Research On Balancing Strategy For Lithium-ion Battery Pack Of Pure Electric Vehicle And Equalization System Implementation

As the energy carrier of the electric vehicle, the Lithium-ion battery pack is made up ofa large amount of battery cells. Because of cell imbalance, the performance of the batterypack is decided by the worst one, thus other better cells could not work at100%of theirpotential and the capacity of the pack decreases. Moreover, imbalance may let the cells beprone to accelerated degradation, which would in some way affect the driving range and costof the electric vehicle. An effective way at present of reducing cell imbalance is to use theequalization system.Battery equalization system is actually one of the most important parts of batterymanagement system (BMS), which would monitor the state of the battery pack in real timeand start balancing program according to certain strategy if needed. Thus a better capacityutilization rate and longer life cycle of the battery pack would be obtained, which are ofimportance to the electric vehicle.Lots of researches on battery balancing have been done at home and abroad, and mostof them focus on balancing strategies and balancing topologies. A reasonable balancingvariable is the premise of an effective strategy, and voltage and SOC are most widely usedvariables now, while the latter one is always affected by its precision. Balancing topologiesfall into two categories, i.e. passive and active balancing and the latter one is a hot area ofresearch now, such methods like switched capacitor, converter and transformer balancingtopologies have been proposed. However there is no balancing topology at present that canmeet the demand of the battery pack used in electric vehicle totally and perfectly.In this paper, some studies on cell imbalance have been done and based on those anequalization system has been established. The main work is as follows:1. Analysis with regards to causes and solutions of cell imbalance was made at first, which showed the importance of equalization system. Then a comparison about imbalanceon open circuit voltage(OCV), closed circuit voltage(CCV) and state of charge(SOC), i.e. theexternal characters of cells in series, was made and with the consideration of SOC’sprecision, OCV and CCV were chosen as the balancing variable;2. Compared the advantages and disadvantages of different balancing topologies indetail and then selected the Buck-Boost converter as the basic balancing circuit. Also voltagerange was introduced here as the evaluation indicator of cell imbalance. After that, four kindsof strategies called “Equal Balance”,“Average Balance”,“Top Balance” and “BottomBalance” adapting to different working status of battery pack were present and simulationsbased on MATLAB/SimPowerSystems proved their effectiveness.3. A host-slave architecture was adopted by the equalization system, both hardware andsoftware designs were completed based on which. Then a calibration protocol based onMODBUS was made for the communication between computer and MCU.4. A series of bench experiments were done to examine the effectiveness of theequalization system. Results of these experiments showed a lower cell imbalance and ahigher capacity utilization rate.