Apparatus and method for battery calibration and state of charge determination (SoC) within battery packs by detecting charge levels in a cell or cells, sending the charge level information to a controller which controls the movement of charge in the battery wherein, the controller initiates the discharge of a sub-unit to a predetermined level into the other sub-units and the charging of said sub-unit in the battery to a predetermined level from the charge held on the other sub-units to improve the accuracy and reliability of battery calibration and provide an accurate State of Charge indication from battery first use to end of life.