balancing current. iv The test results show that the average balancing current will be maximum at the switching frequency of the resonant frequency of the SC circuit, which is 1.2 times, 1.13 times, and 1.28 times higher, respectively, compared to 2, 0.5, and 0.25 times of the resonant frequency. Then by using a DC-DC converter on the SC input side (pack-to-cell topology), the average SC discharge current is 2.66 times higher than the use of the cell with the highest voltage in the SC charging phase (cell-to-cell topology) due to more regulated SC voltage and independent of battery cell voltage. Therefore, in the main test of the balancing system, a resonant frequency is used as the switching frequency and a DC-DC converter on the SC input side to maximize the balancing current. The test results show that the use of two SC lanes results in a 1.75 times faster balancing process and an average balancing current of 1.83 times higher than the use of a single SC in the same configuration. This is because the balancing current from the SC to the target cell is more continuous with the SC discharging phase alternating between the two lines. Therefore, the use of a DCDC converter and two SC lines can be used to optimize a single SC-based active equalizer system.