Research On A SOC Based Active Equalization Method For Power Lithium-ion Battery Pack In Electric Vehicles

Under the double pressure of environmental pollution and energy crisis, developing electric vehicles is becoming an inevitable trend for the sustainable development of automotive industry. As the energy source of electric vehicles, power battery directly affects vehicle performance. Lithium-ion battery is becoming the most promising power source for electric vehicles due to its superior performance. In electric vehicles applications, cells are connected in series and parallel to meet the demand of voltage, capacity and power. The inconsistency between cells will degrade the performance as well as shorten the lifetime of battery string, and even cause security issue. So it’s requisite to balance the serially connected battery string.(1) The basic structure and working principle of Lithium-ion battery is introduced firstly, and its main performance parameters is described, the dynamic characteristics of voltage, internal resistance and capacity are also analyzed. Then the origin of inconsistency is presented, the principle of voltage, internal resistance and capacity inconsistency is introduced. Afterwards, the elimination method for inconsistency is discussed, the screening approach and the equalization approach are analyzed and compared, from which the equalization approach is proven to be essential.(2) On the basis of analysis of design requirements and advantages/disadvantages of various equalization circuits, a modular balancing circuit is proposed. The whole battery pack is divided into several modules, the equalizer of modules balancing circuit is a flyback converter, and different modules can be equalized simultaneously. The equalizer of cells balancing circuit is a shared buck-boost converter, and the highest energy cell and the lowest energy cell are balanced prior. The energy transfer paths of both circuits are from source module/cell directly to target module/cell, respectively. The equalizers of both circuits employ zero-voltage switching and zero-current switching to reduce switching loss, and the specific operational principles are described in detail.(3) The corresponding control strategy of the proposed equalization circuit is designed as well. First, the current equalization criteria is analyzed and SOC is selected and the relative parameters used in balancing judgment and control are provided. Then the balancing processes among modules and within the module are introduced, the overall control principle is presented, and the control algorithm for all possible cases is designed. The energy transfer of modules balancing is the highest energy module to the lowest energy module, the second highest energy module to the second lowest energy module and by this analogy. The energy transfer of cells balancing is from the highest energy cell to the lowest energy cell.(4) At last, the performance of the proposed active equalization method is validated by equalization experiments for modules, cells within a module and the entire battery pack.