Research On Efficient Balancing Strategies For Series Batteries

As secondary energy, battery has been widely used in uninterruptible power supplies and electric vehicles. But the single-cell battery mostly cannot meet the requirements of the load. The series battery string is common. Internal differences during the production process and different external environments result in the differences of the electrical characteristics among batteries, such as the internal resistor and open-circuit voltage. Ignoring the differences will cause serious problems. If so, some batteries will be charged fully earlier while some will be later. Without reasonable control, there will be battery over-charge or over-discharge phenomenon which affects the life of batteries and optimal use of the battery energy. Based on this, balancing strategies for series batteries come into being.Based on research status at home and abroad in balancing strategies, this paper aims to find an efficient balancing strategy, taking simple and feasible control strategy as well as safe and reliable balancing process into account. The paper has completed experimental and improved research of the series-parallel converter balancing strategy, the centralized and distributed resistor balancing strategy as well as the single capacitor balancing strategy. Focused on switch loss during the single-capacitor balancing strategy, the multi-capacitor balancing strategy with the resonant soft-switching technology is presented.The resonant soft-switching technology is applied in the balancing process. Therefore the theoretical analysis is firstly carried out in this paper which validates the technical feasibility of the balancing strategy. Then the conditions of achieving zero current switching loss and the zero voltage gap are proposed. Taking MATLAB/Simulink as the simulation platform, four comparative simulation experiments are designed from different aspects including the internal resistor, the traditional capacitor balancing strategy, the switching frequency and the resonant inductor. Finally, the resonant capacitor balancing experiments for two Li-ion batteries verify the efficiency of the resonant strategy compared to the traditional capacitor balancing strategy. The effects of the switching frequency and inductor are also discussed. The experiment results prove that the resonant capacitor balancing strategy could reduce the switching loss and effectively complete the voltage balancing of series batteries with simple and feasible control strategy as well as reliable and safe balancing process.