A method of charging a battery in a mobile unit by means of an electric charging system provided for contactless charging of the battery of the mobile unit, wherein the electric charging system has a fixed charging station, wherein a primary coil arrangement of the charging station is arranged in a fixed manner and a secondary coil arrangement of the electric charging system is arranged in the mobile unit, wherein the primary coil arrangement and the secondary coil arrangement are designed in such a manner that in a mobile-unit charging position relative to the primary coil arrangement, a charging energy for charging or discharging the battery is transferrable between the primary coil arrangement and the secondary coil arrangement and data for communication between the mobile unit and the charging station can be received and/or sent in a contactless manner, wherein the primary coil arrangement is designed as a primary coil and a first coil device and the secondary coil arrangement as a secondary coil and a second coil device, wherein during a charging procedure or a discharging procedure of the battery, the charging energy is transferrable in a contactless manner between the primary coil and the secondary coil and a data signal is transmittable, preferably bi-directionally, between the first and second coil device, wherein the primary coil, the first coil device, the secondary coil and the second coil device are arranged in such a manner that a magnetic field generated in each case penetrates the primary coil, the first coil device, the secondary coil and the second coil device, wherein the primary coil, the first coil device as well as the secondary coil and the second coil device are in the form of a flat coil, wherein in each case the first coil device is arranged within the primary coil and the second coil device within the secondary coil, characterised in that - the mobile unit activates the electric charging system by travelling over the energy charging mat, - data relating to the battery is transmitted to the charging station, - the battery is charged and simultaneously a value for the state of charge of the battery is transmitted to the charging station, preferably in the data signal, wherein simultaneously with charging, data relating to the maintenance state of the mobile unit is transmitted and evaluated in order to bring about or prepare appropriate maintenance work, wherein the data transmission is operated with a signal with a data carrier frequency in the range from 0.1 MHz, preferably between 0.9 MHz and 1.5 MHz, and the transmission of the charging energy is operated with an energy carrier frequency in the range from 1 to 500 KHz, preferably in the range between 20 and 60 kHz, wherein the data carrier frequency is always greater than the energy transfer frequency, wherein - the state of charge of the battery and at least one actual value for charging current and/or charging voltage are determined, - and a value for the state of charge and the actual value are transmitted to the charging station in an at least partially contactless manner, - the first part of the charging procedure is regulated in a current-led manner by a current regulator and the charging procedure is regulated in a voltage-led manner by a voltage regulator when a switch-over threshold of the state of charge of the battery has been exceeded, wherein the current regulator and the voltage regulator are arranged in a stationary manner, that is in a non-movable manner relative to the charging station. - the mobile unit activates the electric charging system by travelling over the energy charging mat, - data relating to the battery is transmitted to the charging station, - the battery is charged and simultaneously a value for the state of charge of the battery is transmitted to the charging station, preferably in the data signal, - the state of charge of the battery and at least one actual value for charging current and/or charging voltage are determined, - and a value for the state of charge and the actual value are transmitted to the charging station in an at least partially contactless manner, - the first part of the charging procedure is regulated in a current-led manner by a current regulator and the charging procedure is regulated in a voltage-led manner by a voltage regulator when a switch-over threshold of the state of charge of the battery has been exceeded,