An active battery balancing circuit comprising: a first battery unit comprising multiple cells (202a... n); at least one further battery unit comprising multiple cells (202a... n); separate bi-directional DC-AC voltage converters 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 converter; 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 converters; 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 converters; d) convert said excess AC current to DC current using said bidirectional voltage converter and return said converted DC current to a battery unit 202 that requires additional charge, characterized in that said voltage converter is a planar transformer; wherein said circuit further comprises battery balance monitoring means connected between said battery units and said voltage converters and wherein said battery balance monitoring means comprises: monitoring means connected to each said battery unit, switch means connected between said monitoring means and said voltage converter. a first battery unit comprising multiple cells (202a... n); at least one further battery unit comprising multiple cells (202a... n); separate bi-directional DC-AC voltage converters 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 converter; 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 converters; 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 converters; d) convert said excess AC current to DC current using said bidirectional voltage converter and return said converted DC current to a battery unit 202 that requires additional charge, characterized in that said voltage converter is a planar transformer; a) provide excess DC current flow from a battery unit to one of said bidirectional voltage converters; 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 converters; d) convert said excess AC current to DC current using said bidirectional voltage converter and return said converted DC current to a battery unit 202 that requires additional charge, characterized in that said voltage converter is a planar transformer; wherein said circuit further comprises battery balance monitoring means connected between said battery units and said voltage converters and wherein said battery balance monitoring means comprises: monitoring means connected to each said battery unit, switch means connected between said monitoring means and said voltage converter.