Wireless Power Transfer-Based Voltage Equalizer for Scalable Cell-String Charging

Wireless power transfer (WPT)-based voltage equalizer (VE) shows unique advantages due to its physical isolation. Meanwhile, the markets of high-power fast charging have imposed significant needs for high-voltage wireless charging systems. However, a WPT-based charging equalizer cannot support a large-scale cell-string. This article proposes a wireless charging VE with modularity to balance cascaded cell-strings automatically using multiple coils. The WPT system's intrinsic high-frequency power is used to power the voltage multiplier (VM). Only two diodes and one capacitor are required for an extra cell, which enables fewer components and a simple structure. Circuit analysis and a one-transmitter-two-receiver charging equalization experiment validate the system's operating principles. The voltage gaps among eight ultracapacitor (UC) cells are equalized from 1.1 to 0.01 V, with an overall efficiency of 76.5% under a 35.2-V output. Nonideal conditions with coupling mismatch and frequency shifts are also analyzed to reveal the system's characteristics. It contributes several ways to providing general modular wireless charging equalization solutions for large-scale cell-string with lower cost and higher scalability.