Research On Bidirectional Equalization Strategy And System Development For Lithium-ion Power Battery Pack Of Pure Electric Vehicle

When the lithium-ion batteries which are connected in series and parallel are applied topure electric vehicles, due to the battery pack imbalance, it makes battery capacityutilization and driving range decrease; and also battery life reduces, which results in theincrease of the cost of pure electric vehicles. So imbalance of the battery pack is bad forselling more pure electric vehicles. The battery equalization technology studied in the thesismakes batteries balanced by measuring batteries parameters in real time, and the batteryequalization technology can decrease the imbalance of batteries and the capacity utilizationof the battery pack, slow the rate of battery aging and extend the life of the battery pack.The thesis analyzes the mechanism of lithium-ion battery pack imbalance and designan equalization system, and conducts experimental verification. The specific contents are asfollows:1. Analysis with regards to causes of battery imbalance from the process ofproduction，using and storing is made. This thesis summarizes different methods in dealingwith battery imbalance and makes a conclusion that equalization technology is meaningfulfor slowing the rate of battery aging and improving battery imbalance. Then the thesiscompares imbalance on open circuit voltage(OCV), closed circuit voltage(CCV) and stateof charge(SOC) based on lithium-ion battery experiments and does comprehensive analysis,OCV and CCV are chosen as the equalization variable2. The thesis compares advantages and disadvantages of common equalization circuit.It selects the non-dissipative balancing energy topology based on fly-back transformer.Basic parameters of the fly-back transformer are designed and control parameters aredetermined relying on a balanced circuit simulation model.3. The thesis selects the voltage range as the balancing entry and exit judgmentthreshold which is determined according to the relevant requirements of the battery circuit and lithium-ion battery experiments; then the thesis develops different strategies adapting todifferent working status of battery pack and proves their effectiveness by simulation.4. The thesis uses the master-slave architecture as the general framework of theequalization system, and designs hardware circuits such as the voltage acquisition, balancedcircuits. Software design is completed after that.5. The thesis compares and analyses system commissioning results and the simulationresults in the top balance and bottom balance. A series of bench experiments is conducted toexamine the effectiveness of the equalization system.