DEVELOPMENT OF BATTERY STATE OF CHARGE EQUALIZATION CIRCUIT BY TWO COMPLEMENTARY-PHASE SWITCHED-CAPACITORS CONFIGURATION

• Main Author: Prima Wijaya, Trendy
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/51996
• Institution: Institut Teknologi Bandung

Battery energy storage systems typically use a large number of cells in determined series or parallel configuration to reach total voltage and capacity specified. Those cells in series configuration suffer from unequal or unbalanced state of charge distribution. Some cells with different capacity tend to overcharge or overdischarge, hence they need to be protected from potential functional failures or catastrophic accidents. Therefore, battery system need to be equipped with Battery Management System which performs some essential functions, such as State of Charge Balancer. One of hi-efficiency battery state-of-charge balancer topology is based on switched capacitor. Its mechanism consist of two fundamental switching phases. At the first phase, high state-of-charge cell is connected to capacitor so part of its charges will be stored temporarily since the capacitor role is as energy storage intermediary. At the second phase, capacitor is connected to low state-of-charge cell so it can give away the transferred energy from the first phase. The reason of this separation of phases is to prevents short circuit condition between the source and target cells. In this research, some modification of state-of-charge balancer based on switched capacitor have been done by allocating two charge transfer circuits which operated in complementary phase. The switching mechanism is also modified by inserting inductor to deliver Zero Current Switching condition so balancing efficiency can be increased. The balancing algorithm uses state-of-charge condition as an effective goal and overequalization prevention which happen on voltage based algorithm. The state of charge equalization and final cells voltage difference convergence which truly reach near-zero value is known as Zero Voltage Gap condition The scientifical approach of the research are mathematical modeling and simulation aided by Simscape as circuit simulation software. The battery model uses empirical data of Lithium battery LG HG2 18650 which nominal capacity is 3000 mAh and nominal voltage is 3.6 Volt. The balancing circuit includes DC/DC converter and modified switched capacitor which uses feasible and realizable parameter values for each specific components. Charge transfer rate and efficiency from the circuit then be used as performance parameter for integrated simulation on some scenario. The setup of the simulation programs was performed using initial state-of-charge of each cells were set on same differential value, such as 20%, 40%, 60%, and 80%, while the initial voltage was 0.6 Volt. The result shows that two complementary phase network with Zero Current Switching modification have 1.391 times larger rms value of balancing current compared to conventional single network balancer. This soft switching mechanism was capable to reduce switching power losses up to 538 times smaller, hence the efficiency was increased by 4.38%. There are three considerable basic topologies: cell-to-cell, cell-to-pack, and pack-to-cell. The best balancing speed performance was done by cell-to-cell topology which reached Zero Voltage Gap condition in 5007 seconds within 0.01 Volts cells different voltage tolerance value. On the other simulation, the experiment was set up to load the battery with constant 1 Amperes current until some of the cells voltage reach 2.5 Volts cutoff voltage. Cell-to-cell topology was able to prolonged the working time untuil 4502 seconds and small unused remaining capacity (187.4 mAh) since this topology was effectively redistribute the cells state-of-charge. On the other hand, this topology was too expensive because of its large number of switch components and wiring complexity. The cell-to-pack topology reach the same above final condition in relatively long times, i.e. 10132 seconds. Its unusable capacity after current loading experiment was 2521.4 mAh for relatively short working time, i.e. 2546 seconds. The pack-to-cell topology had faster balancing time, i.e. 7510 seconds. This topology was able to prolong the working time on current loading test up to 3799 seconds with unused capacity down to 563,7 mAh. Therefore, pack-to-cell topology was suitable as an alternative choice for smaller components number and lower complexity require-ments compared to cell-to-cell topology.