A Flyback Converter-Based Hybrid Balancing Method for Series-Connected Battery Pack in Electric Vehicles

Abstract

An active balancing method based on two flyback converters is proposed for series-connected battery pack. Balanced energy can be transferred between the whole battery and any single cell. The proposed topology reduces the number of energy storage components, the volume and the cost of the balancing system. And it has the characteristics of fast balancing speed and high balancing efficiency. Based on the topology, a dual-objective hybrid control strategy is proposed, which can reduce the highest voltage and boost the lowest voltage in the charging or discharging process of the cells in one sampling period, so as to improve the balancing speed. Simulation and experimental results show that the proposed method has a good balancing effect and can significantly improve the consistency of series battery pack. This work is potentially significant in terms of improved reliability of battery packs and savings of costs and lives in safety-critical applications.

Design and Control of a Battery Charger/Discharger Based on the Flyback Topology

“…In this work, the design will be focused on the current ripple at the magnetizing inductance and the voltage ripple at the bus voltage, since the first one affects the transformer rating and the second one is defined by the requirements of the devices connected to the bus. The ripple at the magnetizing inductance is calculated from (1), which forces the increment of i m during d • T sw seconds (when u = 1); thus, the current ripple in i m , around the steady-state value given in (15), is given in (17). In such an expression, F sw = 1/T sw is the switching frequency.…”

“…Since the main perturbation sources are the changes on the bus current, i m or K i must be adjusted to compensate the changes on i dc : the relation between i m and i dc , given (15), shows that i dc = 1−d n • i m , thus K i must be defined as given in (33) to ensure the compensation of i dc .…”

“…Considering that these converters have limited voltage gains, a typical value of v b is 12 V [7][8][9], and the values of v dc in microgrid applications vary in wide range, but are significantly greater than v b (e.g., 48 V [10][11][12], 200 V [13]), or higher [14]; then, multiple batteries are connected in series and parallel to reach the required voltage and storage capacity levels, respectively. Those batteries have different electrical characteristics due to manufacturing imperfections, uniform aging, and continuous charging/discharging cycles; these differences produce excessive heat and accelerated degradation of the batteries [15]. That is why there is a necessity for battery balancing systems [15,16] or charging/discharging systems implemented with high-gain power converters to connect each battery, or arrays of parallel batteries, directly to the DC bus.…”

“…Those batteries have different electrical characteristics due to manufacturing imperfections, uniform aging, and continuous charging/discharging cycles; these differences produce excessive heat and accelerated degradation of the batteries [15]. That is why there is a necessity for battery balancing systems [15,16] or charging/discharging systems implemented with high-gain power converters to connect each battery, or arrays of parallel batteries, directly to the DC bus. Such a charging/discharging system would avoid the necessity of a balancing system providing some advantages like a high ESDs modularity, and the connection of batteries with several technologies and electrical characteristics.…”

“…The previous reachability conditions are analyzed by replacing the switching function derivative (22) into inequalities ( 27) and (28), obtaining the design expressions given in ( 29) and (30), respectively, where expressions (15) and ( 16) have also been used.…”