Research On Power Battery Double Layer Active Equalizer Based On Multi-input Transformer

The rapid promotion of new energy vehicles has driven the fast development of power battery industry,and promoted the related product development and frontier research.Among them,the study of on-board lithium-ion battery and its management system are the research hotspot of new energy car companies,universities and research institutions.And there are still many technical problems to be solved.On-board lithium-ion battery pack is composed of hundreds of battery cells in the series-parallel connection so as to meet the needs of the vehicle driving power and range.Because of the parameter differences such as battery self-parameter before leaving the factory,self-discharge,using environment,and working conditions,it is easy to cause the inconsistency of internal and external parameters including internal resistance,voltage,capacity,and so on.And this inconsistency can become worse gradually with the increase of battery pack cycles and ageing,which seriously affects the available capacity and using performance of battery pack,and reduces vehicle safety coefficient.To solve the inconsistency between the different cells or modules,improve the battery life cycle and performance,extend the vehicle range,and ensure the vehicle safety,this paper made analysis and researches on on-board lithium-ion battery equalization system and its control strategy.The main work is as follows:Firstly,the causes and performances of inconsistency for lithium-ion battery was analyzed,and the conclusion that inconsistency caused by working conditions and using environment can be improved through energy balance methods was defined.Secondly,a variety of equalization topology schemes were analyzed and compared.And a double-layer active equalization topology including bottom and top circuit was designed.Considering the limited space and serious cooling condition inside the module,the buck-boost topology was adopted for bottom circuit which has small volume,high efficiency,and easy to modularize.And considering the high power and more module numbers between modules,the top circuit was designed based on multi-input winding transformer,which has fast equalization speed,flexible control modes,and easy to be modularized.Thirdly,the equalization system model was established based on Simulink platform,and the effectiveness of the proposed double layer active equalization topology was validated.Then the hardware platform was designed including equalization unit,main control unit,battery information acquisition unit,etc.Furthermore,the variable duty cycle control was proposed based on battery voltage difference and voltage grade so as to realize the dynamic adjustment for balance current.This control strategy effectively avoided instant overcharge and over discharge caused by big voltage fluctuation,and clearly shorten the equalization time.The over equalization control was proposed based on the battery polarization voltage so as to solve the problem that battery voltage difference would recover after stopping equalization.This control strategy effectively improved the equalization results.And the last phase balance control based on historical charging information and voltage rising rate was proposed so as to effectively avoid wrong equalization for clearly aged batteries.Finally,the equalization experiments including static state and constant current charging condition at room temperature were designed and carried out to verify the proposed equalization circuit topology.Under the constant current charging condition,the experiments with different charging rates(0.25C/0.50C)were carried out respectively to verify the applicability of the equalization circuit.Experimental results showed that the proposed double layer active balancing system had a good balance result,and effectively improved the voltage consistency of battery pack and raised the available capacity.