An alternating current power supply assembly which has an extendable operational period and which provides an external power supply for a tool, the power supply assembly comprising: a power inverter unit for converting direct current (DC) into alternating current (AC) and a removable battery unit which is connectable to the power inverter unit by a coupling to form the power supply assembly and which when so connected forms a discrete unit: characterised in that , the AC power connector is positioned on the power inverter unit ; the removable battery unit comprising a battery , a battery management system which is configured to be removeably connected to the power inverter unit via the coupling , an integral charger and an AC charging connector wherein; the coupling is configured to provide an electrical connection between the power inverter unit and the removeable battery unit and a mechanical coupling which is removeably configured to secure the power inverter unit to the removeable battery unit such that in use, a DC output from the removable battery unit is converted to an AC output from the power supply assembly by the power inverter unit , the operational period of the power supply assembly being extended by replacement of the removeable battery unit when a portion of the energy in the removeable battery unit has been used. a power inverter unit for converting direct current (DC) into alternating current (AC) and a removable battery unit which is connectable to the power inverter unit by a coupling to form the power supply assembly and which when so connected forms a discrete unit: characterised in that , the AC power connector is positioned on the power inverter unit ; the removable battery unit comprising a battery , a battery management system which is configured to be removeably connected to the power inverter unit via the coupling , an integral charger and an AC charging connector wherein; the coupling is configured to provide an electrical connection between the power inverter unit and the removeable battery unit and a mechanical coupling which is removeably configured to secure the power inverter unit to the removeable battery unit such that in use, a DC output from the removable battery unit is converted to an AC output from the power supply assembly by the power inverter unit , the operational period of the power supply assembly being extended by replacement of the removeable battery unit when a portion of the energy in the removeable battery unit has been used. characterised in that , the AC power connector is positioned on the power inverter unit ; the removable battery unit comprising a battery , a battery management system which is configured to be removeably connected to the power inverter unit via the coupling , an integral charger and an AC charging connector wherein; the coupling is configured to provide an electrical connection between the power inverter unit and the removeable battery unit and a mechanical coupling which is removeably configured to secure the power inverter unit to the removeable battery unit such that in use, a DC output from the removable battery unit is converted to an AC output from the power supply assembly by the power inverter unit , the operational period of the power supply assembly being extended by replacement of the removeable battery unit when a portion of the energy in the removeable battery unit has been used. <EOS>