Research On Balancing Strategies And Equalization System Design For Battery Pack Of Pure Electric Vehicle

When the lithium-ion batteries which are connected in series are applied to pure electricvehicles, due to the battery pack imbalance, the performance of the battery pack will decline.It’s available capacity and cycle life will be far lower than the single battery and the batteryperformance not be made full use, which result in the decrease of the range distance and theincrease of the cost of electric vehicles. Battery equalization system is actually one of themost important parts of battery management system (BMS), which can monitor the state ofthe battery pack in real time and start balancing program according to certain strategy ifneeded, then the inconsistency of batteries will decrease. The capacity utilization rate of thebattery pack can also be improved, the same as the range of distance.Based on the fully understanding about the research on battery equalization technologyat home and abroad, this thesis takes the lithium-ion power battery pack in series as theresearch object, studies the equalization topology which can be simply controlled and has ahigh efficiency, and corresponding control strategy.1. Analysis with regards to causes of battery inconsistency from the process ofproduction，using and storing is made at first. State the forms of battery pack inconsistencyreflected in the battery parameters (voltage, resistance, SOC). The thesis compares theadvantages and disadvantages when the voltage or SOC is regarded as the qualizationvariables. Finally, the voltage is chosen as the equalization variable in different workingstages of battery pack.2. The thesis compares advantages and disadvantages of common equalization circuit.Through evaluating the equalization efficiency, speed and difficulty of controlling of thosecircuits, it selects the non-dissipative balancing energy topology based on fly-backtransformer. Basic parameters and improvements of this topological structure are designed,control parameters of the two models are also determined relying on their balanced circuitsimulation model.3. The thesis selects the voltage range as one of the balancing entry and exit judgmentsthreshold which is determined according to the relevant requirements of the battery circuitand lithium-ion battery experiments. In order to avoid the charging time is too long and thesystem makes misjudgments which caused by the irregularity of battery pack dischargingcurrent. Two kinds of strategies called “Charging Balance” and “Parking Balance” adapting to different working status of battery pack are presented and simulations based onMATLAB/SimPowerSystems prove their effectiveness. In addition，the two strategies willonly work at the end of charging and discharging.4. Combining the overall layout and structure of the power battery pack in pure electricvehicles, the thesis puts forward the “master-slave” architecture as the general framework ofthe equalization system, and completes the choice of components of each module, thehardware circuit design. Software design is completed after that. Then the thesis comparesand analyses system commissioning results obtained in bench test platform and thesimulation results in the top and bottom balance models. A series of bench experiments areconducted to examine the effectiveness of the equalization system.