Fast and Compact Voltage Equalizer for Satellite Applications

Abstract

Lithium-ion batteries have now become an essential constituent of the Electrical Power System of solar-powered satellites due to their high energy density, wider operating temperature range , and better radiation tolerance. For the compact realization and better space utilization, the series-parallel connected Li-ion batteries are operated with currents close to the design limit of the cells. This consequently speeds up the increase in the inherent initial imbalance in the individual cell voltages in a series connected stack, demanding fast equalization. Active multicell-to-multicell equalization achieves fast equalization by efficient charge transfer among multiple cells in the series connected stack. PS-MAHB equalizer is a multicell-to-multicell equalizer, with its open-loop control maintaining high equalization current throughout the equalization. Its soft-switched operation and modularization abilities make it an attractive choice for space applications. However, it lacks the necessary protective features and redundancy essential for its use in space applications. Hence, Modified PS-MAHB (MPS-MAHB) equalizer is developed by incorporating necessary protection features and redundancy in the PS-MAHB equalizer. The Failure Mode Impact Analysis of the MPS-MAHB equalizer reveals that during the most likely switch short circuit failure mode, the faulty part of the equalizer is disconnected by the protective device and the circuit redundancy does not let the cell get out of the equalization. The existing static phase shift-based control of the equalizer causes direct dependency of the equalization currents on the cell voltages and limits the equalization current levels to lower than the design equalization current value when the cell voltages are lower. Thus, the control works with a reduced rate of equalization and causes the under-utilization of the equalizer hardware for a significant duration of time in the charge-discharge cycle. A dynamic phase shift-based control is proposed to maximize the equalization current through the cells irrespective of the cell voltages to further increase the rate of equalization, and improve the equalizer hardware utilization. In the simulation, a significant improvement in the equalization rate compared to the static phase shift control is verified with the proposed dynamic phase shift based-control. The compact hardware realization of the equalizer hardware and the voltage sensor board addresses the space-volume constraints in satellite applications. The compact realization of 4-cell equalizer modules is achieved by pushing the switching frequency to 1MHz thereby reducing the values of the passive components. The challenges faced during the PCB design of the 4-cell equalizer module are discussed. A non-isolated high-precision op-amp-based voltage sensing scheme is developed to target the equalization band close to 10mV. The concept of an easy-to-design motherboard-based interface is implemented, which does not require any changes in the design of the 4-cell equalizer module and the voltage sensor board, irrespective of the cell connector geometry. The experimental results verify the operation of the equalizer showing the convergence of cell voltages from the initial imbalance of 300mV to the band of 10mV. The impact of the non-ideal dynamic response of the Li-ion cell voltage on the voltage-sensing-based control algorithm is discussed along with the necessary modifications brought in the control.