The Research In Circuit Topology And Control Algorithm Of Lithium-ion Battery Pack Equalization System

Plenty of fossil resources and energy consumption, cause global environmental pollution and resource depletion. Under the pressure, governments actively promote the industries of low energy consumption and mild pollution, such as electric cars. And they vigorously increase the development of new nergy resources such as wind and solar. Energy storage devices act important roles in these measures implemented. Lithium-ion batterie with its high energy density, no memory effect, and long service life, has been widely utilized in many applications. So it's one of the most promising batteries. Nevertheless, lithium-ion batteries have to be series-connected for high-voltage applications due to their low voltage. The differences existing in batteries result in imbalance problem during battery strings charged and discharged repeatedly. The imbalance problem accelerates asymmetrical cell degradation with aging, and even damages the batteries. Equalization system can make the scheduling of energy, and keep all batteries in the same SOC. That effectively prevents the batteries from overcharge or overdischarge.The paper summarizes domestic and foreign researches, and contrastively analyzes the advantages and shortcomings of equalization approaches. Energy bus-based equalization is employed for the demand of rapidity, high efficiency, flexibility and extensibility. The equalization structure shorts the energy-flow pathway, greatly improve equalization speed and efficiency. And the main work is as follows:(1)Based on Cuk converter, a bi-directional isolated equalizer is proposed. The equalizer realizes the isolation by two capacitors. Its mathematical model is established to analyze the working principle. And a synchronous control strategy with conditions is adopted for higher efficiency.(2)This paper employs double-loop structure by analyzing the mathematical model of lithium-ion batteries. The outside-loop controller with consensus protocol makes the scheduling of energy based on SOC of batteries. And the convergence of protocol has been proved by a Lyapunov function. The inside-loop controller employs fuzzy strategy to regulate equalization currents and to steady bus voltage.(3)The experimental platforms are set up to test the performance of this equalization scheme, such as synchronous condition, equalization efficiency, the validity of equalization control scheme. Based on experiments, the synchronous condition of equalizers has been verified. The equalizers can achieve efficiency above 90% in a wide range of equalization current. And the validity of equalization control scheme has also been proved.In summary, this work presents equalization scheme for the energy bus-based equalization network. The validity has been verified by simulation models and experimental platforms. This scheme with its high performance provides a perfect solution for high-power and super-power applications.