  Batteries are connected in series to meet the voltage requirement in many
applications. A voltage equalizer circuit is necessary to ensure that none of
the batteries is over-charged or over-discharged.
  A novel fast soft-switched cell-to-cell voltage equalizer topology is
proposed in this work. This topology can transfer charge from multiple
over-charged batteries to multiple under-charged batteries simultaneously
avoiding any unnecessary charging or discharging of a battery to achieve fast
voltage equalization. The proposed circuit topology and modulation method
ensure zero voltage switching under all battery conditions. The circuit
operation and soft-switching are analyzed and experimentally verified with a
four battery voltage equalizer prototype. The prototype is tested with a
battery bank and a hybrid ultra-capacitor bank, and a high conversion
efficiency is verified.
