Since reconfigurable battery systems (RBS) can set a desired voltage by engaging / bypassing individual cells, they can fast-charge electric vehicles without battery-to-battery DC-DC converters. However, as individual cell voltages limit the control accuracy, current ripples exceed charging standard (IEC61851-23) tolerances. To eliminate the DC-DC converters without increasing losses, this work investigates the feasibility of using a minor subset of cells with pulse-width modulated (PWM) discharging. The analysis is twofold. Laboratory experiments are designed and demonstrate that switching more frequently than 2 μ s can interfere with switching dynamics. This allows us to choose a PWM frequency, which does not interfere with switching, accelerate degradation or increase impedance, and to choose coils for the battery string to suppress PWM current ripples. The current ripples are validated with laboratory tests. Finally, closed-loop simulations prove the concept. A fast-dynamics simulation is used to prove fast current tracking and to validate a computationally inexpensive theoretical model for current ripples, then a slow-dynamics simulation uses this model to demonstrate that the designed circuit sufficiently reduces the undesired current ripples. The proposed system has similar efficiency to a system with a DC-DC converter, and it is ready for deployment. • A laboratory setup is designed to investigate fast and slow cell, while deploying a RBS of LFP cells. • Laboratory experiments reveal switching dynamics in the nanosecond and microsecond region. • PWM frequency and coil selection considering degradation, oscillations, efficiency and ripples. • Compliance of the current ripple with the charging standard IEC61851-23 is demonstrated. • Mathematical relations help to consider PWM current ripple in the slow dynamics simulation.
Pinter et al. (Fri,) studied this question.