Research On Active Voltage Balancing Topology And Improving The Balance Speed For Series Connected Batteries

As a critical component in renewable energy generation and electric vehicles,energy storage systems directly affect the safety and power density of the system.Therefore,the energy storage system is the core technology to cut down the emission of carbon dioxide.The working process of the battery includes the mutual conversion of electric energy,chemical energy and heat energy.The internal reaction of the battery is complex,and it is susceptible to many factors such as operating temperature,electrode deterioration,and current rate.However,in long-term use,the battery voltage of the series battery pack varies with the battery’s internal voltage due to slight differences in interior electrical characteristics,chemical characteristics,and operating temperature.The maximum capacity of the battery pack depends on the highest voltage monomer,and the voltage imbalance causes the actual capacity of the battery to decrease.If the voltage of one cell is too high or too low,the entire battery pack will be overcharged or over-discharged,which may lead to an explosion or fire.Therefore,the voltage equalizer is widely used for battery packs to maximize the available operating capacity and extend battery life.The thesis focuses on simplifying the battery balance topology and improving the balance speed.A series of researches have been carried out on the battery balance topology of different application scenarios such as recycled battery packs,electric vehicle energy storage systems,automated guided vehicle,and hybrid energy storage systems.The main research work is as follows:(1)A fast resonant switched-capacitor balancing topology is proposed for recycled battery strings.The proposed equalizer uses a pair of fixed-frequency complementary PWM waves to control the resonant capacitor,directly transferring energy from a high-voltage battery to a low-voltage battery.Analyzing the relationship between the current,voltage and operating frequency of the resonant switched-capacitor balancing topology in the working process,and discussing how to eliminate the inrush current by the resonant switched capacitor.Finally,an experimental prototype is used to verify the effectiveness of the resonant switched capacitor battery balancing topology.(2)A charging equalizer based on wireless power transmission is proposed for electric vehicle energy storage systems,which overcomes the problem that the existing wireless equalization system cannot equalize after the coil is offset achieves constant current output.Establish a simplified model of a simplified series-series coupling mechanism,analyzing the model output characteristics and circuit impedance characteristics,optimizing system parameters,and decoupling the output current from the load to achieve constant current output.Combine the switch matrix circuit to discuss the working principle,current flow and control logic of the equalization circuit.Design the charging/equalization control method to realize the constant current charging and constant current battery pack functions.The effectiveness of the charging equalizer based on wireless power transmission is verified by building an experimental prototype.(3)A multi-winding transformer balancing topology is proposed for automated guided vehicle.The balance circuit is integrated into the DC-DC circuit,and the primary side of the multi-winding transformer shares the inductor of the DC-DC.The current ripple generated by the inductor induces balance currents in the secondary winding of the multi-winding transformer.Besides,the balance system and the DC-DC circuit work independently.The voltage and current characteristics of the multi-winding transformer are analyzed,and the relationship between the winding turns ratio,and the variation range of the balance current are discussed.The effectiveness of the multi-winding transformer balancing topology is verified by building an experimental prototype.(4)A charging equalizer based on Zeta circuit for a hybrid energy storage system is proposed.The Zeta circuit generates voltage ripples during the charging process.The voltage ripples drive the voltage multiplier to realize the charging equalization function of the supercapacitor strings.And the current ripple generated by the Zeta circuit is used to drive the multi-winding transformer to realize the battery pack charging and equalization function.The working principle of the voltage multiplier is analyzed,and the voltage and current characteristics of the Zeta circuit are discussed.The effectiveness of the charging equalizer based on Zeta circuit is verified by building an experimental prototype.The thesis aims to simplify the balancing topology,improve the balance speed,and carry out theoretical analysis and experimental verification for the battery balance topology of different application scenarios.