Thesis Defense of Mr. Manish Tathode

Title:  Fast and Compact Voltage Equalizer for Satellite Applications.

Advisor: Prof. Vinod John.

Date and Time: Monday, 27th March 2023, 3.30 pm.
Location: EE-B304, EE Department.

Meeting Link:

Abstract:

Lithium-ion batteries have now become an inevitable 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, speeding up the increase in the inherent initial imbalance in the individual cell voltages in a series-connected stack, demanding fast equalization to avoid underutilization and reduced lifetime. Active multicell-to-multicell equalization achieves fast equalization by efficient simultaneous charge transfer among multiple cells in the series connected stack. PS-MAHB equalizer is one such multicell-to-multicell equalizer with the ability to maintain higher equalization currents irrespective of decreasing differences in the cell voltages. Its open loop, soft-switched operation, and modularization abilities make it an attractive choice for space applications. However, it needs to be modified with the necessary protective features and required redundancy essential for its use in space applications. Hence, the Modified PS-MAHB (M-PS-MAHB) equalizer is developed by incorporating necessary protection features and redundancy in the PS-MAHB equalizer. The flow of development of M-PS-MAHB equalizer is discussed. The Failure Mode Impact Analysis of the M-PS-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 redundancy does not let the cell get out of the equalization. Simulation results of FMIA considering transient and steady state impact of the failure are discussed.

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 which further increases the rate of equalization and improves 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 put by satellite applications. The equalizer hardware is realized as 4-cell equalizer modules, and the compactness of the equalizer hardware is achieved by pushing the switching frequency to 1MHz reducing the values of the passive components. 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 introduced, 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 demonstrating 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 to a step change in current and its impact on the voltage-sensing-based control algorithm is discussed along with the necessary modifications brought in the control to reduce the impact.

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