US Patent 8040103 Battery charging apparatus with planar inductive charging platform

A battery charging apparatus comprises an inductive charging platform including a charging surface on which an electrical device to be charged is to be placed, and a first winding for generating lines of magnetic flux generally perpendicular to the charging surface. To compensate for voltage sag caused by a reduction in the flux generated by the first winding, a second winding is located within an area defined by the first winding for generating an auxiliary magnetic flux generally perpendicular to the charging surface.