Abstract: The invention relates to an active battery balancing circuit comprising: a first battery unit; at least one further battery unit; separate bi-directional DC-AC voltage convertors connected to each battery unit for converting DC to AC in a first direction, and vice versa in the opposite direction; an AC balancing bus connecting each voltage convertor and a control microprocessor; wherein said balancing circuit is configured to a) provide excess DC current flow from a battery unit to one of said bidirectional voltage convertors; b) convert said excess DC current to AC current and provide this to said AC balancing bus; c) provide said excess AC current in said AC balancing bus to one of said bidirectional voltage convertors; d) convert said excess AC current to DC current using said bi-directional voltage convertor and return said converted DC current to a battery unit that requires additional charge.