An electrically driven demolition robot comprising, - a portable power supply system, - a propulsion means with continuous tracks with which the demolition robot can be driven, a manoeuvrable arm intended to support a tool at its free end, - a control unit intended to be carried by an operator who walks beside the demolition robot and controls its various motions in a wireless manner by radio control or through a cable, wherein during normal operation the demolition robot is intended to be connected via an electrical cable to a primary source of power which includes a fixed alternating current electricity distribution grid at the location, for example a voltage of 400 V at 50 Hz, the portable power supply system comprising, - an electric motor that is connected to a hydraulic pump and is intended to supply operating means (8c, 10a) of the demolition robot with a hydraulic medium, characterised in that the portable power supply system further comprising - a secondary source of power that comprises a DC energy-storage arrangement with the ability to store and to supply electrical energy, - a rectifier for the conversion of AC alternating current from the fixed electricity distribution grid at the location to DC direct current for the energy-storage arrangement, - a coupling arrangement (31, 33, 34, 34') arranged between the primary source of power, and the secondary source of power and the electric motor such that it can, when necessary, allow the secondary source of power to be connected to the electric motor in order to drive it, and thus the demolition robot, temporarily, wherein the coupling arrangement (31, 33, 34, 34') further comprises - a DC bus (34, 34') to which the rectifier is connected in order to supply DC direct current from the primary source of power to the DC bus, - a control circuit which is connected to the DC bus and controls and monitors the levels of voltage that are supplied from the DC bus to the electric motor through continuously sending and receiving signals from the units that are connected to the DC bus, and furthers measures the instantaneous state of the DC bus wherein the secondary source of power can be connected in and serve as a supplement to the primary source of power, a switch wherein, the secondary source of power is so dimensioned that it alone, and as a reserve unit, can manage to supply the electrical power that is required to the demolition robot, in any case during a limited period, whereby setting of the demolition robot for operation by means of the secondary source of power can be carried out manually by the operator through the setting of the switch by means of the control unit or automatically through the measurement and recording, by means of the control circuit, of the power surplus or power deficit that arises in the DC bus (34, 34') of the coupling arrangement (31, 33, 34, 34'). - a portable power supply system, - a propulsion means with continuous tracks with which the demolition robot can be driven, a manoeuvrable arm intended to support a tool at its free end, - a control unit intended to be carried by an operator who walks beside the demolition robot and controls its various motions in a wireless manner by radio control or through a cable, - an electric motor that is connected to a hydraulic pump and is intended to supply operating means (8c, 10a) of the demolition robot with a hydraulic medium, - a secondary source of power that comprises a DC energy-storage arrangement with the ability to store and to supply electrical energy, - a rectifier for the conversion of AC alternating current from the fixed electricity distribution grid at the location to DC direct current for the energy-storage arrangement, - a coupling arrangement (31, 33, 34, 34') arranged between the primary source of power, and the secondary source of power and the electric motor such that it can, when necessary, allow the secondary source of power to be connected to the electric motor in order to drive it, and thus the demolition robot, temporarily, wherein the coupling arrangement (31, 33, 34, 34') further comprises - a DC bus (34, 34') to which the rectifier is connected in order to supply DC direct current from the primary source of power to the DC bus, - a control circuit which is connected to the DC bus and controls and monitors the levels of voltage that are supplied from the DC bus to the electric motor through continuously sending and receiving signals from the units that are connected to the DC bus, and furthers measures the instantaneous state of the DC bus wherein the secondary source of power can be connected in and serve as a supplement to the primary source of power,