| ABSTRACT:With wide applications of lithium-ion power batteries in electric vehicles and battery energy storage, the battery grouping technology has become the research hotspot. However, the inconsistency of cells affects many aspects of battery packs, such as capacity, performance, life and safety. This dissertation is focus on the fading mechanisms and grouping optimization analysis of the paralleled lithium-ion battery pack, in which lithium iron phosphate battery and LiMni/3Ni1/3Co1/3O2battery are used as the research objects. Details of the research and achievement are as follows:(1) A parallel battery test platform is established on which different aging batteries are connected in parallel to analyze the imbalanced currents of different branches. Meanwhile, the circulating current after charging and discharging is also considered. Based on the parameters varying with SOC, a calculation method of SOC accumulation effect is proposed. The influence of OCV on the imbalanced currents is also explained according to the comparison of branch current between paralleled LiFePO4and NCM battery packs. The above contents lay the foundation for mechanism analysis of battery aging, and clear the goal of grouping optimization.(2) Making an improved plan for the cycle life test of paralleled lithium-ion batteries, the recession factors of battery packs for different types and different inconsistent degrees are studied by the equivalent circuit parameters, imbalanced currents and dQ/dV curves. Simultaneously, the influence of ohmic resistance, polarization impedance and capacity on imbalanced currents and the influence of imbalanced currents on the aging of battery packs are analyzed in detail. By comparing the difference of action mechanism of different battery types and the difference of degradation mechanism between cell and pack, it comes to a conclusion that the maximum influence factor on decline rate of a battery pack in parallel is the inconsistent degree.(3) Based on the equivalent circuit and charging/discharging curves of different rates, two kinds of parallel-connected battery pack models for simulation are built and the simulation precision of the two methods are also verified by comparing with the actual test data. Indispensably, the conditions and scopes of application for the two methods are listed according to meticulous comparative analysis. Furthermore, the simulation methods of paralleled batteries are extended to the mixed-connected battery pack which is modular.(4) The separately influence of impedance, capacity, initial SOC and current rate is re-analyzed decoupling, and a matching method between capacity and impedance is proposed. In view of the problem of extreme current at the end of charge and discharge, it comes to a using advice that the battery pack is shallow charged and discharge or needs a short-term rest at low SOC and high SOC. The degrees of imbalanced currents for different topology structures which are serial-parallel coupling manner and parallel coupling directly are compared, and based on serial-parallel coupling manner, the paper also evaluates the importance of basic unit modularity. In allusion to different parallel and serial numbers of the smallest unit topology structure, this dissertation proposes a research plan to analyze the influence of connection manner on the battery performance. At last, the influence of series-parallel battery number on maximum current is studied which provides the guidance for optimal selection, optimal grouping, optimal operation of battery packs. |
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