Design and Implementation of a Transformer-Less Voltage Equalizer for Using in Series Connected Lithium-Ion Batteries Packs

Due to the energy storage cells low voltage, such as batteries and super-capacitors, it is often necessary to connect them in series to form a string. Generally, these components are sensitive to over-voltage. Also, due to the inevitable differences in these devices, their charging and discharging processes cause their voltages to be different. This problem leads to inappropriate use of capacity, short life time, and even explosion. To overcome these problems and also to achieve safe and economic operation of these storage devices, employing voltage equalizer converters is necessary, in practice. The proposed converter can overcome the above-mentioned problems and it has less components than the conventional voltage equalizers, smaller dimensions, and lower cost. It operates at a fixed switching frequency under soft switching conditions without any feedback circuit and transformer. Therefore, it is more economic than the conventional voltage equalizers. Here, simulations and analysis of the different operating modes of the converter are given for four series connected lithium-ion batteries. To verify the given analysis and simulations, its prototype has been tested under different conditions. The experimental results are in good agreement with the analysis and simulation results, as well. The batteries voltages maximum differences in simulations and in practice are less than 5 and 10 mV, respectively, which indicate proper circuit operation.