Bidirectional active charge equaliser for series-connected cells

In this study, a new bidirectional charge equaliser for series-connected cells is proposed to equalise the charge of N series-connected cells in static and dynamic states, i.e. while charging, discharging and when charger/load is disconnected. The proposed topology consists of three parts: a multiplexer, H-bridge and transformer. The multiplexer section connects the target cell to the transformer. The transformer acts as temporary energy storage and the H-bridge section sets the polarity of the voltage on the primary of the transformer. In charging/static mode, the energy transfers from charger/string to the low-charged cell and in discharging mode energy transfers from the cell with the highest charge to the load. In fact, energy transfer is in the same direction as the circuit operation, in this way, the speeds of charge and discharge of series cells are increased. Also due to the existence of only one small transformer and totem-pole arrangement of bidirectional switches, which halved the switch drivers, the cost and size of the circuit is reduced. A prototype of the proposed circuit to balance the charge of four series lithium-ion battery cells is designed and implemented. The experimental results confirm the correct operation of the circuit in all three states.