Bidirectional flyback converter design for energy transfer applications in battery management systems with active balancing method

The cells forming the battery pack show non-equivalent charging and discharging behaviours due to manufacturing tolerances and environmental conditions. These non equivalent behaviours between the cells cause safety problems by exposing the cells forming the battery pack to overcharge and overdischarge. At the same time, it causes the battery pack to be used inefficiently and to complete its service life early. In order to eliminate these negative situations, the cells that make up the battery pack must be balanced. Cell balancing systems are the most basic component of battery management systems. Cell balancing systems are divided into two as passive and active cell balancing. In passive cell balancing, the balancing process is performed by expending the excess energy that causes the imbalance between the cells that make up the battery pack, while in active balancing, the cell balancing process is performed by transferring the excess energy that causes the imbalance from the high energy cell to the low energy cell. While passive cell balancing system has the advantage of simple design and low cost, it has the disadvantage of low balancing speed and low balancing efficiency. While active cell balancing has the advantage of high balancing speed and high balancing efficiency, it has the disadvantages of high cost and large system size. In this thesis, a transformer based active cell balancing system has been developed for battery packs containing three or more lithium ion cells, which can perform cell-to pack or pack-to-cell bidirectional cell balancing according to the charge or discharge state of the battery pack. The cell balancing system consists of a switching array for switching on the cells in the battery pack, isolated and non-isolated driver circuits for controlling the switching elements, current sensing circuits for determining the charging and discharging currents, voltage sensing circuits for determining the cell and battery pack voltages, and a flyback converter for energy transfer.The operating principle and operating modes of the cell balancing system and the flyback converter are analysed in detail. The mathematical equations required for the design of the flyback converter are given. Firstly, the cell balancing system is simulated in Matlab/Simulink environment. Then, the real-time application circuit is designed and the performance of the balancing system is analysed in the laboratory environment. Simulation and real-time application findings are compared and found to confirm each other.It was determined that the cell balancing system is superior to other cell balancing systems with its ease of control and design, high balancing speed and having few magnetic components.