Power Electronic Converters for Condition Monitoring and Voltage Equalization of Batteries

Abstract

Power converters are used in battery-based storage systems in many applications. Apart from the task of regulating the charging and discharging, the power electronic converters can also help to monitor battery condition and to avoid over-charge or over-discharge of any battery cell. One approach to monitoring the cell condition is by measuring its impedance. The power converter for charging and discharging of the cell stack can be used for online measurement of cell impedances. The challenges involved in control, measurement, and the hardware requirements for impedance measurement are analyzed in this work, and suitable solutions are proposed. A Proportional Integral Resonant (PIR) controller-based control scheme and a DAC based measurement method are proposed for impedance measurement over the required frequency range. Two di erent approaches are proposed to achieve su cient output voltage resolution for generating small amplitude voltage perturbation. One approach achieves high voltage resolution by replacing the single-leg buck converter with a multi-leg interleaved converter. The other approach uses a low-power rated auxiliary converter in series with the main converter to achieve high voltage resolution. Both of the methods are experimentally veri ed and compared with commercial equipment and the advantages of each approach are evaluated. A voltage equalizer is a power electronic circuit that equalizes the cell voltages in a series- connected cell stack to avoid over-charge and over-discharge of any individual cell. A low- cost voltage equalizer using selection switches for a cell to cell equalization is proposed. This equalizer uses capacitive voltage level shifting to avoid bulky and lossy isolation transformer and to reduce cost. A new approach with a lower number of low-frequency selection switches further reduces the equalizer cost. A high-performance voltage equalizer is also proposed to achieve fast equalization by direct multi-cell to multi-cell charge transfer. This topology is shown to provide soft-switching with high e ciency. The equalizer is controlled in an open loop. The equalization currents do not reduce with progress in voltage equalization, making this topology faster than the existing open-loop multi-cell to multi-cell topologies. A modularization method is proposed for this topology to provide a direct path for charge transfer from any cell in one module to any cell in another module. The operation of both the equalizers and the modularization technique are experimentally veri ed which con rms the theoretical analysis.