Research On Any-cell-to-any-cell Equalization Technology For Series-Connected Lithium-Ion Battery Packs

The large-scale development of electric vehicles is an important strategic initiative to respond to the climate change and energy crisis,and it is also the key way for China to become a powerful country in the automobile industry.As the core of electric vehicles,lithium-ion batteries are critical to the performance,economy,and safety of the entire vehicle.However,the capacity and terminal voltage of lithium-ion battery cells are low due to the constraints of the electrochemical mechanism,production materials,and manufacturing process.The cells usually need to be connected in series and parallel to form a battery pack.The energy mismatch between series-connected cells is very easy to occur,resulting in a reduction in the overall available capacity and reliability of the battery pack.In addition,the aging difference between the cells will also aggravate the degree of energy mismatch.The life of the battery pack will be greatly shortened under the vicious cycle.Energy equalization technology can effectively alleviate this problem,especially the method based on AC2 AC equalization has the advantages of flexible equalization mode,low flowback energy and high-power level.Therefore,Combining with AC2 AC equalization mode,this paper conducts research in terms of battery equalizer,modular equalization structure,and estimation method of cell health state.The main research contents are shown as:(1)For the automatic battery equalizer,the strong correlation between the balancing current and cell voltage difference will constrain the global equalization performance.To solve this problem,an automatic equalizer with inherent power limitation is proposed.The linear relationship between balancing current and voltage difference is changed by the inherent power limitation characteristic to achieve faster global balancing speed and ensure safe operation.In addition,high conversion efficiency is achieved due to the soft-switching operations of the switches.Although a multiwinding transformer is used in the equalizer,the impact of parameter mismatch can be mitigated by designing appropriate driving signals.(2)The any-cell-to-any-cell(AC2AC)active equalizers suffer from the problems of high cost,large size,and difficult control.To overcome this problem,an AC2 AC equalizer based on the half-bridge LC converter is proposed.By adjusting the duty cycle of the corresponding half-bridge for each cell,flexible and direct energy equalization is achieved between any cells.An enumeration-based duty cycle assignment strategy is proposed,and cell-to-cell,cell-to-multicell,multicell-to-cell,and multicell-to-multicell balancing modes can be flexibly realized regardless of the number and location of the target cells.Compared with existing equalizers in the same category,the proposed method uses the small-size passive elements,which is important for the compact design of the system.In addition,an improved method for estimating cell SOH within the battery pack is proposed in conjunction with the equalizer.This estimation method is simple and effective,and does not require the configuration of adaptive models for each cell.It reduces the computational complexity and difficulty of estimating all cell SOHs within the pack.(3)The winding leakage inductance of traditional modular methods based on auxiliary windings will hinder energy transmission between modules.To address this issue,a modular method based on the auxiliary resonant tanks is proposed in conjunction with the AC2 AC equalizer.Additional auxiliary capacitors,inductors,and winding leakage inductance are added in the auxiliary winding to jointly construct a resonant tank.The resonance frequency and switching frequency are set to be matched.The impact of leakage on the equalization between modules is indirectly eliminated.Experimental platforms for the two proposed equalizers and two modular methods are built.The feasibility and superiority of the proposed method are verified through a series of comparative experiments.(4)The degree of mismatch between modules in a battery pack is usually higher than that within a module.Hence,a modular method based on phase shift modulation is further proposed in conjunction with the AC2 AC automatic equalizer.By adjusting the phase shift ratio between the driving signals of each individual level equalizer,the current on the winding leakage inductance is modulated.The winding leakage inductance is used as a carrier for energy transmission.Compared with the traditional method,it significantly improves the energy balancing speed between modules without increasing too much system cost.In addition,a shift ratio allocation strategy based on cluster analysis is proposed,which simplifies the difficulty of calculating the shift ratio for each individual level equalizer.Finally,the dissertation summarizes the whole work and combines the problems encountered in the research work to make an expectation of future research directions.