Interview

Abstract

Dr Xiaolong Li from Southwest Jiaotong University, China, talks to Electronics Letters about the background to his paper ‘Two-switch equalizer for series-connected battery stack using Zeta type converter and symmetrical capacitor-diode circuit’, page 1600. Xiaolong Li Energy storage systems are widely applied in various applications, including electric vehicles, renewable energy systems, etc. In most energy storage systems, lithium-ion batteries and super-capacitors are used. However, lithium-ion batteries and super-capacitors are usually connected in series to provide high-voltage, and voltages of these cells are gradually unbalanced because of non-uniform individual cell properties such as capacitance, internal impedance and self-discharge rate. Therefore, equalisation circuits are required to prevent over-charging and over-discharging of series-connected lithium-ion battery and super-capacitor stacks. Our team focuses on voltage equalisation to extend battery life and I think it is one of the most interesting areas to work in within the battery-circuit field. Various designs of battery equalisers have been proposed. However, most of them require multiple switches and/or a multi-winding transformer, thus they have issues in terms of circuit and/or control complexity/reliability, modularity, or size. In our Letter, a two-switch equaliser for series-connected battery stack using Zeta-type converter and symmetrical capacitor-diode circuit is proposed for voltage equalisation of series-connected batteries. The proposed equaliser consists of a Zeta-type front-end DC-DC converter and symmetrical capacitor-diode circuit, which only uses two active switches and two magnetic components. Thus, the design benefits from small circuit volume and is flexible for extension. Furthermore, the equaliser can work under open-loop, which makes its control simple. High operational efficiency is achieved due to zero voltage switching of all switches. In my opinion, the novelty of the work is significant. To avoid short-circuit events of battery cells during the equalisation process, most active equalisers require multiple switches and/or multi-winding transformers to achieve equalisation, thus they have the disadvantages of high cost or complex control or large size. With rectifier bridges and DC blocking capacitors, our proposed equaliser can efficiently prevent short-circuit events in battery cells. Furthermore, without multiple switches and multi-winding transformers, this equaliser benefits from small size, control simplicity and low cost. First, in order to avoid using multiple switches or a multi-winding transformer, a symmetrical capacitor-diode bridge was designed as the back-end circuit. Through the analysis of the characteristics of DC-DC converters and simulation of the equaliser, a Zeta converter is designed as the front-end circuit. After that, the operation mode of the proposed equaliser is analysed and its equivalent circuit is presented to simplify circuit analysis and parameters design. There are lots of considerations that are required when designing battery equalisation schemes, for example, the question of which front-end circuit should be chosen, how to design the parameters and so on. The final device has been tested and demonstrated in our lab using our energy management platform and three-battery equaliser prototype. We think our equaliser provides a new idea for other scholars. Our equaliser can be directly used in energy storage systems which need low cost and small size equalisation circuits. Some improved circuits and ideas with smaller size and higher efficiency may be derived from the proposed circuit. We also hope that the research will be used in the battery designs of electric vehicles. The use of diodes can reduce the control complexity and number of switches, but it also increases the loss of the circuit. We have therefore been working on how to increase the efficiency of the equaliser. With the development and use of batteries and super-capacitors in energy storage systems, voltage equalisation circuits will be one of the most important circuit components. Therefore, we predict that a low-cost, high-efficiency and small size equalisation circuit will be more significant and popular.