Research On Lithium-ion Battery Equalization Based On Quasi-resonant Converter

Nowadays, lithium-ion batteries are widely used as an energy storage devicefor electric vehicles due to its excellent performance. However, the wo rkingvoltage of electric vehicle is up to hundreds of volts, single lithium-ion can notreach. Therefore, the single battery in series and parallel as battery group couldbe used to meet the demand of users. Owing to the environment impacts andmanufacturing process limitations, the inconsistencies, which could lead to lowerbattery’s capacity and reducing the battery life or even effecting the battery safetyperformance, will gradually present the charge state between the batterymonomer of the series group. Hence, this paper introduces the batteryequalization technology in order to improve the cell inconsistency betweenmonomers and also reduce its negative effect on the cell performance.In fact, the equalization of the series lithium-ion battery are to reach energyequilibrium between monomers through specific approaches. This paper takes adeep research on the characteristics of lithium-ion battery and SOC estimatingmethod, and summarizing the relationship between terminal voltage and batter ySOC, which could provide the basis for equilibrium. Based on the deep study ofthe exiting equilibrium circuit structure, this paper designs the equilibriumsystem which consists of the main control module, information collection module,battery equalization module, PC and display module, etc. Based on quasi resonantconverter, equalization module as the core of the system uses the centralizedequilibrium structure. Quasi resonant fly-back power converter uses the wholegroup battery energy as input and supplies energy to the single cell with minimalpower, in this way battery energy equilibrium could be achieved. The othermodules of the equilibrium system are used for collecting the battery voltage,temperature and current information, communication between modules, and themain circuit protection, etc. This paper designs the overall structure of the system,especially focus on the hardware and software design of quasi resonant fly-backconverter, the structure of relay switching network, battery informationacquisition circuit, the main control circuit, and compiles the upper monitor andIPC screen display program.This paper establishes the lithium-ion battery equalization experimentalplatform which could do the voltage acquis ition, temperature acquisition and equilibrium experiments, etc. The experimental results show that the designedcollecting devices could precisely collect the battery voltage, temperature andother battery parameters, and the whole system could achieve battery e nergyequalization in batteries according to the designed equilibrium strategy.This paper demonstrates the effectiveness of SOC estimation method and thehigh precis ion of voltage acquisition circuit which sampling error is less than10mV. The upper monitor could display the battery status information in real time,which could achieve human-computer interaction control. This paper also provesthat the equalization module has obvious equilibrium effect, and could achievepower equilibrium between monomers, extend the battery life, and also increasethe available battery capacity.